The Origin of Species by means of Natural Selection
Charles Darwin

The Origin of Species by means of Natural Selection
Table of Contents
The Origin of Species by means of Natural Selection; or the Preservation of Favoured Races in the
Struggle for Life.......................................................................................................................................................1
Charles Darwin..............................................................................................................................................2
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES,
PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK..........................3
INTRODUCTION.........................................................................................................................................8
CHAPTER I. VARIATION UNDER DOMESTICATION........................................................................10
CHAPTER II. VARIATION UNDER NATURE.......................................................................................24
.
CHAPTER III. STRUGGLE FOR EXISTENCE........................................................................................32
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.............................39
SUMMARY OF CHAPTER. ......................................................................................................................60
.
CHAPTER V. LAWS OF VARIATION.....................................................................................................62
CHAPTER VI. DIFFICULTIES OF THE THEORY. ................................................................................76
.
CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL
SELECTION...............................................................................................................................................94
CHAPTER VIII. INSTINCT.....................................................................................................................113
CHAPTER IX. HYBRIDISM....................................................................................................................128
SUMMARY OF CHAPTER. ....................................................................................................................142
.
CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD....................................144
CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. .............................158 .
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.......................................................170
CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.............................................................................172
CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION−−continued.......................................................186
SUMMARY OF THE LAST AND PRESENT CHAPTERS. ..................................................................195
.
CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY −−
EMBRYOLOGY −− RUDIMENTARY ORGANS.................................................................................197
CHAPTER XV. RECAPITULATION AND CONCLUSION..................................................................218
GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.......231
INDEX.......................................................................................................................................................240
i

The Origin of Species by means of Natural Selection;
or the Preservation of Favoured Races in the
Struggle for Life.
The Origin of Species by means of Natural Selection; or the Preservation of Favoured Races in the Struggle for
1

The Origin of Species by means of Natural Selection
Charles Darwin
"But with regard to the material world, we can at least go so far as this−− we can perceive that events are
brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."−−Whewell: "Bridgewater Treatise".
"The only distinct meaning of the word 'natural' is STATED, FIXED or SETTLED; since what is natural as
much requires and presupposes an intelligent agent to render it so, i.e., to effect it continually or at stated times, as
what is supernatural or miraculous does to effect it for once."−−Butler: "Analogy of Revealed Religion".
"To conclude, therefore, let no man out of a weak conceit of sobriety, or an ill−applied moderation, think or
maintain, that a man can search too far or be too well studied in the book of God's word, or in the book of God's
works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both."−−Bacon:
"Advancement of Learning".
Charles Darwin 2

The Origin of Species by means of Natural Selection
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE
ORIGIN OF SPECIES, PREVIOUSLY TO THE PUBLICATION OF THE FIRST
EDITION OF THIS WORK.
I will here give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great
majority of naturalists believed that species were immutable productions, and had been separately created. This
view has been ably maintained by many authors. Some few naturalists, on the other hand, have believed that
species undergo modification, and that the existing forms of life are the descendants by true generation of pre
existing forms. Passing over allusions to the subject in the classical writers (Aristotle, in his "Physicae
Auscultationes" (lib.2, cap.8, s.2), after remarking that rain does not fall in order to make the corn grow, any more
than it falls to spoil the farmer's corn when threshed out of doors, applies the same argument to organisation; and
adds (as translated by Mr. Clair Grece, who first pointed out the passage to me), "So what hinders the different
parts (of the body) from having this merely accidental relation in nature? as the teeth, for example, grow by
necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the
food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to
other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together
(that is all the parts of one whole) happened like as if they were made for the sake of something, these were
preserved, having been appropriately constituted by an internal spontaneity; and whatsoever things were not thus
constituted, perished and still perish." We here see the principle of natural selection shadowed forth, but how little
Aristotle fully comprehended the principle, is shown by his remarks on the formation of the teeth.), the first
author who in modern times has treated it in a scientific spirit was Buffon. But as his opinions fluctuated greatly
at different periods, and as he does not enter on the causes or means of the transformation of species, I need not
here enter on details.
Lamarck was the first man whose conclusions on the subject excited much attention. This justly celebrated
naturalist first published his views in 1801; he much enlarged them in 1809 in his "Philosophie Zoologique", and
subsequently, 1815, in the Introduction to his "Hist. Nat. des Animaux sans Vertebres". In these works he up
holds the doctrine that all species, including man, are descended from other species. He first did the eminent
service of arousing attention to the probability of all change in the organic, as well as in the inorganic world,
being the result of law, and not of miraculous interposition. Lamarck seems to have been chiefly led to his
conclusion on the gradual change of species, by the difficulty of distinguishing species and varieties, by the
almost perfect gradation of forms in certain groups, and by the analogy of domestic productions. With respect to
the means of modification, he attributed something to the direct action of the physical conditions of life,
something to the crossing of already existing forms, and much to use and disuse, that is, to the effects of habit. To
this latter agency he seems to attribute all the beautiful adaptations in nature; such as the long neck of the giraffe
for browsing on the branches of trees. But he likewise believed in a law of progressive development, and as all the
forms of life thus tend to progress, in order to account for the existence at the present day of simple productions,
he maintains that such forms are now spontaneously generated. (I have taken the date of the first publication of
Lamarck from Isidore Geoffroy Saint− Hilaire's ("Hist. Nat. Generale", tom. ii. page 405, 1859) excellent history
of opinion on this subject. In this work a full account is given of Buffon's conclusions on the same subject. It is
curious how largely my grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of opinion
of Lamarck in his "Zoonomia" (vol. i. pages 500−510), published in 1794. According to Isid. Geoffroy there is no
doubt that Goethe was an extreme partisan of similar views, as shown in the introduction to a work written in
1794 and 1795, but not published till long afterward; he has pointedly remarked ("Goethe als Naturforscher", von
Dr. Karl Meding, s. 34) that the future question for naturalists will be how, for instance, cattle got their horns and
not for what they are used. It is rather a singular instance of the manner in which similar views arise at about the
same time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint−Hilaire (as we shall immediately
see) in France, came to the same conclusion on the origin of species, in the years 1794−5.)
Geoffroy Saint−Hilaire, as is stated in his "Life", written by his son, suspected, as early as 1795, that what we
call species are various degenerations of the same type. It was not until 1828 that he published his conviction that
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY
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The Origin of Species by means of Natural Selection
the same forms have not been perpetuated since the origin of all things. Geoffroy seems to have relied chiefly on
the conditions of life, or the "monde ambiant" as the cause of change. He was cautious in drawing conclusions,
and did not believe that existing species are now undergoing modification; and, as his son adds, "C'est donc un
probleme a reserver entierement a l'avenir, suppose meme que l'avenir doive avoir prise sur lui."
In 1813 Dr. W.C. Wells read before the Royal Society "An Account of a White Female, part of whose skin
resembles that of a Negro"; but his paper was not published until his famous "Two Essays upon Dew and Single
Vision" appeared in 1818. In this paper he distinctly recognises the principle of natural selection, and this is the
first recognition which has been indicated; but he applies it only to the races of man, and to certain characters
alone. After remarking that negroes and mulattoes enjoy an immunity from certain tropical diseases, he observes,
firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated
animals by selection; and then, he adds, but what is done in this latter case "by art, seems to be done with equal
efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which
they inhabit. Of the accidental varieties of man, which would occur among the first few and scattered inhabitants
of the middle regions of Africa, some one would be better fitted than others to bear the diseases of the country.
This race would consequently multiply, while the others would decrease; not only from their in ability to sustain
the attacks of disease, but from their incapacity of contending with their more vigorous neighbours. The colour of
this vigorous race I take for granted, from what has been already said, would be dark. But the same disposition to
form varieties still existing, a darker and a darker race would in the course of time occur: and as the darkest would
be the best fitted for the climate, this would at length become the most prevalent, if not the only race, in the
particular country in which it had originated." He then extends these same views to the white inhabitants of colder
climates. I am indebted to Mr. Rowley, of the United States, for having called my attention, through Mr. Brace, to
the above passage of Dr. Wells' work.
The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the fourth volume of the "Horticultural
Transactions", 1822, and in his work on the "Amaryllidaceae" (1837, pages 19, 339), declares that "horticultural
experiments have established, beyond the possibility of refutation, that botanical species are only a higher and
more permanent class of varieties." He extends the same view to animals. The dean believes that single species of
each genus were created in an originally highly plastic condition, and that these have produced, chiefly by
inter−crossing, but likewise by variation, all our existing species.
In 1826 Professor Grant, in the concluding paragraph in his well−known paper ("Edinburgh Philosophical
Journal", vol. XIV, page 283) on the Spongilla, clearly declares his belief that species are descended from other
species, and that they become improved in the course of modification. This same view was given in his Fifty−fifth
Lecture, published in the "Lancet" in 1834.
In 1831 Mr. Patrick Matthew published his work on "Naval Timber and Arboriculture", in which he gives
precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace
and myself in the "Linnean Journal", and as that enlarged in the present volume. Unfortunately the view was
given by Mr. Matthew very briefly in scattered passages in an appendix to a work on a different subject, so that it
remained unnoticed until Mr. Matthew himself drew attention to it in the "Gardeners' Chronicle", on April 7,
1860. The differences of Mr. Matthew's views from mine are not of much importance: he seems to consider that
the world was nearly depopulated at successive periods, and then restocked; and he gives as an alternative, that
new forms may be generated "without the presence of any mold or germ of former aggregates." I am not sure that
I understand some passages; but it seems that he attributes much influence to the direct action of the conditions of
life. He clearly saw, however, the full force of the principle of natural selection.
The celebrated geologist and naturalist, Von Buch, in his excellent "Description Physique des Isles Canaries"
(1836, page 147), clearly expresses his belief that varieties slowly become changed into permanent species, which
are no longer capable of intercrossing.
Rafinesque, in his "New Flora of North America", published in 1836, wrote (page 6) as follows: "All species
might have been varieties once, and many varieties are gradually becoming species by assuming constant and
peculiar characters;" but further on (page 18) he adds, "except the original types or ancestors of the genus."
In 1843−44 Professor Haldeman ("Boston Journal of Nat. Hist. U. States", vol. iv, page 468) has ably given
the arguments for and against the hypothesis of the development and modification of species: he seems to lean
toward the side of change.
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY
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The Origin of Species by means of Natural Selection
The "Vestiges of Creation" appeared in 1844. In the tenth and much improved edition (1853) the anonymous
author says (page 155): "The proposition determined on after much consideration is, that the several series of
animated beings, from the simplest and oldest up to the highest and most recent, are, under the providence of God,
the results, FIRST, of an impulse which has been imparted to the forms of life, advancing them, in definite times,
by generation, through grades of organisation terminating in the highest dicotyledons and vertebrata, these grades
being few in number, and generally marked by intervals of organic character, which we find to be a practical
difficulty in ascertaining affinities; SECOND, of another impulse connected with the vital forces, tending, in the
course of generations, to modify organic structures in accordance with external circumstances, as food, the nature
of the habitat, and the meteoric agencies, these being the 'adaptations' of the natural theologian." The author
apparently believes that organisation progresses by sudden leaps, but that the effects produced by the conditions
of life are gradual. He argues with much force on general grounds that species are not immutable productions. But
I cannot see how the two supposed "impulses" account in a scientific sense for the numerous and beautiful
coadaptations which we see throughout nature; I cannot see that we thus gain any insight how, for instance, a
woodpecker has become adapted to its peculiar habits of life. The work, from its powerful and brilliant style,
though displaying in the early editions little accurate knowledge and a great want of scientific caution,
immediately had a very wide circulation. In my opinion it has done excellent service in this country in calling
attention to the subject, in removing prejudice, and in thus preparing the ground for the reception of analogous
views.
In 1846 the veteran geologist M.J. d'Omalius d'Halloy published in an excellent though short paper ("Bulletins
de l'Acad. Roy. Bruxelles", tom. xiii, page 581) his opinion that it is more probable that new species have been
produced by descent with modification than that they have been separately created: the author first promulgated
this opinion in 1831.
Professor Owen, in 1849 ("Nature of Limbs", page 86), wrote as follows: "The archetypal idea was manifested
in the flesh under diverse such modifications, upon this planet, long prior to the existence of those animal species
that actually exemplify it. To what natural laws or secondary causes the orderly succession and progression of
such organic phenomena may have been committed, we, as yet, are ignorant." In his address to the British
Association, in 1858, he speaks (page li) of "the axiom of the continuous operation of creative power, or of the
ordained becoming of living things." Further on (page xc), after referring to geographical distribution, he adds,
"These phenomena shake our confidence in the conclusion that the Apteryx of New Zealand and the Red Grouse
of England were distinct creations in and for those islands respectively. Always, also, it may be well to bear in
mind that by the word 'creation' the zoologist means 'a process he knows not what.'" He amplifies this idea by
adding that when such cases as that of the Red Grouse are "enumerated by the zoologist as evidence of distinct
creation of the bird in and for such islands, he chiefly expresses that he knows not how the Red Grouse came to be
there, and there exclusively; signifying also, by this mode of expressing such ignorance, his belief that both the
bird and the islands owed their origin to a great first Creative Cause." If we interpret these sentences given in the
same address, one by the other, it appears that this eminent philosopher felt in 1858 his confidence shaken that the
Apteryx and the Red Grouse first appeared in their respective homes "he knew not how," or by some process "he
knew not what."
This address was delivered after the papers by Mr. Wallace and myself on the Origin of Species, presently to
be referred to, had been read before the Linnean Society. When the first edition of this work was published, I was
so completely deceived, as were many others, by such expressions as "the continuous operation of creative
power," that I included Professor Owen with other palaeontologists as being firmly convinced of the immutability
of species; but it appears ("Anat. of Vertebrates", vol. iii, page 796) that this was on my part a preposterous error.
In the last edition of this work I inferred, and the inference still seems to me perfectly just, from a passage
beginning with the words "no doubt the type− form," etc.(Ibid., vol. i, page xxxv), that Professor Owen admitted
that natural selection may have done something in the formation of a new species; but this it appears (Ibid., vol.
iii. page 798) is inaccurate and without evidence. I also gave some extracts from a correspondence between
Professor Owen and the editor of the "London Review", from which it appeared manifest to the editor as well as
to myself, that Professor Owen claimed to have promulgated the theory of natural selection before I had done so;
and I expressed my surprise and satisfaction at this announcement; but as far as it is possible to understand certain
recently published passages (Ibid., vol. iii. page 798) I have either partially or wholly again fallen into error. It is
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY
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The Origin of Species by means of Natural Selection
consolatory to me that others find Professor Owen's controversial writings as difficult to understand and to
reconcile with each other, as I do. As far as the mere enunciation of the principle of natural selection is concerned,
it is quite immaterial whether or not Professor Owen preceded me, for both of us, as shown in this historical
sketch, were long ago preceded by Dr. Wells and Mr. Matthews.
M. Isidore Geoffroy Saint−Hilaire, in his lectures delivered in 1850 (of which a Resume appeared in the
"Revue et Mag. de Zoolog.", Jan., 1851), briefly gives his reason for believing that specific characters "sont fixes,
pour chaque espece, tant qu'elle se perpetue au milieu des memes circonstances: ils se modifient, si les
circonstances ambiantes viennent a changer. En resume, L'OBSERVATION des animaux sauvages demontre deja
la variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux sauvages devenus domestiques, et sur
les animaux domestiques redevenus sauvages, la demontrent plus clairment encore. Ces memes experiences
prouvent, de plus, que les differences produites peuvent etre de VALEUR GENERIQUE." In his "Hist. Nat.
Generale" (tom. ii, page 430, 1859) he amplifies analogous conclusions.
>From a circular lately issued it appears that Dr. Freke, in 1851 ("Dublin Medical Press", page 322),
propounded the doctrine that all organic beings have descended from one primordial form. His grounds of belief
and treatment of the subject are wholly different from mine; but as Dr. Freke has now (1861) published his Essay
on the "Origin of Species by means of Organic Affinity", the difficult attempt to give any idea of his views would
be superfluous on my part.
Mr. Herbert Spencer, in an Essay (originally published in the "Leader", March, 1852, and republished in his
"Essays", in 1858), has contrasted the theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic productions, from the changes which the
embryos of many species undergo, from the difficulty of distinguishing species and varieties, and from the
principle of general gradation, that species have been modified; and he attributes the modification to the change of
circumstances. The author (1855) has also treated Psychology on the principle of the necessary acquirement of
each mental power and capacity by gradation.
In 1852 M. Naudin, a distinguished botanist, expressly stated, in an admirable paper on the Origin of Species
("Revue Horticole", page 102; since partly republished in the "Nouvelles Archives du Museum", tom. i, page
171), his belief that species are formed in an analogous manner as varieties are under cultivation; and the latter
process he attributes to man's power of selection. But he does not show how selection acts under nature. He
believes, like Dean Herbert, that species, when nascent, were more plastic than at present. He lays weight on what
he calls the principle of finality, "puissance mysterieuse, indeterminee; fatalite pour les uns; pour les autres
volonte providentielle, dont l'action incessante sur les etres vivantes determine, a toutes les epoques de l'existence
du monde, la forme, le volume, et la duree de chacun d'eux, en raison de sa destinee dans l'ordre de choses dont il
fait partie. C'est cette puissance qui harmonise chaque membre a l'ensemble, en l'appropriant a la fonction qu'il
doit remplir dans l'organisme general de la nature, fonction qui est pour lui sa raison d'etre." (From references in
Bronn's "Untersuchungen uber die Entwickelungs−Gesetze", it appears that the celebrated botanist and
palaeontologist Unger published, in 1852, his belief that species undergo development and modification. Dalton,
likewise, in Pander and Dalton's work on Fossil Sloths, expressed, in 1821, a similar belief. Similar views have, as
is well known, been maintained by Oken in his mystical "Natur−Philosophie". From other references in Godron's
work "Sur l'Espece", it seems that Bory St. Vincent, Burdach, Poiret and Fries, have all admitted that new species
are continually being produced. I may add, that of the thirty−four authors named in this Historical Sketch, who
believe in the modification of species, or at least disbelieve in separate acts of creation, twenty−seven have
written on special branches of natural history or geology.)
In 1853 a celebrated geologist, Count Keyserling ("Bulletin de la Soc. Geolog.", 2nd Ser., tom. x, page 357),
suggested that as new diseases, supposed to have been caused by some miasma have arisen and spread over the
world, so at certain periods the germs of existing species may have been chemically affected by circumambient
molecules of a particular nature, and thus have given rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet ("Verhand. des Naturhist. Vereins
der Preuss. Rheinlands", etc.), in which he maintains the development of organic forms on the earth. He infers that
many species have kept true for long periods, whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. "Thus living plants and animals are not separated
from the extinct by new creations, but are to be regarded as their descendants through continued reproduction."
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY
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The Origin of Species by means of Natural Selection
A well−known French botanist, M. Lecoq, writes in 1854 ("Etudes sur Geograph. Bot. tom. i, page 250), "On
voit que nos recherches sur la fixite ou la variation de l'espece, nous conduisent directement aux idees emises par
deux hommes justement celebres, Geoffroy Saint−Hilaire et Goethe." Some other passages scattered through M.
Lecoq's large work make it a little doubtful how far he extends his views on the modification of species.
The "Philosophy of Creation" has been treated in a masterly manner by the Rev. Baden Powell, in his "Essays
on the Unity of Worlds", 1855. Nothing can be more striking than the manner in which he shows that the
introduction of new species is "a regular, not a casual phenomenon," or, as Sir John Herschel expresses it, "a
natural in contradistinction to a miraculous process."
The third volume of the "Journal of the Linnean Society" contains papers, read July 1, 1858, by Mr. Wallace
and myself, in which, as stated in the introductory remarks to this volume, the theory of Natural Selection is
promulgated by Mr. Wallace with admirable force and clearness.
Von Baer, toward whom all zoologists feel so profound a respect, expressed about the year 1859 (see Prof.
Rudolph Wagner, "Zoologisch−Anthropologische Untersuchungen", 1861, s. 51) his conviction, chiefly grounded
on the laws of geographical distribution, that forms now perfectly distinct have descended from a single
parent−form.
In June, 1859, Professor Huxley gave a lecture before the Royal Institution on the "Persistent Types of Animal
Life". Referring to such cases, he remarks, "It is difficult to comprehend the meaning of such facts as these, if we
suppose that each species of animal and plant, or each great type of organisation, was formed and placed upon the
surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an
assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of nature. If, on the
other hand, we view "Persistent Types" in relation to that hypothesis which supposes the species living at any
time to be the result of the gradual modification of pre−existing species, a hypothesis which, though unproven,
and sadly damaged by some of its supporters, is yet the only one to which physiology lends any countenance;
their existence would seem to show that the amount of modification which living beings have undergone during
geological time is but very small in relation to the whole series of changes which they have suffered."
In December, 1859, Dr. Hooker published his "Introduction to the Australian Flora". In the first part of this
great work he admits the truth of the descent and modification of species, and supports this doctrine by many
original observations.
The first edition of this work was published on November 24, 1859, and the second edition on January 7,
1860.
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, PREVIOUSLY
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The Origin of Species by means of Natural Selection
INTRODUCTION.
When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the
organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of
that continent. These facts, as will be seen in the latter chapters of this volume, seemed to throw some light on the
origin of species−−that mystery of mysteries, as it has been called by one of our greatest philosophers. On my
return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently
accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years'
work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily
pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to
show that I have not been hasty in coming to a decision.
My work is now (1859) nearly finished; but as it will take me many more years to complete it, and as my
health is far from strong, I have been urged to publish this abstract. I have more especially been induced to do
this, as Mr. Wallace, who is now studying the natural history of the Malay Archipelago, has arrived at almost
exactly the same general conclusions that I have on the origin of species. In 1858 he sent me a memoir on this
subject, with a request that I would forward it to Sir Charles Lyell, who sent it to the Linnean Society, and it is
published in the third volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, who both knew of my
work−−the latter having read my sketch of 1844−−honoured me by thinking it advisable to publish, with Mr.
Wallace's excellent memoir, some brief extracts from my manuscripts.
This abstract, which I now publish, must necessarily be imperfect. I cannot here give references and
authorities for my several statements; and I must trust to the reader reposing some confidence in my accuracy. No
doubt errors may have crept in, though I hope I have always been cautious in trusting to good authorities alone. I
can here give only the general conclusions at which I have arrived, with a few facts in illustration, but which, I
hope, in most cases will suffice. No one can feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been grounded; and I hope in a future work to
do this. For I am well aware that scarcely a single point is discussed in this volume on which facts cannot be
adduced, often apparently leading to conclusions directly opposite to those at which I have arrived. A fair result
can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and
this is here impossible.
I much regret that want of space prevents my having the satisfaction of acknowledging the generous
assistance which I have received from very many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep obligations to Dr. Hooker, who, for the last fifteen
years, has aided me in every possible way by his large stores of knowledge and his excellent judgment.
In considering the origin of species, it is quite conceivable that a naturalist, reflecting on the mutual affinities
of organic beings, on their embryological relations, their geographical distribution, geological succession, and
other such facts, might come to the conclusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be
unsatisfactory, until it could be shown how the innumerable species, inhabiting this world have been modified, so
as to acquire that perfection of structure and coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. In one
limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external
conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the mistletoe, which draws its nourishment from
certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by
the effects of external conditions, or of habit, or of the volition of the plant itself.
It is, therefore, of the highest importance to gain a clear insight into the means of modification and
INTRODUCTION. 8

The Origin of Species by means of Natural Selection
coadaptation. At the commencement of my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem.
Nor have I been disappointed; in this and in all other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the best and safest clue. I may venture to
express my conviction of the high value of such studies, although they have been very commonly neglected by
naturalists.
>From these considerations, I shall devote the first chapter of this abstract to variation under domestication.
We shall thus see that a large amount of hereditary modification is at least possible; and, what is equally or more
important, we shall see how great is the power of man in accumulating by his selection successive slight
variations. I will then pass on to the variability of species in a state of nature; but I shall, unfortunately, be
compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts.
We shall, however, be enabled to discuss what circumstances are most favourable to variation. In the next chapter
the struggle for existence among all organic beings throughout the world, which inevitably follows from the high
geometrical ratio of their increase, will be considered. This is the doctrine of Malthus, applied to the whole animal
and vegetable kingdoms. As many more individuals of each species are born than can possibly survive; and as,
consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however
slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have
a better chance of surviving, and thus be NATURALLY SELECTED. From the strong principle of inheritance,
any selected variety will tend to propagate its new and modified form.
This fundamental subject of natural selection will be treated at some length in the fourth chapter; and we shall
then see how natural selection almost inevitably causes much extinction of the less improved forms of life, and
leads to what I have called divergence of character. In the next chapter I shall discuss the complex and little
known laws of variation. In the five succeeding chapters, the most apparent and gravest difficulties in accepting
the theory will be given: namely, first, the difficulties of transitions, or how a simple being or a simple organ can
be changed and perfected into a highly developed being or into an elaborately constructed organ; secondly the
subject of instinct, or the mental powers of animals; thirdly, hybridism, or the infertility of species and the fertility
of varieties when intercrossed; and fourthly, the imperfection of the geological record. In the next chapter I shall
consider the geological succession of organic beings throughout time; in the twelfth and thirteenth, their
geographical distribution throughout space; in the fourteenth, their classification or mutual affinities, both when
mature and in an embryonic condition. In the last chapter I shall give a brief recapitulation of the whole work, and
a few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and
varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many
beings which live around us. Who can explain why one species ranges widely and is very numerous, and why
another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they
determine the present welfare and, as I believe, the future success and modification of every inhabitant of this
world. Still less do we know of the mutual relations of the innumerable inhabitants of the world during the many
past geological epochs in its history. Although much remains obscure, and will long remain obscure, I can
entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained, and which I formerly entertained−−namely, that each
species has been independently created−−is erroneous. I am fully convinced that species are not immutable; but
that those belonging to what are called the same genera are lineal descendants of some other and generally extinct
species, in the same manner as the acknowledged varieties of any one species are the descendants of that species.
Furthermore, I am convinced that natural selection has been the most important, but not the exclusive, means of
modification.
INTRODUCTION. 9

The Origin of Species by means of Natural Selection
CHAPTER I. VARIATION UNDER DOMESTICATION.
Causes of Variability −− Effects of Habit and the use and disuse of Parts −− Correlated Variation −−
Inheritance −− Character of Domestic Varieties −− Difficulty of distinguishing between Varieties and Species −−
Origin of Domestic Varieties from one or more Species −− Domestic Pigeons, their Differences and Origin −−
Principles of Selection, anciently followed, their Effects −− Methodical and Unconscious Selection −− Unknown
Origin of our Domestic Productions −− Circumstances favourable to Man's power of Selection.
CAUSES OF VARIABILITY.
When we compare the individuals of the same variety or sub−variety of our older cultivated plants and
animals, one of the first points which strikes us is, that they generally differ more from each other than do the
individuals of any one species or variety in a state of nature. And if we reflect on the vast diversity of the plants
and animals which have been cultivated, and which have varied during all ages under the most different climates
and treatment, we are driven to conclude that this great variability is due to our domestic productions having been
raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species
had been exposed under nature. There is, also, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems clear that organic beings must be exposed
during several generations to new conditions to cause any great amount of variation; and that, when the
organisation has once begun to vary, it generally continues varying for many generations. No case is on record of
a variable organism ceasing to vary under cultivation. Our oldest cultivated plants, such as wheat, still yield new
varieties: our oldest domesticated animals are still capable of rapid improvement or modification.
As far as I am able to judge, after long attending to the subject, the conditions of life appear to act in two
ways−−directly on the whole organisation or on certain parts alone and in directly by affecting the reproductive
system. With respect to the direct action, we must bear in mind that in every case, as Professor Weismann has
lately insisted, and as I have incidently shown in my work on "Variation under Domestication," there are two
factors: namely, the nature of the organism and the nature of the conditions. The former seems to be much the
more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions;
and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform. The effects
on the offspring are either definite or in definite. They may be considered as definite when all or nearly all the
offspring of individuals exposed to certain conditions during several generations are modified in the same
manner. It is extremely difficult to come to any conclusion in regard to the extent of the changes which have been
thus definitely induced. There can, however, be little doubt about many slight changes, such as size from the
amount of food, colour from the nature of the food, thickness of the skin and hair from climate, etc. Each of the
endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same
cause were to act uniformly during a long series of generations on many individuals, all probably would be
modified in the same manner. Such facts as the complex and extraordinary out growths which variably follow
from the insertion of a minute drop of poison by a gall−producing insect, shows us what singular modifications
might result in the case of plants from a chemical change in the nature of the sap.
In definite variability is a much more common result of changed conditions than definite variability, and has
probably played a more important part in the formation of our domestic races. We see in definite variability in the
endless slight peculiarities which distinguish the individuals of the same species, and which cannot be accounted
for by inheritance from either parent or from some more remote ancestor. Even strongly−marked differences
occasionally appear in the young of the same litter, and in seedlings from the same seed−capsule. At long
intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food,
deviations of structure so strongly pronounced as to deserve to be called monstrosities arise; but monstrosities
cannot be separated by any distinct line from slighter variations. All such changes of structure, whether extremely
slight or strongly marked, which appear among many individuals living together, may be considered as the in
definite effects of the conditions of life on each individual organism, in nearly the same manner as the chill effects
different men in an in definite manner, according to their state of body or constitution, causing coughs or colds,
rheumatism, or inflammation of various organs.
CHAPTER I. VARIATION UNDER DOMESTICATION. 10

The Origin of Species by means of Natural Selection
With respect to what I have called the in direct action of changed conditions, namely, through the reproductive
system of being affected, we may infer that variability is thus induced, partly from the fact of this system being
extremely sensitive to any change in the conditions, and partly from the similarity, as Kolreuter and others have
remarked, between the variability which follows from the crossing of distinct species, and that which may be
observed with plants and animals when reared under new or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very slight changes in the surrounding conditions. Nothing is
more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement,
even when the male and female unite. How many animals there are which will not breed, though kept in an almost
free state in their native country! This is generally, but erroneously attributed to vitiated instincts. Many cultivated
plants display the utmost vigour, and yet rarely or never seed! In some few cases it has been discovered that a
very trifling change, such as a little more or less water at some particular period of growth, will determine
whether or not a plant will produce seeds. I cannot here give the details which I have collected and elsewhere
published on this curious subject; but to show how singular the laws are which determine the reproduction of
animals under confinement, I may mention that carnivorous animals, even from the tropics, breed in this country
pretty freely under confinement, with the exception of the plantigrades or bear family, which seldom produce
young; whereas, carnivorous birds, with the rarest exception, hardly ever lay fertile eggs. Many exotic plants have
pollen utterly worthless, in the same condition as in the most sterile hybrids. When, on the one hand, we see
domesticated animals and plants, though often weak and sickly, breeding freely under confinement; and when, on
the other hand, we see individuals, though taken young from a state of nature perfectly tamed, long−lived, and
healthy (of which I could give numerous instances), yet having their reproductive system so seriously affected by
unperceived causes as to fail to act, we need not be surprised at this system, when it does act under confinement,
acting irregularly, and producing offspring somewhat unlike their parents. I may add that as some organisms
breed freely under the most unnatural conditions−−for instance, rabbits and ferrets kept in hutches−−showing that
their reproductive organs are not easily affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly−−perhaps hardly more than in a state of nature.
Some naturalists have maintained that all variations are connected with the act of sexual reproduction; but this
is certainly an error; for I have given in another work a long list of "sporting plants;" as they are called by
gardeners; that is, of plants which have suddenly produced a single bud with a new and sometimes widely
different character from that of the other buds on the same plant. These bud variations, as they may be named, can
be propagated by grafts, offsets, etc., and sometimes by seed. They occur rarely under nature, but are far from rare
under culture. As a single bud out of many thousands produced year after year on the same tree under uniform
conditions, has been known suddenly to assume a new character; and as buds on distinct trees, growing under
different conditions, have sometimes yielded nearly the same variety−−for instance, buds on peach− trees
producing nectarines, and buds on common roses producing moss−roses−− we clearly see that the nature of the
conditions is of subordinate importance in comparison with the nature of the organism in determining each
particular form of variation; perhaps of not more importance than the nature of the spark, by which a mass of
combustible matter is ignited, has in determining the nature of the flames.
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION;
INHERITANCE.
Changed habits produce an inherited effect as in the period of the flowering of plants when transported from
one climate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I
find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the
whole skeleton, than do the same bones in the wild duck; and this change may be safely attributed to the domestic
duck flying much less, and walking more, than its wild parents. The great and inherited development of the udders
in cows and goats in countries where they are habitually milked, in comparison with these organs in other
countries, is probably another instance of the effects of use. Not one of our domestic animals can be named which
has not in some country drooping ears; and the view which has been suggested that the drooping is due to disuse
of the muscles of the ear, from the animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed. I
will here only allude to what may be called correlated variation. Important changes in the embryo or larva will
probably entail changes in the mature animal. In monstrosities, the correlations between quite distinct parts are
CHAPTER I. VARIATION UNDER DOMESTICATION. 11

The Origin of Species by means of Natural Selection
very curious; and many instances are given in Isidore Geoffroy St. Hilaire's great work on this subject. Breeders
believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are
quite whimsical; thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately
stated by Mr. Tait that this is confined to the males. Colour and constitutional peculiarities go together, of which
many remarkable cases could be given among animals and plants. From facts collected by Heusinger, it appears
that white sheep and pigs are injured by certain plants, while dark−coloured individuals escape: Professor Wyman
has recently communicated to me a good illustration of this fact; on asking some farmers in Virginia how it was
that all their pigs were black, they informed him that the pigs ate the paint−root (Lachnanthes), which coloured
their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers"
(i.e. Virginia squatters) added, "we select the black members of a litter for raising, as they alone have a good
chance of living." Hairless dogs have imperfect teeth; long−haired and coarse−haired animals are apt to have, as
is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short
beaks have small feet, and those with long beaks large feet. Hence if man goes on selecting, and thus augmenting,
any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the
mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and
diversified. It is well worth while carefully to study the several treatises on some of our old cultivated plants, as
on the hyacinth, potato, even the dahlia, etc.; and it is really surprising to note the endless points of structure and
constitution in which the varieties and sub−varieties differ slightly from each other. The whole organisation seems
to have become plastic, and departs in a slight degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable
deviations of structure, both those of slight and those of considerable physiological importance, are endless. Dr.
Prosper Lucas' treatise, in two large volumes, is the fullest and the best on this subject. No breeder doubts how
strong is the tendency to inheritance; that like produces like is his fundamental belief: doubts have been thrown on
this principle only by theoretical writers. When any deviation of structure often appears, and we see it in the father
and child, we cannot tell whether it may not be due to the same cause having acted on both; but when among
individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary
combination of circumstances, appears in the parent−−say, once among several million individuals−−and it
reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance.
Every one must have heard of cases of albinism, prickly skin, hairy bodies, etc., appearing in several members of
the same family. If strange and rare deviations of structure are truly inherited, less strange and commoner
deviations may be freely admitted to be inheritable. Perhaps the correct way of viewing the whole subject would
be, to look at the inheritance of every character whatever as the rule, and non−inheritance as the anomaly.
The laws governing inheritance are for the most part unknown; no one can say why the same peculiarity in
different individuals of the same species, or in different species, is sometimes inherited and sometimes not so;
why the child often reverts in certain characteristics to its grandfather or grandmother or more remote ancestor;
why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not
exclusively to the like sex. It is a fact of some importance to us, that peculiarities appearing in the males of our
domestic breeds are often transmitted, either exclusively or in a much greater degree, to the males alone. A much
more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it
tends to reappear in the offspring at a corresponding age, though sometimes earlier. In many cases this could not
be otherwise; thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silk−worm are known to appear at the corresponding caterpillar or cocoon stage. But
hereditary diseases and some other facts make me believe that the rule has a wider extension, and that, when there
is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of course confined to the first
APPEARANCE of the peculiarity, and not to the primary cause which may have acted on the ovules or on the
male element; in nearly the same manner as the increased length of the horns in the offspring from a short−horned
cow by a long−horned bull, though appearing late in life, is clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists−−namely,
CHAPTER I. VARIATION UNDER DOMESTICATION. 12

The Origin of Species by means of Natural Selection
that our domestic varieties, when run wild, gradually but invariably revert in character to their aboriginal stocks.
Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been
made. There would be great difficulty in proving its truth: we may safely conclude that very many of the most
strongly marked domestic varieties could not possibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. It would be
necessary, in order to prevent the effects of intercrossing, that only a single variety should be turned loose in its
new home. Nevertheless, as our varieties certainly do occasionally revert in some of their characters to ancestral
forms, it seems to me not improbable that if we could succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very poor soil−−in which case, however, some
effect would have to be attributed to the DEFINITE action of the poor soil −−that they would, to a large extent, or
even wholly, revert to the wild aboriginal stock. Whether or not the experiment would succeed is not of great
importance for our line of argument; for by the experiment itself the conditions of life are changed. If it could be
shown that our domestic varieties manifested a strong tendency to reversion−−that is, to lose their acquired
characters, while kept under the same conditions and while kept in a considerable body, so that free intercrossing
might check, by blending together, any slight deviations in their structure, in such case, I grant that we could
deduce nothing from domestic varieties in regard to species. But there is not a shadow of evidence in favour of
this view: to assert that we could not breed our cart and race−horses, long and short−horned cattle, and poultry of
various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all
experience.
CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE SPECIES.
When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with
closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of
character than in true species. Domestic races often have a somewhat monstrous character; by which I mean, that,
although differing from each other and from other species of the same genus, in several trifling respects, they
often differ in an extreme degree in some one part, both when compared one with another, and more especially
when compared with the species under nature to which they are nearest allied. With these exceptions (and with
that of the perfect fertility of varieties when crossed−−a subject hereafter to be discussed), domestic races of the
same species differ from each other in the same manner as do the closely allied species of the same genus in a
state of nature, but the differences in most cases are less in degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere varieties. If any well marked distinction existed between a
domestic race and a species, this source of doubt would not so perpetually recur. It has often been stated that
domestic races do not differ from each other in characters of generic value. It can be shown that this statement is
not correct; but naturalists differ much in determining what characters are of generic value; all such valuations
being at present empirical. When it is explained how genera originate under nature, it will be seen that we have no
right to expect often to find a generic amount of difference in our domesticated races.
In attempting to estimate the amount of structural difference between allied domestic races, we are soon
involved in doubt, from not knowing whether they are descended from one or several parent species. This point, if
it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound,
terrier, spaniel and bull−dog, which we all know propagate their kind truly, were the offspring of any single
species, then such facts would have great weight in making us doubt about the immutability of the many closely
allied natural species−−for instance, of the many foxes−−inhabiting the different quarters of the world. I do not
believe, as we shall presently see, that the whole amount of difference between the several breeds of the dog has
been produced under domestication; I believe that a small part of the difference is due to their being descended
from distinct species. In the case of strongly marked races of some other domesticated species, there is
presumptive or even strong evidence that all are descended from a single wild stock.
It has often been assumed that man has chosen for domestication animals and plants having an extraordinary
inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions; but how could a savage possibly know, when
CHAPTER I. VARIATION UNDER DOMESTICATION. 13

The Origin of Species by means of Natural Selection
he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other
climates? Has the little variability of the ass and goose, or the small power of endurance of warmth by the
reindeer, or of cold by the common camel, prevented their domestication? I cannot doubt that if other animals and
plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries,
were taken from a state of nature, and could be made to breed for an equal number of generations under
domestication, they would on an average vary as largely as the parent species of our existing domesticated
productions have varied.
In the case of most of our anciently domesticated animals and plants, it is not possible to come to any definite
conclusion, whether they are descended from one or several wild species. The argument mainly relied on by those
who believe in the multiple origin of our domestic animals is, that we find in the most ancient times, on the
monuments of Egypt, and in the lake− habitations of Switzerland, much diversity in the breeds; and that some of
these ancient breeds closely resemble, or are even identical with, those still existing. But this only throws far
backward the history of civilisation, and shows that animals were domesticated at a much earlier period than has
hitherto been supposed. The lake−inhabitants of Switzerland cultivated several kinds of wheat and barley, the pea,
the poppy for oil and flax; and they possessed several domesticated animals. They also carried on commerce with
other nations. All this clearly shows, as Heer has remarked, that they had at this early age progressed considerably
in civilisation; and this again implies a long continued previous period of less advanced civilisation, during which
the domesticated animals, kept by different tribes in different districts, might have varied and given rise to distinct
races. Since the discovery of flint tools in the superficial formations of many parts of the world, all geologists
believe that barbarian men existed at an enormously remote period; and we know that at the present day there is
hardly a tribe so barbarous as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably forever remain vague. But I may here state that,
looking to the domestic dogs of the whole world, I have, after a laborious collection of all known facts, come to
the conclusion that several wild species of Canidae have been tamed, and that their blood, in some cases mingled
together, flows in the veins of our domestic breeds. In regard to sheep and goats I can form no decided opinion.
From facts communicated to me by Mr. Blyth, on the habits, voice, constitution and structure of the humped
Indian cattle, it is almost certain that they are descended from a different aboriginal stock from our European
cattle; and some competent judges believe that these latter have had two or three wild progenitors, whether or not
these deserve to be called species. This conclusion, as well as that of the specific distinction between the humped
and common cattle, may, indeed, be looked upon as established by the admirable researches of Professor
Rutimeyer. With respect to horses, from reasons which I cannot here give, I am doubtfully inclined to believe, in
opposition to several authors, that all the races belong to the same species. Having kept nearly all the English
breeds of the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost
certain that all are the descendants of the wild Indian fowl, Gallus bankiva; and this is the conclusion of Mr.
Blyth, and of others who have studied this bird in India. In regard to ducks and rabbits, some breeds of which
differ much from each other, the evidence is clear that they are all descended from the common duck and wild
rabbit.
The doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an
absurd extreme by some authors. They believe that every race which breeds true, let the distinctive characters be
ever so slight, has had its wild prototype. At this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats, in Europe alone, and several even within Great Britain. One author
believes that there formerly existed eleven wild species of sheep peculiar to Great Britain! When we bear in mind
that Britain has now not one peculiar mammal, and France but few distinct from those of Germany, and so with
Hungary, Spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we
must admit that many domestic breeds must have originated in Europe; for whence otherwise could they have
been derived? So it is in India. Even in the case of the breeds of the domestic dog throughout the world, which I
admit are descended from several wild species, it cannot be doubted that there has been an immense amount of
inherited variation; for who will believe that animals closely resembling the Italian greyhound, the bloodhound,
the bull−dog, pug−dog, or Blenheim spaniel, etc.−−so unlike all wild Canidae−−ever existed in a state of nature?
It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal
species; but by crossing we can only get forms in some degree intermediate between their parents; and if we
CHAPTER I. VARIATION UNDER DOMESTICATION. 14

The Origin of Species by means of Natural Selection
account for our several domestic races by this process, we must admit the former existence of the most extreme
forms, as the Italian greyhound, bloodhound, bull−dog, etc., in the wild state. Moreover, the possibility of making
distinct races by crossing has been greatly exaggerated. Many cases are on record showing that a race may be
modified by occasional crosses if aided by the careful selection of the individuals which present the desired
character; but to obtain a race intermediate between two quite distinct races would be very difficult. Sir J.
Sebright expressly experimented with this object and failed. The offspring from the first cross between two pure
breeds is tolerably and sometimes (as I have found with pigeons) quite uniform in character, and every thing
seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two
of them are alike, and then the difficulty of the task becomes manifest.
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN.
Believing that it is always best to study some special group, I have, after deliberation, taken up domestic
pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favoured with skins
from several quarters of the world, more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray
from Persia. Many treatises in different languages have been published on pigeons, and some of them are very
important, as being of considerable antiquity. I have associated with several eminent fanciers, and have been
permitted to join two of the London Pigeon Clubs. The diversity of the breeds is something astonishing. Compare
the English carrier and the short−faced tumbler, and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the
wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated
eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short−faced tumbler has a beak
in outline almost like that of a finch; and the common tumbler has the singular inherited habit of flying at a great
height in a compact flock, and tumbling in the air head over heels. The runt is a bird of great size, with long,
massive beak and large feet; some of the sub−breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the carrier, but, instead of a long beak, has a very short and
broad one. The pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it
glories in inflating, may well excite astonishment and even laughter. The turbit has a short and conical beak, with
a line of reversed feathers down the breast; and it has the habit of continually expanding, slightly, the upper part
of the oesophagus. The Jacobin has the feathers so much reversed along the back of the neck that they form a
hood, and it has, proportionally to its size, elongated wing and tail feathers. The trumpeter and laugher, as their
names express, utter a very different coo from the other breeds. The fantail has thirty or even forty tail−feathers,
instead of twelve or fourteen, the normal number in all the members of the great pigeon family: these feathers are
kept expanded and are carried so erect that in good birds the head and tail touch: the oil−gland is quite aborted.
Several other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the bones of the face, in length and breadth and
curvature, differs enormously. The shape, as well as the breadth and length of the ramus of the lower jaw, varies
in a highly remarkable manner. The caudal and sacral vertebrae vary in number; as does the number of the ribs,
together with their relative breadth and the presence of processes. The size and shape of the apertures in the
sternum are highly variable; so is the degree of divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the
tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the
oesophagus; the development and abortion of the oil−gland; the number of the primary wing and caudal feathers;
the relative length of the wing and tail to each other and to the body; the relative length of the leg and foot; the
number of scutellae on the toes, the development of skin between the toes, are all points of structure which are
variable. The period at which the perfect plumage is acquired varies, as does the state of the down with which the
nestling birds are clothed when hatched. The shape and size of the eggs vary. The manner of flight, and in some
breeds the voice and disposition, differ remarkably. Lastly, in certain breeds, the males and females have come to
differ in a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told
that they were wild birds, would certainly be ranked by him as well−defined species. Moreover, I do not believe
that any ornithologist would in this case place the English carrier, the short−faced tumbler, the runt, the barb,
pouter, and fantail in the same genus; more especially as in each of these breeds several truly−inherited
CHAPTER I. VARIATION UNDER DOMESTICATION. 15

The Origin of Species by means of Natural Selection
sub−breeds, or species, as he would call them, could be shown him.
Great as are the differences between the breeds of the pigeon, I am fully convinced that the common opinion
of naturalists is correct, namely, that all are descended from the rock−pigeon (Columba livia), including under this
term several geographical races or sub−species, which differ from each other in the most trifling respects. As
several of the reasons which have led me to this belief are in some degree applicable in other cases, I will here
briefly give them. If the several breeds are not varieties, and have not proceeded from the rock−pigeon, they must
have descended from at least seven or eight aboriginal stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for instance, could a pouter be produced by crossing two breeds
unless one of the parent−stocks possessed the characteristic enormous crop? The supposed aboriginal stocks must
all have been rock−pigeons, that is, they did not breed or willingly perch on trees. But besides C. livia, with its
geographical sub−species, only two or three other species of rock−pigeons are known; and these have not any of
the characters of the domestic breeds. Hence the supposed aboriginal stocks must either still exist in the countries
where they were originally domesticated, and yet be unknown to ornithologists; and this, considering their size,
habits and remarkable characters, seems improbable; or they must have become extinct in the wild state. But birds
breeding on precipices, and good flyers, are unlikely to be exterminated; and the common rock−pigeon, which has
the same habits with the domestic breeds, has not been exterminated even on several of the smaller British islets,
or on the shores of the Mediterranean. Hence the supposed extermination of so many species having similar habits
with the rock−pigeon seems a very rash assumption. Moreover, the several above−named domesticated breeds
have been transported to all parts of the world, and, therefore, some of them must have been carried back again
into their native country; but not one has become wild or feral, though the dovecot−pigeon, which is the
rock−pigeon in a very slightly altered state, has become feral in several places. Again, all recent experience shows
that it is difficult to get wild animals to breed freely under domestication; yet on the hypothesis of the multiple
origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly domesticated in
ancient times by half−civilized man, as to be quite prolific under confinement.
An argument of great weight, and applicable in several other cases, is, that the above−specified breeds, though
agreeing generally with the wild rock−pigeon in constitution, habits, voice, colouring, and in most parts of their
structure, yet are certainly highly abnormal in other parts; we may look in vain through the whole great family of
Columbidae for a beak like that of the English carrier, or that of the short−faced tumbler, or barb; for reversed
feathers like those of the Jacobin; for a crop like that of the pouter; for tail−feathers like those of the fantail.
Hence it must be assumed, not only that half−civilized man succeeded in thoroughly domesticating several
species, but that he intentionally or by chance picked out extraordinarily abnormal species; and further, that these
very species have since all become extinct or unknown. So many strange contingencies are improbable in the
highest degree.
Some facts in regard to the colouring of pigeons well deserve consideration. The rock−pigeon is of a
slaty−blue, with white loins; but the Indian sub−species, C. intermedia of Strickland, has this part bluish. The tail
has a terminal dark bar, with the outer feathers externally edged at the base with white. The wings have two black
bars. Some semi−domestic breeds, and some truly wild breeds, have, besides the two black bars, the wings
chequered with black. These several marks do not occur together in any other species of the whole family. Now,
in every one of the domestic breeds, taking thoroughly well−bred birds, all the above marks, even to the white
edging of the outer tail−feathers, sometimes concur perfectly developed. Moreover, when birds belonging to two
or more distinct breeds are crossed, none of which are blue or have any of the above−specified marks, the
mongrel offspring are very apt suddenly to acquire these characters. To give one instance out of several which I
have observed: I crossed some white fantails, which breed very true, with some black barbs−− and it so happens
that blue varieties of barbs are so rare that I never heard of an instance in England; and the mongrels were black,
brown and mottled. I also crossed a barb with a spot, which is a white bird with a red tail and red spot on the
forehead, and which notoriously breeds very true; the mongrels were dusky and mottled. I then crossed one of the
mongrel barb−fantails with a mongrel barb−spot, and they produced a bird of as beautiful a blue colour, with the
white loins, double black wing−bar, and barred and white−edged tail−feathers, as any wild rock−pigeon! We can
understand these facts, on the well−known principle of reversion to ancestral characters, if all the domestic breeds
are descended from the rock−pigeon. But if we deny this, we must make one of the two following highly
improbable suppositions. Either, first, that all the several imagined aboriginal stocks were coloured and marked
CHAPTER I. VARIATION UNDER DOMESTICATION. 16

The Origin of Species by means of Natural Selection
like the rock−pigeon, although no other existing species is thus coloured and marked, so that in each separate
breed there might be a tendency to revert to the very same colours and markings. Or, secondly, that each breed,
even the purest, has within a dozen, or at most within a score, of generations, been crossed by the rock−pigeon: I
say within a dozen or twenty generations, for no instance is known of crossed descendants reverting to an ancestor
of foreign blood, removed by a greater number of generations. In a breed which has been crossed only once the
tendency to revert to any character derived from such a cross will naturally become less and less, as in each
succeeding generation there will be less of the foreign blood; but when there has been no cross, and there is a
tendency in the breed to revert to a character which was lost during some former generation, this tendency, for all
that we can see to the contrary, may be transmitted undiminished for an indefinite number of generations. These
two distinct cases of reversion are often confounded together by those who have written on inheritance.
Lastly, the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile, as I can state
from my own observations, purposely made, on the most distinct breeds. Now, hardly any cases have been
ascertained with certainty of hybrids from two quite distinct species of animals being perfectly fertile. Some
authors believe that long−continued domestication eliminates this strong tendency to sterility in species. >From
the history of the dog, and of some other domestic animals, this conclusion is probably quite correct, if applied to
species closely related to each other. But to extend it so far as to suppose that species, aboriginally as distinct as
carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile, inter se, seems to me rash
in the extreme.
>From these several reasons, namely, the improbability of man having formerly made seven or eight supposed
species of pigeons to breed freely under domestication−−these supposed species being quite unknown in a wild
state, and their not having become anywhere feral−−these species presenting certain very abnormal characters, as
compared with all other Columbidae, though so like the rock−pigeon in most other respects−−the occasional
reappearance of the blue colour and various black marks in all the breeds, both when kept pure and when
crossed−−and lastly, the mongrel offspring being perfectly fertile−−from these several reasons, taken together, we
may safely conclude that all our domestic breeds are descended from the rock− pigeon or Columba livia with its
geographical sub−species.
In favour of this view, I may add, firstly, that the wild C. livia has been found capable of domestication in
Europe and in India; and that it agrees in habits and in a great number of points of structure with all the domestic
breeds. Secondly, that although an English carrier or a short−faced tumbler differs immensely in certain
characters from the rock−pigeon, yet that by comparing the several sub−breeds of these two races, more
especially those brought from distant countries, we can make, between them and the rock−pigeon, an almost
perfect series; so we can in some other cases, but not with all the breeds. Thirdly, those characters which are
mainly distinctive of each breed are in each eminently variable, for instance, the wattle and length of beak of the
carrier, the shortness of that of the tumbler, and the number of tail−feathers in the fantail; and the explanation of
this fact will be obvious when we treat of selection. Fourthly, pigeons have been watched and tended with the
utmost care, and loved by many people. They have been domesticated for thousands of years in several quarters
of the world; the earliest known record of pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me that pigeons are given in a bill of fare in the
previous dynasty. In the time of the Romans, as we hear from Pliny, immense prices were given for pigeons; "nay,
they are come to this pass, that they can reckon up their pedigree and race." Pigeons were much valued by Akber
Khan in India, about the year 1600; never less than 20,000 pigeons were taken with the court. "The monarchs of
Iran and Turan sent him some very rare birds;" and, continues the courtly historian, "His Majesty, by crossing the
breeds, which method was never practised before, has improved them astonishingly." About this same period the
Dutch were as eager about pigeons as were the old Romans. The paramount importance of these considerations in
explaining the immense amount of variation which pigeons have undergone, will likewise be obvious when we
treat of selection. We shall then, also, see how it is that the several breeds so often have a somewhat monstrous
character. It is also a most favourable circumstance for the production of distinct breeds, that male and female
pigeons can be easily mated for life; and thus different breeds can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when
I first kept pigeons and watched the several kinds, well knowing how truly they breed, I felt fully as much
difficulty in believing that since they had been domesticated they had all proceeded from a common parent, as any
CHAPTER I. VARIATION UNDER DOMESTICATION. 17

The Origin of Species by means of Natural Selection
naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of
birds, in nature. One circumstance has struck me much; namely, that nearly all the breeders of the various
domestic animals and the cultivators of plants, with whom I have conversed, or whose treatises I have read, are
firmly convinced that the several breeds to which each has attended, are descended from so many aboriginally
distinct species. Ask, as I have asked, a celebrated raiser of Hereford cattle, whether his cattle might not have
descended from Long−horns, or both from a common parent− stock, and he will laugh you to scorn. I have never
met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully convinced that each main breed was
descended from a distinct species. Van Mons, in his treatise on pears and apples, shows how utterly he disbelieves
that the several sorts, for instance a Ribston−pippin or Codlin−apple, could ever have proceeded from the seeds of
the same tree. Innumerable other examples could be given. The explanation, I think, is simple: from
long−continued study they are strongly impressed with the differences between the several races; and though they
well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they
ignore all general arguments, and refuse to sum up in their minds slight differences accumulated during many
successive generations. May not those naturalists who, knowing far less of the laws of inheritance than does the
breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet admit that
many of our domestic races are descended from the same parents−−may they not learn a lesson of caution, when
they deride the idea of species in a state of nature being lineal descendants of other species?
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS.
Let us now briefly consider the steps by which domestic races have been produced, either from one or from
several allied species. Some effect may be attributed to the direct and definite action of the external conditions of
life, and some to habit; but he would be a bold man who would account by such agencies for the differences
between a dray and race−horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of the most
remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal's or
plant's own good, but to man's use or fancy. Some variations useful to him have probably arisen suddenly, or by
one step; many botanists, for instance, believe that the fuller's teasel, with its hooks, which can not be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus; and this amount of change may have suddenly
arisen in a seedling. So it has probably been with the turnspit dog; and this is known to have been the case with
the ancon sheep. But when we compare the dray−horse and race−horse, the dromedary and camel, the various
breeds of sheep fitted either for cultivated land or mountain pasture, with the wool of one breed good for one
purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for
man in different ways; when we compare the game−cock, so pertinacious in battle, with other breeds so little
quarrelsome, with "everlasting layers" which never desire to sit, and with the bantam so small and elegant; when
we compare the host of agricultural, culinary, orchard, and flower−garden races of plants, most useful to man at
different seasons and for different purposes, or so beautiful in his eyes, we must, I think, look further than to mere
variability. We can not suppose that all the breeds were suddenly produced as perfect and as useful as we now see
them; indeed, in many cases, we know that this has not been their history. The key is man's power of
accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him.
In this sense he may be said to have made for himself useful breeds.
The great power of this principle of selection is not hypothetical. It is certain that several of our eminent
breeders have, even within a single lifetime, modified to a large extent their breeds of cattle and sheep. In order
fully to realise what they have done it is almost necessary to read several of the many treatises devoted to this
subject, and to inspect the animals. Breeders habitually speak of an animal's organisation as something plastic,
which they can model almost as they please. If I had space I could quote numerous passages to this effect from
highly competent authorities. Youatt, who was probably better acquainted with the works of agriculturalists than
almost any other individual, and who was himself a very good judge of animals, speaks of the principle of
selection as "that which enables the agriculturist, not only to modify the character of his flock, but to change it
altogether. It is the magician's wand, by means of which he may summon into life whatever form and mould he
pleases." Lord Somerville, speaking of what breeders have done for sheep, says: "It would seem as if they had
chalked out upon a wall a form perfect in itself, and then had given it existence." In Saxony the importance of the
principle of selection in regard to merino sheep is so fully recognised, that men follow it as a trade: the sheep are
placed on a table and are studied, like a picture by a connoisseur; this is done three times at intervals of months,
CHAPTER I. VARIATION UNDER DOMESTICATION. 18

The Origin of Species by means of Natural Selection
and the sheep are each time marked and classed, so that the very best may ultimately be selected for breeding.
What English breeders have actually effected is proved by the enormous prices given for animals with a good
pedigree; and these have been exported to almost every quarter of the world. The improvement is by no means
generally due to crossing different breeds; all the best breeders are strongly opposed to this practice, except
sometimes among closely allied sub−breeds. And when a cross has been made, the closest selection is far more
indispensable even than in ordinary cases. If selection consisted merely in separating some very distinct variety
and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direction, during successive generations, of differences
absolutely inappreciable by an uneducated eye−−differences which I for one have vainly attempted to appreciate.
Not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. If gifted
with these qualities, and he studies his subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he wants any of these qualities, he will
assuredly fail. Few would readily believe in the natural capacity and years of practice requisite to become even a
skilful pigeon−fancier.
The same principles are followed by horticulturists; but the variations are here often more abrupt. No one
supposes that our choicest productions have been produced by a single variation from the aboriginal stock. We
have proofs that this is not so in several cases in which exact records have been kept; thus, to give a very trifling
instance, the steadily increasing size of the common gooseberry may be quoted. We see an astonishing
improvement in many florists' flowers, when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty well established, the seed−raisers do not pick
out the best plants, but merely go over their seed−beds, and pull up the "rogues," as they call the plants that
deviate from the proper standard. With animals this kind of selection is, in fact, likewise followed; for hardly any
one is so careless as to breed from his worst animals.
In regard to plants, there is another means of observing the accumulated effects of selection−−namely, by
comparing the diversity of flowers in the different varieties of the same species in the flower−garden; the diversity
of leaves, pods, or tubers, or whatever part is valued, in the kitchen−garden, in comparison with the flowers of the
same varieties; and the diversity of fruit of the same species in the orchard, in comparison with the leaves and
flowers of the same set of varieties. See how different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and hairiness, and yet the flowers present very slight
differences. It is not that the varieties which differ largely in some one point do not differ at all in other points;
this is hardly ever−−I speak after careful observation−−perhaps never, the case. The law of correlated variation,
the importance of which should never be overlooked, will ensure some differences; but, as a general rule, it
cannot be doubted that the continued selection of slight variations, either in the leaves, the flowers, or the fruit,
will produce races differing from each other chiefly in these characters.
It may be objected that the principle of selection has been reduced to methodical practice for scarcely more
than three−quarters of a century; it has certainly been more attended to of late years, and many treatises have been
published on the subject; and the result has been, in a corresponding degree, rapid and important. But it is very far
from true that the principle is a modern discovery. I could give several references to works of high antiquity, in
which the full importance of the principle is acknowledged. In rude and barbarous periods of English history
choice animals were often imported, and laws were passed to prevent their exportation: the destruction of horses
under a certain size was ordered, and this may be compared to the "roguing" of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese encyclopaedia. Explicit rules are laid down by
some of the Roman classical writers. From passages in Genesis, it is clear that the colour of domestic animals was
at that early period attended to. Savages now sometimes cross their dogs with wild canine animals, to improve the
breed, and they formerly did so, as is attested by passages in Pliny. The savages in South Africa match their
draught cattle by colour, as do some of the Esquimaux their teams of dogs. Livingstone states that good domestic
breeds are highly valued by the negroes in the interior of Africa who have not associated with Europeans. Some
of these facts do not show actual selection, but they show that the breeding of domestic animals was carefully
attended to in ancient times, and is now attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of good and bad qualities is so obvious.
CHAPTER I. VARIATION UNDER DOMESTICATION. 19

The Origin of Species by means of Natural Selection
UNCONSCIOUS SELECTION.
At the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a
new strain or sub−breed, superior to anything of the kind in the country. But, for our purpose, a form of selection,
which may be called unconscious, and which results from every one trying to possess and breed from the best
individual animals, is more important. Thus, a man who intends keeping pointers naturally tries to get as good
dogs as he can, and afterwards breeds from his own best dogs, but he has no wish or expectation of permanently
altering the breed. Nevertheless we may infer that this process, continued during centuries, would improve and
modify any breed, in the same way as Bakewell, Collins, etc., by this very same process, only carried on more
methodically, did greatly modify, even during their lifetimes, the forms and qualities of their cattle. Slow and
insensible changes of this kind could never be recognised unless actual measurements or careful drawings of the
breeds in question have been made long ago, which may serve for comparison. In some cases, however,
unchanged, or but little changed, individuals of the same breed exist in less civilised districts, where the breed has
been less improved. There is reason to believe that King Charles' spaniel has been unconsciously modified to a
large extent since the time of that monarch. Some highly competent authorities are convinced that the setter is
directly derived from the spaniel, and has probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this case the change has, it is believed, been
chiefly effected by crosses with the foxhound; but what concerns us is, that the change has been effected
unconsciously and gradually, and yet so effectually that, though the old Spanish pointer certainly came from
Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in Spain like our pointer.
By a similar process of selection, and by careful training, English race− horses have come to surpass in
fleetness and size the parent Arabs, so that the latter, by the regulations for the Goodwood Races, are favoured in
the weights which they carry. Lord Spencer and others have shown how the cattle of England have increased in
weight and in early maturity, compared with the stock formerly kept in this country. By comparing the accounts
given in various old treatises of the former and present state of carrier and tumbler pigeons in Britain, India, and
Persia, we can trace the stages through which they have insensibly passed, and come to differ so greatly from the
rock−pigeon.
Youatt gives an excellent illustration of the effects of a course of selection which may be considered as
unconscious, in so far that the breeders could never have expected, or even wished, to produce the result which
ensued−−namely, the production of the distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley
and Mr. Burgess, as Mr. Youatt remarks, "Have been purely bred from the original stock of Mr. Bakewell for
upwards of fifty years. There is not a suspicion existing in the mind of any one at all acquainted with the subject
that the owner of either of them has deviated in any one instance from the pure blood of Mr. Bakewell's flock, and
yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance
of being quite different varieties."
If there exist savages so barbarous as never to think of the inherited character of the offspring of their
domestic animals, yet any one animal particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones; so that in this case there would be a kind of unconscious
selection going on. We see the value set on animals even by the barbarians of Tierra del Fuego, by their killing
and devouring their old women, in times of dearth, as of less value than their dogs.
In plants the same gradual process of improvement through the occasional preservation of the best individuals,
whether or not sufficiently distinct to be ranked at their first appearance as distinct varieties, and whether or not
two or more species or races have become blended together by crossing, may plainly be recognised in the
increased size and beauty which we now see in the varieties of the heartsease, rose, pelargonium, dahlia, and other
plants, when compared with the older varieties or with their parent−stocks. No one would ever expect to get a
first−rate heartsease or dahlia from the seed of a wild plant. No one would expect to raise a first−rate melting pear
from the seed of a wild pear, though he might succeed from a poor seedling growing wild, if it had come from a
garden−stock. The pear, though cultivated in classical times, appears, from Pliny's description, to have been a
fruit of very inferior quality. I have seen great surprise expressed in horticultural works at the wonderful skill of
gardeners in having produced such splendid results from such poor materials; but the art has been simple, and, as
far as the final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating
CHAPTER I. VARIATION UNDER DOMESTICATION. 20

The Origin of Species by means of Natural Selection
the best known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so
onwards. But the gardeners of the classical period, who cultivated the best pears which they could procure, never
thought what splendid fruit we should eat; though we owe our excellent fruit in some small degree to their having
naturally chosen and preserved the best varieties they could anywhere find.
A large amount of change, thus slowly and unconsciously accumulated, explains, as I believe, the well−known
fact, that in a number of cases we cannot recognise, and therefore do not know, the wild parent−stocks of the
plants which have been longest cultivated in our flower and kitchen gardens. If it has taken centuries or thousands
of years to improve or modify most of our plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope, nor any other region inhabited by quite
uncivilised man, has afforded us a single plant worth culture. It is not that these countries, so rich in species, do
not by a strange chance possess the aboriginal stocks of any useful plants, but that the native plants have not been
improved by continued selection up to a standard of perfection comparable with that acquired by the plants in
countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it should not be overlooked that they almost always
have to struggle for their own food, at least during certain seasons. And in two countries very differently
circumstanced, individuals of the same species, having slightly different constitutions or structure, would often
succeed better in the one country than in the other, and thus by a process of "natural selection," as will hereafter
be more fully explained, two sub−breeds might be formed. This, perhaps, partly explains why the varieties kept
by savages, as has been remarked by some authors, have more of the character of true species than the varieties
kept in civilised countries.
On the view here given of the important part which selection by man has played, it becomes at once obvious,
how it is that our domestic races show adaptation in their structure or in their habits to man's wants or fancies. We
can, I think, further understand the frequently abnormal character of our domestic races, and likewise their
differences being so great in external characters, and relatively so slight in internal parts or organs. Man can
hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and
indeed he rarely cares for what is internal. He can never act by selection, excepting on variations which are first
given to him in some slight degree by nature. No man would ever try to make a fantail till he saw a pigeon with a
tail developed in some slight degree in an unusual manner, or a pouter till he saw a pigeon with a crop of
somewhat unusual size; and the more abnormal or unusual any character was when it first appeared, the more
likely it would be to catch his attention. But to use such an expression as trying to make a fantail is, I have no
doubt, in most cases, utterly incorrect. The man who first selected a pigeon with a slightly larger tail, never
dreamed what the descendants of that pigeon would become through long−continued, partly unconscious and
partly methodical, selection. Perhaps the parent bird of all fantails had only fourteen tail−feathers somewhat
expanded, like the present Java fantail, or like individuals of other and distinct breeds, in which as many as
seventeen tail−feathers have been counted. Perhaps the first pouter−pigeon did not inflate its crop much more than
the turbit now does the upper part of its oesophagus−−a habit which is disregarded by all fanciers, as it is not one
of the points of the breed.
Nor let it be thought that some great deviation of structure would be necessary to catch the fancier's eye: he
perceives extremely small differences, and it is in human nature to value any novelty, however slight, in one's
own possession. Nor must the value which would formerly have been set on any slight differences in the
individuals of the same species, be judged of by the value which is now set on them, after several breeds have
fairly been established. It is known that with pigeons many slight variations now occasionally appear, but these
are rejected as faults or deviations from the standard of perfection in each breed. The common goose has not
given rise to any marked varieties; hence the Toulouse and the common breed, which differ only in colour, that
most fleeting of characters, have lately been exhibited as distinct at our poultry−shows.
These views appear to explain what has sometimes been noticed, namely, that we know hardly anything about
the origin or history of any of our domestic breeds. But, in fact, a breed, like a dialect of a language, can hardly be
said to have a distinct origin. A man preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals, and thus improves them, and the improved
animals slowly spread in the immediate neighbourhood. But they will as yet hardly have a distinct name, and
from being only slightly valued, their history will have been disregarded. When further improved by the same
CHAPTER I. VARIATION UNDER DOMESTICATION. 21

The Origin of Species by means of Natural Selection
slow and gradual process, they will spread more widely, and will be recognised as something distinct and
valuable, and will then probably first receive a provincial name. In semi−civilised countries, with little free
communication, the spreading of a new sub−breed will be a slow process. As soon as the points of value are once
acknowledged, the principle, as I have called it, of unconscious selection will always tend−−perhaps more at one
period than at another, as the breed rises or falls in fashion−−perhaps more in one district than in another,
according to the state of civilisation of the inhabitants−−slowly to add to the characteristic features of the breed,
whatever they may be. But the chance will be infinitely small of any record having been preserved of such slow,
varying, and insensible changes.
CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION.
I will now say a few words on the circumstances, favourable or the reverse, to man's power of selection. A
high degree of variability is obviously favourable, as freely giving the materials for selection to work on; not that
mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as variations manifestly useful or pleasing to man
appear only occasionally, the chance of their appearance will be much increased by a large number of individuals
being kept. Hence number is of the highest importance for success. On this principle Marshall formerly remarked,
with respect to the sheep of part of Yorkshire, "As they generally belong to poor people, and are mostly IN
SMALL LOTS, they never can be improved." On the other hand, nurserymen, from keeping large stocks of the
same plant, are generally far more successful than amateurs in raising new and valuable varieties. A large number
of individuals of an animal or plant can be reared only where the conditions for its propagation are favourable.
When the individuals are scanty all will be allowed to breed, whatever their quality may be, and this will
effectually prevent selection. But probably the most important element is that the animal or plant should be so
highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure.
Unless such attention be paid nothing can be effected. I have seen it gravely remarked, that it was most fortunate
that the strawberry began to vary just when gardeners began to attend to this plant. No doubt the strawberry had
always varied since it was cultivated, but the slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again
picked out the best seedlings and bred from them, then (with some aid by crossing distinct species) those many
admirable varieties of the strawberry were raised which have appeared during the last half−century.
With animals, facility in preventing crosses is an important element in the formation of new races−−at least, in
a country which is already stocked with other races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one breed of the same species. Pigeons can be
mated for life, and this is a great convenience to the fancier, for thus many races may be improved and kept true,
though mingled in the same aviary; and this circumstance must have largely favoured the formation of new
breeds. Pigeons, I may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be
freely rejected, as when killed they serve for food. On the other hand, cats, from their nocturnal rambling habits,
can not be easily matched, and, although so much valued by women and children, we rarely see a distinct breed
long kept up; such breeds as we do sometimes see are almost always imported from some other country. Although
I do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of the
cat, the donkey, peacock, goose, etc., may be attributed in main part to selection not having been brought into
play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by poor people, and little
attention paid to their breeding; for recently in certain parts of Spain and of the United States this animal has been
surprisingly modified and improved by careful selection; in peacocks, from not being very easily reared and a
large stock not kept; in geese, from being valuable only for two purposes, food and feathers, and more especially
from no pleasure having been felt in the display of distinct breeds; but the goose, under the conditions to which it
is exposed when domesticated, seems to have a singularly inflexible organisation, though it has varied to a slight
extent, as I have elsewhere described.
Some authors have maintained that the amount of variation in our domestic productions is soon reached, and
can never afterward be exceeded. It would be somewhat rash to assert that the limit has been attained in any one
case; for almost all our animals and plants have been greatly improved in many ways within a recent period; and
this implies variation. It would be equally rash to assert that characters now increased to their utmost limit, could
not, after remaining fixed for many centuries, again vary under new conditions of life. No doubt, as Mr. Wallace
CHAPTER I. VARIATION UNDER DOMESTICATION. 22

The Origin of Species by means of Natural Selection
has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of
any terrestrial animal, as this will be determined by the friction to be overcome, the weight of the body to be
carried, and the power of contraction in the muscular fibres. But what concerns us is that the domestic varieties of
the same species differ from each other in almost every character, which man has attended to and selected, more
than do the distinct species of the same genera. Isidore Geoffroy St. Hilaire has proved this in regard to size, and
so it is with colour, and probably with the length of hair. With respect to fleetness, which depends on many bodily
characters, Eclipse was far fleeter, and a dray−horse is comparably stronger, than any two natural species
belonging to the same genus. So with plants, the seeds of the different varieties of the bean or maize probably
differ more in size than do the seeds of the distinct species in any one genus in the same two families. The same
remark holds good in regard to the fruit of the several varieties of the plum, and still more strongly with the
melon, as well as in many other analogous cases.
To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the
highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting
the reproductive system. It is not probable that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion determine whether variations shall endure.
Variability is governed by many unknown laws, of which correlated growth is probably the most important.
Something, but how much we do not know, may be attributed to the definite action of the conditions of life.
Some, perhaps a great, effect may be attributed to the increased use or disuse of parts. The final result is thus
rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have
played an important part in the origin of our breeds. When several breeds have once been formed in any country,
their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new
sub−breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those
plants which are propagated by seed. With plants which are temporarily propagated by cuttings, buds, etc., the
importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids
and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for
their endurance is only temporary. Over all these causes of change, the accumulative action of selection, whether
applied methodically and quickly, or unconsciously and slowly, but more efficiently, seems to have been the
predominant power.
CHAPTER I. VARIATION UNDER DOMESTICATION. 23

The Origin of Species by means of Natural Selection
CHAPTER II. VARIATION UNDER NATURE.
Variability −− Individual differences −− Doubtful species −− Wide ranging, much diffused, and common
species, vary most −− Species of the larger genera in each country vary more frequently than the species of the
smaller genera −− Many of the species of the larger genera resemble varieties in being very closely, but
unequally, related to each other, and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any variation. To treat this subject properly, a long catalogue of
dry facts ought to be given; but these I shall reserve for a future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one definition has satisfied all naturalists; yet every
naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown
element of a distinct act of creation. The term "variety" is almost equally difficult to define; but here community
of descent is almost universally implied, though it can rarely be proved. We have also what are called
monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some considerable deviation
of structure, generally injurious, or not useful to the species. Some authors use the term "variation" in a technical
sense, as implying a modification directly due to the physical conditions of life; and "variations" in this sense are
supposed not to be inherited; but who can say that the dwarfed condition of shells in the brackish waters of the
Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an animal from far northwards, would not in
some cases be inherited for at least a few generations? And in this case I presume that the form would be called a
variety.
It may be doubted whether sudden and considerable deviations of structure, such as we occasionally see in our
domestic productions, more especially with plants, are ever permanently propagated in a state of nature. Almost
every part of every organic being is so beautifully related to its complex conditions of life that it seems as
improbable that any part should have been suddenly produced perfect, as that a complex machine should have
been invented by man in a perfect state. Under domestication monstrosities sometimes occur which resemble
normal structures in widely different animals. Thus pigs have occasionally been born with a sort of proboscis, and
if any wild species of the same genus had naturally possessed a proboscis, it might have been argued that this had
appeared as a monstrosity; but I have as yet failed to find, after diligent search, cases of monstrosities resembling
normal structures in nearly allied forms, and these alone bear on the question. If monstrous forms of this kind ever
do appear in a state of nature and are capable of reproduction (which is not always the case), as they occur rarely
and singly, their preservation would depend on unusually favourable circumstances. They would, also, during the
first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost
inevitably be lost. But I shall have to return in a future chapter to the preservation and perpetuation of single or
occasional variations.
INDIVIDUAL DIFFERENCES.
The many slight differences which appear in the offspring from the same parents, or which it may be
presumed have thus arisen, from being observed in the individuals of the same species inhabiting the same
confined locality, may be called individual differences. No one supposes that all the individuals of the same
species are cast in the same actual mould. These individual differences are of the highest importance for us, for
they are often inherited, as must be familiar to every one; and they thus afford materials for natural selection to act
on and accumulate, in the same manner as man accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally affect what naturalists consider unimportant
parts; but I could show, by a long catalogue of facts, that parts which must be called important, whether viewed
under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even
in important parts of structure, which he could collect on good authority, as I have collected, during a course of
years. It should be remembered that systematists are far from being pleased at finding variability in important
characters, and that there are not many men who will laboriously examine internal and important organs, and
compare them in many specimens of the same species. It would never have been expected that the branching of
CHAPTER II. VARIATION UNDER NATURE. 24

The Origin of Species by means of Natural Selection
the main nerves close to the great central ganglion of an insect would have been variable in the same species; it
might have been thought that changes of this nature could have been effected only by slow degrees; yet Sir J.
Lubbock has shown a degree of variability in these main nerves in Coccus, which may almost be compared to the
irregular branching of the stem of a tree. This philosophical naturalist, I may add, has also shown that the muscles
in the larvae of certain insects are far from uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank those parts as important (as some few
naturalists have honestly confessed) which do not vary; and, under this point of view, no instance will ever be
found of an important part varying; but under any other point of view many instances assuredly can be given.
There is one point connected with individual differences which is extremely perplexing: I refer to those genera
which have been called "protean" or "polymorphic," in which species present an inordinate amount of variation.
With respect to many of these forms, hardly two naturalists agree whether to rank them as species or as varieties.
We may instance Rubus, Rosa, and Hieracium among plants, several genera of insects, and of Brachiopod shells.
In most polymorphic genera some of the species have fixed and definite characters. Genera which are
polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and likewise,
judging from Brachiopod shells, at former periods of time. These facts are very perplexing, for they seem to show
that this kind of variability is independent of the conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or disservice to the species, and which
consequently have not been seized on and rendered definite by natural selection, as hereafter to be explained.
Individuals of the same species often present, as is known to every one, great differences of structure,
independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or
workers among insects, and in the immature and larval states of many of the lower animals. There are, also, cases
of dimorphism and trimorphism, both with animals and plants. Thus, Mr. Wallace, who has lately called attention
to the subject, has shown that the females of certain species of butterflies, in the Malayan Archipelago, regularly
appear under two or even three conspicuously distinct forms, not connected by intermediate varieties. Fritz Muller
has described analogous but more extraordinary cases with the males of certain Brazilian Crustaceans: thus, the
male of a Tanais regularly occurs under two distinct forms; one of these has strong and differently shaped pincers,
and the other has antennae much more abundantly furnished with smelling−hairs. Although in most of these
cases, the two or three forms, both with animals and plants, are not now connected by intermediate gradations, it
is possible that they were once thus connected. Mr. Wallace, for instance, describes a certain butterfly which
presents in the same island a great range of varieties connected by intermediate links, and the extreme links of the
chain closely resemble the two forms of an allied dimorphic species inhabiting another part of the Malay
Archipelago. Thus also with ants, the several worker−castes are generally quite distinct; but in some cases, as we
shall hereafter see, the castes are connected together by finely graduated varieties. So it is, as I have myself
observed, with some dimorphic plants. It certainly at first appears a highly remarkable fact that the same female
butterfly should have the power of producing at the same time three distinct female forms and a male; and that an
hermaphrodite plant should produce from the same seed− capsule three distinct hermaphrodite forms, bearing
three different kinds of females and three or even six different kinds of males. Nevertheless these cases are only
exaggerations of the common fact that the female produces offspring of two sexes which sometimes differ from
each other in a wonderful manner.
DOUBTFUL SPECIES.
The forms which possess in some considerable degree the character of species, but which are so closely
similar to other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to
rank them as distinct species, are in several respects the most important for us. We have every reason to believe
that many of these doubtful and closely allied forms have permanently retained their characters for a long time;
for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite by means of
intermediate links any two forms, he treats the one as a variety of the other, ranking the most common, but
sometimes the one first described as the species, and the other as the variety. But cases of great difficulty, which I
will not here enumerate, sometimes arise in deciding whether or not to rank one form as a variety of another, even
when they are closely connected by intermediate links; nor will the commonly assumed hybrid nature of the
intermediate forms always remove the difficulty. In very many cases, however, one form is ranked as a variety of
another, not because the intermediate links have actually been found, but because analogy leads the observer to
CHAPTER II. VARIATION UNDER NATURE. 25

The Origin of Species by means of Natural Selection
suppose either that they do now somewhere exist, or may formerly have existed; and here a wide door for the
entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists
having sound judgment and wide experience seems the only guide to follow. We must, however, in many cases,
decide by a majority of naturalists, for few well−marked and well−known varieties can be named which have not
been ranked as species by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be disputed. Compare the several floras
of Great Britain, of France, or of the United States, drawn up by different botanists, and see what a surprising
number of forms have been ranked by one botanist as good species, and by another as mere varieties. Mr. H.C.
Watson, to whom I lie under deep obligation for assistance of all kinds, has marked for me 182 British plants,
which are generally considered as varieties, but which have all been ranked by botanists as species; and in making
this list he has omitted many trifling varieties, but which nevertheless have been ranked by some botanists as
species, and he has entirely omitted several highly polymorphic genera. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112−−a difference of 139
doubtful forms! Among animals which unite for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but
are common in separated areas. How many of the birds and insects in North America and Europe, which differ
very slightly from each other, have been ranked by one eminent naturalist as undoubted species, and by another as
varieties, or, as they are often called, geographical races! Mr. Wallace, in several valuable papers on the various
animals, especially on the Lepidoptera, inhabiting the islands of the great Malayan Archipelago, shows that they
may be classed under four heads, namely, as variable forms, as local forms, as geographical races or sub−species,
and as true representative species. The first or variable forms vary much within the limits of the same island. The
local forms are moderately constant and distinct in each separate island; but when all from the several islands are
compared together, the differences are seen to be so slight and graduated that it is impossible to define or describe
them, though at the same time the extreme forms are sufficiently distinct. The geographical races or sub−species
are local forms completely fixed and isolated; but as they do not differ from each other by strongly marked and
important characters, "There is no possible test but individual opinion to determine which of them shall be
considered as species and which as varieties." Lastly, representative species fill the same place in the natural
economy of each island as do the local forms and sub−species; but as they are distinguished from each other by a
greater amount of difference than that between the local forms and sub−species, they are almost universally
ranked by naturalists as true species. Nevertheless, no certain criterion can possibly be given by which variable
forms, local forms, sub species and representative species can be recognised.
Many years ago, when comparing, and seeing others compare, the birds from the closely neighbouring islands
of the Galapagos Archipelago, one with another, and with those from the American mainland, I was much struck
how entirely vague and arbitrary is the distinction between species and varieties. On the islets of the little Madeira
group there are many insects which are characterized as varieties in Mr. Wollaston's admirable work, but which
would certainly be ranked as distinct species by many entomologists. Even Ireland has a few animals, now
generally regarded as varieties, but which have been ranked as species by some zoologists. Several experienced
ornithologists consider our British red grouse as only a strongly marked race of a Norwegian species, whereas the
greater number rank it as an undoubted species peculiar to Great Britain. A wide distance between the homes of
two doubtful forms leads many naturalists to rank them as distinct species; but what distance, it has been well
asked, will suffice if that between America and Europe is ample, will that between Europe and the Azores, or
Madeira, or the Canaries, or between the several islets of these small archipelagos, be sufficient?
Mr. B.D. Walsh, a distinguished entomologist of the United States, has described what he calls Phytophagic
varieties and Phytophagic species. Most vegetable−feeding insects live on one kind of plant or on one group of
plants; some feed indiscriminately on many kinds, but do not in consequence vary. In several cases, however,
insects found living on different plants, have been observed by Mr. Walsh to present in their larval or mature
state, or in both states, slight, though constant differences in colour, size, or in the nature of their secretions. In
some instances the males alone, in other instances, both males and females, have been observed thus to differ in a
slight degree. When the differences are rather more strongly marked, and when both sexes and all ages are
affected, the forms are ranked by all entomologists as good species. But no observer can determine for another,
CHAPTER II. VARIATION UNDER NATURE. 26

The Origin of Species by means of Natural Selection
even if he can do so for himself, which of these Phytophagic forms ought to be called species and which varieties.
Mr. Walsh ranks the forms which it may be supposed would freely intercross, as varieties; and those which appear
to have lost this power, as species. As the differences depend on the insects having long fed on distinct plants, it
cannot be expected that intermediate links connecting the several forms should now be found. The naturalist thus
loses his best guide in determining whether to rank doubtful forms as varieties or species. This likewise
necessarily occurs with closely allied organisms, which inhabit distinct continents or islands. When, on the other
hand, an animal or plant ranges over the same continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a good chance that intermediate forms will be
discovered which will link together the extreme states; and these are then degraded to the rank of varieties.
Some few naturalists maintain that animals never present varieties; but then these same naturalists rank the
slightest difference as of specific value; and when the same identical form is met with in two distant countries, or
in two geological formations, they believe that two distinct species are hidden under the same dress. The term
species thus comes to be a mere useless abstraction, implying and assuming a separate act of creation. It is certain
that many forms, considered by highly competent judges to be varieties, resemble species so completely in
character that they have been thus ranked by other highly competent judges. But to discuss whether they ought to
be called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat
the air.
Many of the cases of strongly marked varieties or doubtful species well deserve consideration; for several
interesting lines of argument, from geographical distribution, analogical variation, hybridism, etc., have been
brought to bear in the attempt to determine their rank; but space does not here permit me to discuss them. Close
investigation, in many cases, will no doubt bring naturalists to agree how to rank doubtful forms. Yet it must be
confessed that it is in the best known countries that we find the greatest number of them. I have been struck with
the fact that if any animal or plant in a state of nature be highly useful to man, or from any cause closely attracts
his attention, varieties of it will almost universally be found recorded. These varieties, moreover, will often be
ranked by some authors as species. Look at the common oak, how closely it has been studied; yet a German
author makes more than a dozen species out of forms, which are almost universally considered by other botanists
to be varieties; and in this country the highest botanical authorities and practical men can be quoted to show that
the sessile and pedunculated oaks are either good and distinct species or mere varieties.
I may here allude to a remarkable memoir lately published by A. de Candolle, on the oaks of the whole world.
No one ever had more ample materials for the discrimination of the species, or could have worked on them with
more zeal and sagacity. He first gives in detail all the many points of structure which vary in the several species,
and estimates numerically the relative frequency of the variations. He specifies above a dozen characters which
may be found varying even on the same branch, sometimes according to age or development, sometimes without
any assignable reason. Such characters are not of course of specific value, but they are, as Asa Gray has remarked
in commenting on this memoir, such as generally enter into specific definitions. De Candolle then goes on to say
that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never
found connected by intermediate states. After this discussion, the result of so much labour, he emphatically
remarks: "They are mistaken, who repeat that the greater part of our species are clearly limited, and that the
doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and
its species were founded upon a few specimens, that is to say, were provisional. Just as we come to know them
better, intermediate forms flow in, and doubts as to specific limits augment." He also adds that it is the best
known species which present the greatest number of spontaneous varieties and sub−varieties. Thus Quercus robur
has twenty−eight varieties, all of which, excepting six, are clustered round three sub− species, namely Q.
pedunculata, sessiliflora and pubescens. The forms which connect these three sub−species are comparatively rare;
and, as Asa Gray again remarks, if these connecting forms which are now rare were to become totally extinct the
three sub−species would hold exactly the same relation to each other as do the four or five provisionally admitted
species which closely surround the typical Quercus robur. Finally, De Candolle admits that out of the 300 species,
which will be enumerated in his Prodromus as belonging to the oak family, at least two−thirds are provisional
species, that is, are not known strictly to fulfil the definition above given of a true species. It should be added that
De Candolle no longer believes that species are immutable creations, but concludes that the derivative theory is
the most natural one, "and the most accordant with the known facts in palaeontology, geographical botany and
CHAPTER II. VARIATION UNDER NATURE. 27

The Origin of Species by means of Natural Selection
zoology, of anatomical structure and classification."
When a young naturalist commences the study of a group of organisms quite unknown to him he is at first
much perplexed in determining what differences to consider as specific and what as varietal; for he knows nothing
of the amount and kind of variation to which the group is subject; and this shows, at least, how very generally
there is some variation. But if he confine his attention to one class within one country he will soon make up his
mind how to rank most of the doubtful forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry fancier before alluded to, with the amount of difference in the
forms which he is continually studying; and he has little general knowledge of analogical variation in other groups
and in other countries by which to correct his first impressions. As he extends the range of his observations he
will meet with more cases of difficulty; for he will encounter a greater number of closely−allied forms. But if his
observations be widely extended he will in the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the truth of this admission will often be disputed
by other naturalists. When he comes to study allied forms brought from countries not now continuous, in which
case he cannot hope to find intermediate links, he will be compelled to trust almost entirely to analogy, and his
difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between species and sub−species−−that is, the
forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species;
or, again, between sub−species and well−marked varieties, or between lesser varieties and individual differences.
These differences blend into each other by an insensible series; and a series impresses the mind with the idea of
an actual passage.
Hence I look at individual differences, though of small interest to the systematist, as of the highest importance
for us, as being the first step towards such slight varieties as are barely thought worth recording in works on
natural history. And I look at varieties which are in any degree more distinct and permanent, as steps toward more
strongly marked and permanent varieties; and at the latter, as leading to sub−species, and then to species. The
passage from one stage of difference to another may, in many cases, be the simple result of the nature of the
organism and of the different physical conditions to which it has long been exposed; but with respect to the more
important and adaptive characters, the passage from one stage of difference to another may be safely attributed to
the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or
disuse of parts. A well−marked variety may therefore be called an incipient species; but whether this belief is
justifiable must be judged by the weight of the various facts and considerations to be given throughout this work.
It need not be supposed that all varieties or incipient species attain the rank of species. They may become
extinct, or they may endure as varieties for very long periods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land−shells in Madeira, and with plants by Gaston de Saporta. If a variety were
to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the
variety; or it might come to supplant and exterminate the parent species; or both might co−exist, and both rank as
independent species. But we shall hereafter return to this subject.
>From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of
convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the
term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison
with mere individual differences, is also applied arbitrarily, for convenience sake.
WIDE−RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.
Guided by theoretical considerations, I thought that some interesting results might be obtained in regard to the
nature and relations of the species which vary most, by tabulating all the varieties in several well−worked floras.
At first this seemed a simple task; but Mr. H.C. Watson, to whom I am much indebted for valuable advice and
assistance on this subject, soon convinced me that there were many difficulties, as did subsequently Dr. Hooker,
even in stronger terms. I shall reserve for a future work the discussion of these difficulties, and the tables of the
proportional numbers of the varying species. Dr. Hooker permits me to add that after having carefully read my
manuscript, and examined the tables, he thinks that the following statements are fairly well established. The
whole subject, however, treated as it necessarily here is with much brevity, is rather perplexing, and allusions
cannot be avoided to the "struggle for existence," "divergence of character," and other questions, hereafter to be
discussed.
CHAPTER II. VARIATION UNDER NATURE. 28

The Origin of Species by means of Natural Selection
Alphonse de Candolle and others have shown that plants which have very wide ranges generally present
varieties; and this might have been expected, as they are exposed to diverse physical conditions, and as they come
into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of
organic beings. But my tables further show that, in any limited country, the species which are the most common,
that is abound most in individuals, and the species which are most widely diffused within their own country (and
this is a different consideration from wide range, and to a certain extent from commonness), oftenest give rise to
varieties sufficiently well−marked to have been recorded in botanical works. Hence it is the most flourishing, or,
as they may be called, the dominant species−−those which range widely, are the most diffused in their own
country, and are the most numerous in individuals−−which oftenest produce well−marked varieties, or, as I
consider them, incipient species. And this, perhaps, might have been anticipated; for, as varieties, in order to
become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the
species which are already dominant will be the most likely to yield offspring, which, though in some slight degree
modified, still inherit those advantages that enabled their parents to become dominant over their compatriots. In
these remarks on predominence, it should be understood that reference is made only to the forms which come into
competition with each other, and more especially to the members of the same genus or class having nearly similar
habits of life. With respect to the number of individuals or commonness of species, the comparison of course
relates only to the members of the same group. One of the higher plants may be said to be dominant if it be more
numerous in individuals and more widely diffused than the other plants of the same country, which live under
nearly the same conditions. A plant of this kind is not the less dominant because some conferva inhabiting the
water or some parasitic fungus is infinitely more numerous in individuals, and more widely diffused. But if the
conferva or parasitic fungus exceeds its allies in the above respects, it will then be dominant within its own class.
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THE
SPECIES OF THE SMALLER GENERA.
If the plants inhabiting a country as described in any Flora, be divided into two equal masses, all those in the
larger genera (i.e., those including many species) being placed on one side, and all those in the smaller genera on
the other side, the former will be found to include a somewhat larger number of the very common and much
diffused or dominant species. This might have been anticipated, for the mere fact of many species of the same
genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those
including many species, a larger proportional number of dominant species. But so many causes tend to obscure
this result, that I am surprised that my tables show even a small majority on the side of the larger genera. I will
here allude to only two causes of obscurity. Fresh water and salt−loving plants generally have very wide ranges
and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has
little or no relation to the size of the genera to which the species belong. Again, plants low in the scale of
organisation are generally much more widely diffused than plants higher in the scale; and here again there is no
close relation to the size of the genera. The cause of lowly−organised plants ranging widely will be discussed in
our chapter on Geographical Distribution.
>From looking at species as only strongly marked and well−defined varieties, I was led to anticipate that the
species of the larger genera in each country would oftener present varieties, than the species of the smaller genera;
for wherever many closely related species (i.e., species of the same genus) have been formed, many varieties or
incipient species ought, as a general rule, to be now forming. Where many large trees grow, we expect to find
saplings. Where many species of a genus have been formed through variation, circumstances have been
favourable for variation; and hence we might expect that the circumstances would generally still be favourable to
variation. On the other hand, if we look at each species as a special act of creation, there is no apparent reason
why more varieties should occur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous
insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the
smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species
on the side of the larger genera presented varieties, than on the side of the smaller genera. Moreover, the species
of the large genera which present any varieties, invariably present a larger average number of varieties than do the
species of the small genera. Both these results follow when another division is made, and when all the least
CHAPTER II. VARIATION UNDER NATURE. 29

The Origin of Species by means of Natural Selection
genera, with from only one to four species, are altogether excluded from the tables. These facts are of plain
signification on the view that species are only strongly marked and permanent varieties; for wherever many
species of the same genus have been formed, or where, if we may use the expression, the manufactory of species
has been active, we ought generally to find the manufactory still in action, more especially as we have every
reason to believe the process of manufacturing new species to be a slow one. And this certainly holds true if
varieties be looked at as incipient species; for my tables clearly show, as a general rule, that, wherever many
species of a genus have been formed, the species of that genus present a number of varieties, that is, of incipient
species, beyond the average. It is not that all large genera are now varying much, and are thus increasing in the
number of their species, or that no small genera are now varying and increasing; for if this had been so, it would
have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time
often increased greatly in size; and that large genera have often come to their maxima, declined, and disappeared.
All that we want to show is, that where many species of a genus have been formed, on an average many are still
forming; and this certainly holds good.
MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES IN
BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING
RESTRICTED RANGES.
There are other relations between the species of large genera and their recorded varieties which deserve
notice. We have seen that there is no infallible criterion by which to distinguish species and well−marked
varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to
come to a determination by the amount of difference between them, judging by analogy whether or not the
amount suffices to raise one or both to the rank of species. Hence the amount of difference is one very important
criterion in settling whether two forms should be ranked as species or varieties. Now Fries has remarked in regard
to plants, and Westwood in regard to insects, that in large genera the amount of difference between the species is
often exceedingly small. I have endeavoured to test this numerically by averages, and, as far as my imperfect
results go, they confirm the view. I have also consulted some sagacious and experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore, the species of the larger genera resemble varieties,
more than do the species of the smaller genera. Or the case may be put in another way, and it may be said, that in
the larger genera, in which a number of varieties or incipient species greater than the average are now
manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they
differ from each other by a less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any
one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from
each other; they may generally be divided into sub−genera, or sections, or lesser groups. As Fries has well
remarked, little groups of species are generally clustered like satellites around other species. And what are
varieties but groups of forms, unequally related to each other, and clustered round certain forms−−that is, round
their parent−species. Undoubtedly there is one most important point of difference between varieties and species,
namely, that the amount of difference between varieties, when compared with each other or with their
parent−species, is much less than that between the species of the same genus. But when we come to discuss the
principle, as I call it, of divergence of character, we shall see how this may be explained, and how the lesser
differences between varieties tend to increase into the greater differences between species.
There is one other point which is worth notice. Varieties generally have much restricted ranges. This
statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of its
supposed parent−species, their denominations would be reversed. But there is reason to believe that the species
which are very closely allied to other species, and in so far resemble varieties, often have much restricted ranges.
For instance, Mr. H.C. Watson has marked for me in the well−sifted London catalogue of Plants (4th edition)
sixty−three plants which are therein ranked as species, but which he considers as so closely allied to other species
as to be of doubtful value: these sixty−three reputed species range on an average over 6.9 of the provinces into
which Mr. Watson has divided Great Britain. Now, in this same catalogue, fifty−three acknowledged varieties are
recorded, and these range over 7.7 provinces; whereas, the species to which these varieties belong range over 14.3
provinces. So that the acknowledged varieties have very nearly the same restricted average range, as have the
closely allied forms, marked for me by Mr. Watson as doubtful species, but which are almost universally ranked
CHAPTER II. VARIATION UNDER NATURE. 30

The Origin of Species by means of Natural Selection
by British botanists as good and true species.
SUMMARY.
Finally, varieties cannot be distinguished from species−−except, first, by the discovery of intermediate linking
forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very
little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of
difference considered necessary to give to any two forms the rank of species cannot be defined. In genera having
more than the average number of species in any country, the species of these genera have more than the average
number of varieties. In large genera the species are apt to be closely but unequally allied together, forming little
clusters round other species. Species very closely allied to other species apparently have restricted ranges. In all
these respects the species of large genera present a strong analogy with varieties. And we can clearly understand
these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly
inexplicable if species are independent creations.
We have also seen that it is the most flourishing or dominant species of the larger genera within each class
which on an average yield the greatest number of varieties, and varieties, as we shall hereafter see, tend to become
converted into new and distinct species. Thus the larger genera tend to become larger; and throughout nature the
forms of life which are now dominant tend to become still more dominant by leaving many modified and
dominant descendants. But, by steps hereafter to be explained, the larger genera also tend to break up into smaller
genera. And thus, the forms of life throughout the universe become divided into groups subordinate to groups.
CHAPTER II. VARIATION UNDER NATURE. 31

The Origin of Species by means of Natural Selection
CHAPTER III. STRUGGLE FOR EXISTENCE.
Its bearing on natural selection −− The term used in a wide sense −− Geometrical ratio of increase −− Rapid
increase of naturalised animals and plants −− Nature of the checks to increase −− Competition universal −−
Effects of climate −− Protection from the number of individuals −− Complex relations of all animals and plants
throughout nature −− Struggle for life most severe between individuals and varieties of the same species: often
severe between species of the same genus −− The relation of organism to organism the most important of all
relations.
Before entering on the subject of this chapter I must make a few preliminary remarks to show how the
struggle for existence bears on natural selection. It has been seen in the last chapter that among organic beings in a
state of nature there is some individual variability: indeed I am not aware that this has ever been disputed. It is
immaterial for us whether a multitude of doubtful forms be called species or sub−species or varieties; what rank,
for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any
well−marked varieties be admitted. But the mere existence of individual variability and of some few well−marked
varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in
nature. How have all those exquisite adaptations of one part of the organisation to another part, and to the
conditions of life and of one organic being to another being, been perfected? We see these beautiful
co−adaptations most plainly in the woodpecker and the mistletoe; and only a little less plainly in the humblest
parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives
through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful
adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately
converted into good and distinct species, which in most cases obviously differ from each other far more than do
the varieties of the same species? How do those groups of species, which constitute what are called distinct genera
and which differ from each other more than do the species of the same genus, arise? All these results, as we shall
more fully see in the next chapter, follow from the struggle for life. Owing to this struggle, variations, however
slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in
their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the
preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus
have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by
the term natural selection, in order to mark its relation to man's power of selection. But the expression often used
by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient. We
have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses,
through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural
Selection, we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's
feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence. In my future work this subject will be
treated, as it well deserves, at greater length. The elder De Candolle and Lyell have largely and philosophically
shown that all organic beings are exposed to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently the result of his great
horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or
more difficult−−at least I found it so−−than constantly to bear this conclusion in mind. Yet unless it be thoroughly
engrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction,
and variation, will be dimly seen or quite misunderstood. We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we forget that the birds which are idly singing round us
mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters,
or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that,
though food may be now superabundant, it is not so at all seasons of each recurring year.
CHAPTER III. STRUGGLE FOR EXISTENCE. 32

The Origin of Species by means of Natural Selection
THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE.
I should premise that I use this term in a large and metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the life of the individual, but success in leaving
progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get
food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more
properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of
which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same
and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees,
but can only in a far−fetched sense be said to struggle with these trees, for, if too many of these parasites grow on
the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch,
may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence
depends on them; and it may metaphorically be said to struggle with other fruit−bearing plants, in tempting the
birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for
convenience sake the general term of Struggle for Existence.
GEOMETRICAL RATIO OF INCREASE.
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase.
Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some
period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country could support the product. Hence, as
more individuals are produced than can possibly survive, there must in every case be a struggle for existence,
either one individual with another of the same species, or with the individuals of distinct species, or with the
physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from
marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for
the world would not hold them.
There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not
destroyed, the earth would soon be covered by the progeny of a single pair. Even slow−breeding man has doubled
in twenty−five years, and at this rate, in less than a thousand years, there would literally not be standing room for
his progeny. Linnaeus has calculated that if an annual plant produced only two seeds−−and there is no plant so
unproductive as this−−and their seedlings next year produced two, and so on, then in twenty years there would be
a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains
to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when
thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving
till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen
million elephants alive descended from the first pair.
But we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded
cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been
favourable to them during two or three following seasons. Still more striking is the evidence from our domestic
animals of many kinds which have run wild in several parts of the world; if the statements of the rate of increase
of slow−breeding cattle and horses in South America, and latterly in Australia, had not been well authenticated,
they would have been incredible. So it is with plants; cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten years. Several of the plants, such as the
cardoon and a tall thistle, which are now the commonest over the wide plains of La Plata, clothing square leagues
of surface almost to the exclusion of every other plant, have been introduced from Europe; and there are plants
which now range in India, as I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which have been
imported from America since its discovery. In such cases, and endless others could be given, no one supposes that
the fertility of the animals or plants has been suddenly and temporarily increased in any sensible degree. The
obvious explanation is that the conditions of life have been highly favourable, and that there has consequently
been less destruction of the old and young and that nearly all the young have been enabled to breed. Their
geometrical ratio of increase, the result of which never fails to be surprising, simply explains their extraordinarily
rapid increase and wide diffusion in their new homes.
CHAPTER III. STRUGGLE FOR EXISTENCE. 33

The Origin of Species by means of Natural Selection
In a state of nature almost every full−grown plant annually produces seed, and among animals there are very
few which do not annually pair. Hence we may confidently assert that all plants and animals are tending to
increase at a geometrical ratio−−that all would rapidly stock every station in which they could any how exist, and
that this geometrical tendency to increase must be checked by destruction at some period of life. Our familiarity
with the larger domestic animals tends, I think, to mislead us; we see no great destruction falling on them, and we
do not keep in mind that thousands are annually slaughtered for food, and that in a state of nature an equal number
would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or seeds by the thousand, and those
which produce extremely few, is, that the slow breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The condor lays a couple of eggs and the ostrich a
score, and yet in the same country the condor may be the more numerous of the two. The Fulmar petrel lays but
one egg, yet it is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs, and
another, like the hippobosca, a single one. But this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some importance to those species which
depend on a fluctuating amount of food, for it allows them rapidly to increase in number. But the real importance
of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the
great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number
may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed, many must
be produced or the species will become extinct. It would suffice to keep up the full number of a tree, which lived
on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this
seed were never destroyed and could be ensured to germinate in a fitting place; so that, in all cases, the average
number of any animal or plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing considerations always in mind−−never to
forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each
lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old
during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the
number of the species will almost instantaneously increase to any amount.
NATURE OF THE CHECKS TO INCREASE.
The causes which check the natural tendency of each species to increase are most obscure. Look at the most
vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further. We know
not exactly what the checks are even in a single instance. Nor will this surprise any one who reflects how ignorant
we are on this head, even in regard to mankind, although so incomparably better known than any other animal.
This subject of the checks to increase has been ably treated by several authors, and I hope in a future work to
discuss it at considerable length, more especially in regard to the feral animals of South America. Here I will
make only a few remarks, just to recall to the reader's mind some of the chief points. Eggs or very young animals
seem generally to suffer most, but this is not invariably the case. With plants there is a vast destruction of seeds,
but from some observations which I have made it appears that the seedlings suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are destroyed in vast numbers by various
enemies; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could
be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357
no less than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown, and the case
would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually
kill the less vigorous, though fully grown plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species being allowed to grow up freely.
The amount of food for each species, of course, gives the extreme limit to which each can increase; but very
frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average
number of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not one head of game were shot during the next
twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be
less game than at present, although hundreds of thousands of game animals are now annually shot. On the other
hand, in some cases, as with the elephant, none are destroyed by beasts of prey; for even the tiger in India most
CHAPTER III. STRUGGLE FOR EXISTENCE. 34

The Origin of Species by means of Natural Selection
rarely dares to attack a young elephant protected by its dam.
Climate plays an important part in determining the average numbers of a species, and periodical seasons of
extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the greatly reduced
numbers of nests in the spring) that the winter of 1854−5 destroyed four−fifths of the birds in my own grounds;
and this is a tremendous destruction, when we remember that ten per cent. is an extraordinarily severe mortality
from epidemics with man. The action of climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the
individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when
climate, for instance, extreme cold, acts directly, it will be the least vigorous individuals, or those which have got
least food through the advancing winter, which will suffer the most. When we travel from south to north, or from
a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing;
and the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But
this is a false view; we forget that each species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competitors for the same place and food; and if these
enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in
numbers; and as each area is already fully stocked with inhabitants, the other species must decrease. When we
travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in
other species being favoured, as in this one being hurt. So it is when we travel northward, but in a somewhat
lesser degree, for the number of species of all kinds, and therefore of competitors, decreases northward; hence in
going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the DIRECTLY
injurious action of climate, than we do in proceeding southward or in descending a mountain. When we reach the
Arctic regions, or snow−capped summits, or absolute deserts, the struggle for life is almost exclusively with the
elements.
That climate acts in main part indirectly by favouring other species we clearly see in the prodigious number of
plants which in our gardens can perfectly well endure our climate, but which never become naturalised, for they
cannot compete with our native plants nor resist destruction by our native animals.
When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract,
epidemics−−at least, this seems generally to occur with our game animals−−often ensue; and here we have a
limiting check independent of the struggle for life. But even some of these so−called epidemics appear to be due
to parasitic worms, which have from some cause, possibly in part through facility of diffusion among the crowded
animals, been disproportionally favoured: and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of
its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn and rape−seed,
etc., in our fields, because the seeds are in great excess compared with the number of birds which feed on them;
nor can the birds, though having a superabundance of food at this one season, increase in number proportionally
to the supply of seed, as their numbers are checked during the winter; but any one who has tried knows how
troublesome it is to get seed from a few wheat or other such plants in a garden; I have in this case lost every single
seed. This view of the necessity of a large stock of the same species for its preservation, explains, I believe, some
singular facts in nature such as that of very rare plants being sometimes extremely abundant, in the few spots
where they do exist; and that of some social plants being social, that is abounding in individuals, even on the
extreme verge of their range. For in such cases, we may believe, that a plant could exist only where the conditions
of its life were so favourable that many could exist together, and thus save the species from utter destruction. I
should add that the good effects of intercrossing, and the ill effects of close interbreeding, no doubt come into
play in many of these cases; but I will not here enlarge on this subject.
COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLE
FOR EXISTENCE.
Many cases are on record showing how complex and unexpected are the checks and relations between organic
beings, which have to struggle together in the same country. I will give only a single instance, which, though a
simple one, interested me. In Staffordshire, on the estate of a relation, where I had ample means of investigation,
there was a large and extremely barren heath, which had never been touched by the hand of man; but several
hundred acres of exactly the same nature had been enclosed twenty−five years previously and planted with Scotch
CHAPTER III. STRUGGLE FOR EXISTENCE. 35

The Origin of Species by means of Natural Selection
fir. The change in the native vegetation of the planted part of the heath was most remarkable, more than is
generally seen in passing from one quite different soil to another: not only the proportional numbers of the
heath−plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in
the plantations, which could not be found on the heath. The effect on the insects must have been still greater, for
six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the
heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of
the introduction of a single tree, nothing whatever else having been done, with the exception of the land having
been enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near
Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hill−tops:
within the last ten years large spaces have been enclosed, and self−sown firs are now springing up in multitudes,
so close together that all cannot live. When I ascertained that these young trees had not been sown or planted I
was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of
acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps. But
on looking closely between the stems of the heath, I found a multitude of seedlings and little trees, which had
been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one
of the old clumps, I counted thirty−two little trees; and one of them, with twenty−six rings of growth, had, during
many years tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land
was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely
barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually
searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts of the world
insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here
neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral
state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly,
which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they
are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain
insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would
lessen the number of the navel−frequenting flies−−then cattle and horses would become feral, and this would
certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onwards
in ever−increasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle
within battle must be continually recurring with varying success; and yet in the long−run the forces are so nicely
balanced that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so profound is our ignorance, and so high our
presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause,
we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals, remote in the scale of nature, are
bound together by a web of complex relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens is never visited in my garden by insects, and consequently, from its peculiar structure, never sets a seed.
Nearly all our orchidaceous plants absolutely require the visits of insects to remove their pollen−masses and thus
to fertilise them. I find from experiments that humble−bees are almost indispensable to the fertilisation of the
heartsease (Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are
necessary for the fertilisation of some kinds of clover; for instance twenty heads of Dutch clover (Trifolium
repens) yielded 2,290 seeds, but twenty other heads, protected from bees, produced not one. Again, 100 heads of
red clover (T. pratense) produced 2,700 seeds, but the same number of protected heads produced not a single
seed. Humble bees alone visit red clover, as other bees cannot reach the nectar. It has been suggested that moths
may fertilise the clovers; but I doubt whether they could do so in the case of the red clover, from their weight not
being sufficient to depress the wing petals. Hence we may infer as highly probable that, if the whole genus of
humble−bees became extinct or very rare in England, the heartsease and red clover would become very rare, or
wholly disappear. The number of humble−bees in any district depends in a great measure upon the number of
field−mice, which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of
CHAPTER III. STRUGGLE FOR EXISTENCE. 36

The Origin of Species by means of Natural Selection
humble−bees, believes that "more than two−thirds of them are thus destroyed all over England." Now the number
of mice is largely dependent, as every one knows, on the number of cats; and Colonel Newman says, "Near
villages and small towns I have found the nests of humble−bees more numerous than elsewhere, which I attribute
to the number of cats that destroy the mice." Hence it is quite credible that the presence of a feline animal in large
numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of
certain flowers in that district!
In the case of every species, many different checks, acting at different periods of life, and during different
seasons or years, probably come into play; some one check or some few being generally the most potent, but all
will concur in determining the average number, or even the existence of the species. In some cases it can be
shown that widely−different checks act on the same species in different districts. When we look at the plants and
bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we
call chance. But how false a view is this! Every one has heard that when an American forest is cut down, a very
different vegetation springs up; but it has been observed that ancient Indian ruins in the Southern United States,
which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds
as in the surrounding virgin forests. What a struggle must have gone on during long centuries between the several
kinds of trees, each annually scattering its seeds by the thousand; what war between insect and insect−−between
insects, snails, and other animals with birds and beasts of prey−−all striving to increase, all feeding on each other,
or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus checked
the growth of the trees. Throw up a handful of feathers, and all fall to the ground according to definite laws; but
how simple is the problem where each shall fall compared to that of the action and reaction of the innumerable
plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees
now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings
remote in the scale of nature. This is likewise sometimes the case with those which may strictly be said to struggle
with each other for existence, as in the case of locusts and grass−feeding quadrupeds. But the struggle will almost
invariably be most severe between the individuals of the same species, for they frequent the same districts, require
the same food, and are exposed to the same dangers. In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest soon decided: for instance, if several
varieties of wheat be sown together, and the mixed seed be resown, some of the varieties which best suit the soil
or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a
few years supplant the other varieties. To keep up a mixed stock of even such extremely close varieties as the
variously coloured sweet−peas, they must be each year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease in number and disappear. So again with the varieties
of sheep: it has been asserted that certain mountain−varieties will starve out other mountain−varieties, so that they
cannot be kept together. The same result has followed from keeping together different varieties of the medicinal
leech. It may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the
same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being prevented)
could be kept up for half−a−dozen generations, if they were allowed to struggle together, in the same manner as
beings in a state of nature, and if the seed or young were not annually preserved in due proportion.
STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME
SPECIES.
As the species of the same genus usually have, though by no means invariably, much similarity in habits and
constitution, and always in structure, the struggle will generally be more severe between them, if they come into
competition with each other, than between the species of distinct genera. We see this in the recent extension over
parts of the United States of one species of swallow having caused the decrease of another species. The recent
increase of the missel−thrush in parts of Scotland has caused the decrease of the song−thrush. How frequently we
hear of one species of rat taking the place of another species under the most different climates! In Russia the small
Asiatic cockroach has everywhere driven before it its great congener. In Australia the imported hive−bee is
rapidly exterminating the small, stingless native bee. One species of charlock has been known to supplant another
species; and so in other cases. We can dimly see why the competition should be most severe between allied forms,
which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say
CHAPTER III. STRUGGLE FOR EXISTENCE. 37

The Origin of Species by means of Natural Selection
why one species has been victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure
of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings,
with which it comes into competition for food or residence, or from which it has to escape, or on which it preys.
This is obvious in the structure of the teeth and talons of the tiger; and in that of the legs and claws of the parasite
which clings to the hair on the tiger's body. But in the beautifully plumed seed of the dandelion, and in the
flattened and fringed legs of the water−beetle, the relation seems at first confined to the elements of air and water.
Yet the advantage of the plumed seeds no doubt stands in the closest relation to the land being already thickly
clothed with other plants; so that the seeds may be widely distributed and fall on unoccupied ground. In the
water−beetle, the structure of its legs, so well adapted for diving, allows it to compete with other aquatic insects,
to hunt for its own prey, and to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to
other plants. But from the strong growth of young plants produced from such seeds, as peas and beans, when
sown in the midst of long grass, it may be suspected that the chief use of the nutriment in the seed is to favour the
growth of the seedlings, whilst struggling with other plants growing vigorously all around.
Look at a plant in the midst of its range! Why does it not double or quadruple its numbers? We know that it
can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly
hotter or colder, damper or drier districts. In this case we can clearly see that if we wish in imagination to give the
plant the power of increasing in numbers, we should have to give it some advantage over its competitors, or over
the animals which prey on it. On the confines of its geographical range, a change of constitution with respect to
climate would clearly be an advantage to our plant; but we have reason to believe that only a few plants or
animals range so far, that they are destroyed exclusively by the rigour of the climate. Not until we reach the
extreme confines of life, in the Arctic regions or on the borders of an utter desert, will competition cease. The land
may be extremely cold or dry, yet there will be competition between some few species, or between the individuals
of the same species, for the warmest or dampest spots.
Hence we can see that when a plant or animal is placed in a new country, among new competitors, the
conditions of its life will generally be changed in an essential manner, although the climate may be exactly the
same as in its former home. If its average numbers are to increase in its new home, we should have to modify it in
a different way to what we should have had to do in its native country; for we should have to give it some
advantage over a different set of competitors or enemies.
It is good thus to try in imagination to give any one species an advantage over another. Probably in no single
instance should we know what to do. This ought to convince us of our ignorance on the mutual relations of all
organic beings; a conviction as necessary, as it is difficult to acquire. All that we can do is to keep steadily in
mind that each organic being is striving to increase in a geometrical ratio; that each, at some period of its life,
during some season of the year, during each generation, or at intervals, has to struggle for life and to suffer great
destruction. When we reflect on this struggle we may console ourselves with the full belief that the war of nature
is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the
happy survive and multiply.
CHAPTER III. STRUGGLE FOR EXISTENCE. 38

The Origin of Species by means of Natural Selection
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection −− its power compared with man's selection −− its power on characters of trifling
importance −− its power at all ages and on both sexes −− Sexual Selection −− On the generality of intercrosses
between individuals of the same species −− Circumstances favourable and unfavourable to the results of Natural
Selection, namely, intercrossing, isolation, number of individuals −− Slow action −− Extinction caused by Natural
Selection −− Divergence of Character, related to the diversity of inhabitants of any small area and to
naturalisation −− Action of Natural Selection, through Divergence of Character and Extinction, on the
descendants from a common parent −− Explains the Grouping of all organic beings −− Advance in organisation
−− Low forms preserved −− Convergence of character −− Indefinite multiplication of species −− Summary.
How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? Can the
principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall
see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring
in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the
strength of the hereditary tendency. Under domestication, it may truly be said that the whole organisation
becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic
productions is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither
originate varieties nor prevent their occurrence; he can only preserve and accumulate such as do occur.
Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but
similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely
complex and close−fitting are the mutual relations of all organic beings to each other and to their physical
conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being
under changing conditions of life. Can it then be thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of
life, should occur in the course of many successive generations? If such do occur, can we doubt (remembering
that many more individuals are born than can possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we
may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of
favourable individual differences and variations, and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by
natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species,
or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.
Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that
natural selection induces variability, whereas it implies only the preservation of such variations as arise and are
beneficial to the being under its conditions of life. No one objects to agriculturists speaking of the potent effects
of man's selection; and in this case the individual differences given by nature, which man for some object selects,
must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals
which become modified; and it has even been urged that, as plants have no volition, natural selection is not
applicable to them! In the literal sense of the word, no doubt, natural selection is a false term; but who ever
objected to chemists speaking of the elective affinities of the various elements?−−and yet an acid cannot strictly
be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as
an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the
movements of the planets? Every one knows what is meant and is implied by such metaphorical expressions; and
they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by
nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as
ascertained by us. With a little familiarity such superficial objections will be forgotten.
We shall best understand the probable course of natural selection by taking the case of a country undergoing
some slight physical change, for instance, of climate. The proportional numbers of its inhabitants will almost
immediately undergo a change, and some species will probably become extinct. We may conclude, from what we
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. 39

The Origin of Species by means of Natural Selection
have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that
any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would
seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this
would likewise seriously disturb the relations of some of the former inhabitants. Let it be remembered how
powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island,
or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we
should then have places in the economy of nature which would assuredly be better filled up if some of the original
inhabitants were in some manner modified; for, had the area been open to immigration, these same places would
have been seized on by intruders. In such cases, slight modifications, which in any way favoured the individuals
of any species, by better adapting them to their altered conditions, would tend to be preserved; and natural
selection would have free scope for the work of improvement.
We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a
tendency to increased variability; and in the foregoing cases the conditions the changed, and this would manifestly
be favourable to natural selection, by affording a better chance of the occurrence of profitable variations. Unless
such occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere
individual differences are included. As man can produce a great result with his domestic animals and plants by
adding up in any given direction individual differences, so could natural selection, but far more easily from
having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any
unusual degree of isolation, to check immigration, is necessary in order that new and unoccupied places should be
left for natural selection to fill up by improving some of the varying inhabitants. For as all the inhabitants of each
country are struggling together with nicely balanced forces, extremely slight modifications in the structure or
habits of one species would often give it an advantage over others; and still further modifications of the same kind
would often still further increase the advantage, as long as the species continued under the same conditions of life
and profited by similar means of subsistence and defence. No country can be named in which all the native
inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that
none of them could be still better adapted or improved; for in all countries, the natives have been so far conquered
by naturalised productions that they have allowed some foreigners to take firm possession of the land. And as
foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might
have been modified with advantage, so as to have better resisted the intruders.
As man can produce, and certainly has produced, a great result by his methodical and unconscious means of
selection, what may not natural selection effect? Man can act only on external and visible characters: Nature, if I
may be allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances,
except in so far as they are useful to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that
of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their
selection. Man keeps the natives of many climates in the same country. He seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a short−beaked pigeon on the same food; he
does not exercise a long−backed or long−legged quadruped in any peculiar manner; he exposes sheep with long
and short wool to the same climate; does not allow the most vigorous males to struggle for the females; he does
not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half−monstrous form, or at least by some modification
prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest differences of
structure or constitution may well turn the nicely−balanced scale in the struggle for life, and so be preserved. How
fleeting are the wishes and efforts of man! How short his time, and consequently how poor will be his results,
compared with those accumulated by Nature during whole geological periods! Can we wonder, then, that Nature's
productions should be far "truer" in character than man's productions; that they should be infinitely better adapted
to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, the
slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly
working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at the improvement of each organic
being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress,
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The Origin of Species by means of Natural Selection
until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long−past
geological ages that we see only that the forms of life are now different from what they formerly were.
In order that any great amount of modification should be effected in a species, a variety, when once formed
must again, perhaps after a long interval of time, vary or present individual differences of the same favourable
nature as before; and these must again be preserved, and so onward, step by step. Seeing that individual
differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption. But
whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly
limited quantity, is likewise a simple assumption.
Although natural selection can act only through and for the good of each being, yet characters and structures,
which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf−eating
insects green, and bark−feeders mottled−grey; the alpine ptarmigan white in winter, the red−grouse the colour of
heather, we must believe that these tints are of service to these birds and insects in preserving them from danger.
Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by eyesight to their prey,−−so much so that on parts of the
continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural
selection might be effective in giving the proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any
particular colour would produce little effect; we should remember how essential it is in a flock of white sheep to
destroy a lamb with the faintest trace of black. We have seen how the colour of hogs, which feed on the
"paint−root" in Virginia, determines whether they shall live or die. In plants, the down on the fruit and the colour
of the flesh are considered by botanists as characters of the most trifling importance; yet we hear from an
excellent horticulturist, Downing, that in the United States smooth−skinned fruits suffer far more from a beetle, a
Curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums;
whereas another disease attacks yellow−fleshed peaches far more than those with other coloured flesh. If, with all
the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a
state of nature, where the trees would have to struggle with other trees and with a host of enemies, such
differences would effectually settle which variety, whether a smooth or downy, a yellow or a purple−fleshed fruit,
should succeed.
In looking at many small points of difference between species, which, as far as our ignorance permits us to
judge, seem quite unimportant, we must not forget that climate, food, etc., have no doubt produced some direct
effect. It is also necessary to bear in mind that, owing to the law of correlation, when one part varies and the
variations are accumulated through natural selection, other modifications, often of the most unexpected nature,
will ensue.
As we see that those variations which, under domestication, appear at any particular period of life, tend to
reappear in the offspring at the same period; for instance, in the shape, size and flavour of the seeds of the many
varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the
silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and
cattle when nearly adult; so in a state of nature natural selection will be enabled to act on and modify organic
beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a
corresponding age. If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see
no greater difficulty in this being effected through natural selection, than in the cotton−planter increasing and
improving by selection the down in the pods on his cotton−trees. Natural selection may modify and adapt the
larva of an insect to a score of contingencies, wholly different from those which concern the mature insect; and
these modifications may affect, through correlation, the structure of the adult. So, conversely, modifications in the
adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be
injurious: for if they were so, the species would become extinct.
Natural selection will modify the structure of the young in relation to the parent and of the parent in relation to
the young. In social animals it will adapt the structure of each individual for the benefit of the whole community;
if the community profits by the selected change. What natural selection cannot do, is to modify the structure of
one species, without giving it any advantage, for the good of another species; and though statements to this effect
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The Origin of Species by means of Natural Selection
may be found in works of natural history, I cannot find one case which will bear investigation. A structure used
only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection; for
instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon−−or the hard tip to
the beak of unhatched birds, used for breaking the eggs. It has been asserted, that of the best short−beaked
tumbler−pigeons a greater number perish in the egg than are able to get out of it; so that fanciers assist in the act
of hatching. Now, if nature had to make the beak of a full−grown pigeon very short for the bird's own advantage,
the process of modification would be very slow, and there would be simultaneously the most rigorous selection of
all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell
being known to vary like every other structure.
It may be well here to remark that with all beings there must be much fortuitous destruction, which can have
little or no influence on the course of natural selection. For instance, a vast number of eggs or seeds are annually
devoured, and these could be modified through natural selection only if they varied in some manner which
protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded
individuals better adapted to their conditions of life than any of those which happened to survive. So again a vast
number of mature animals and plants, whether or not they be the best adapted to their conditions, must be
annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of
structure or constitution which would in other ways be beneficial to the species. But let the destruction of the
adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes−−or
again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are
developed−−yet of those which do survive, the best adapted individuals, supposing that there is any variability in
a favourable direction, will tend to propagate their kind in larger numbers than the less well adapted. If the
numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will
be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in
other ways; for we are far from having any reason to suppose that many species ever undergo modification and
improvement at the same time in the same area.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that
sex, so no doubt it will be under nature. Thus it is rendered possible for the two sexes to be modified through
natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say a few words on what I have called sexual
selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to
external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession
of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection
is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted
for their places in nature, will leave most progeny. But in many cases victory depends not so much on general
vigour, but on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a
poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely
give indomitable courage, length of spur, and strength to the wing to strike in the spurred leg, in nearly the same
manner as does the brutal cockfighter by the careful selection of his best cocks. How low in the scale of nature the
law of battle descends I know not; male alligators have been described as fighting, bellowing, and whirling round,
like Indians in a war−dance, for the possession of the females; male salmons have been observed fighting all day
long; male stag−beetles sometimes bear wounds from the huge mandibles of other males; the males of certain
hymenopterous insects have been frequently seen by that inimitable observer M. Fabre, fighting for a particular
female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror. The
war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though to them and to others, special means
of defence may be given through means of sexual selection, as the mane of the lion, and the hooked jaw to the
male salmon; for the shield may be as important for victory as the sword or spear.
Among birds, the contest is often of a more peaceful character. All those who have attended to the subject,
believe that there is the severest rivalry between the males of many species to attract, by singing, the females. The
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The Origin of Species by means of Natural Selection
rock−thrush of Guiana, birds of paradise, and some others, congregate, and successive males display with the
most elaborate care, and show off in the best manner, their gorgeous plumage; they likewise perform strange
antics before the females, which, standing by as spectators, at last choose the most attractive partner. Those who
have closely attended to birds in confinement well know that they often take individual preferences and dislikes:
thus Sir R. Heron has described how a pied peacock was eminently attractive to all his hen birds. I cannot here
enter on the necessary details; but if man can in a short time give beauty and an elegant carriage to his bantams,
according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might
produce a marked effect. Some well−known laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained through the action of sexual selection on
variations occurring at different ages, and transmitted to the males alone or to both sexes at corresponding ages;
but I have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but
differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection: that is, by
individual males having had, in successive generations, some slight advantage over other males, in their weapons,
means of defence, or charms; which they have transmitted to their male offspring alone. Yet, I would not wish to
attribute all sexual differences to this agency: for we see in our domestic animals peculiarities arising and
becoming attached to the male sex, which apparently have not been augmented through selection by man. The tuft
of hair on the breast of the wild turkey−cock cannot be of any use, and it is doubtful whether it can be ornamental
in the eyes of the female bird; indeed, had the tuft appeared under domestication it would have been called a
monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.
In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two
imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from
any change in the country increased in numbers, or that other prey had decreased in numbers, during that season
of the year when the wolf was hardest pressed for food. Under such circumstances the swiftest and slimmest
wolves have the best chance of surviving, and so be preserved or selected, provided always that they retained
strength to master their prey at this or some other period of the year, when they were compelled to prey on other
animals. I can see no more reason to doubt that this would be the result, than that man should be able to improve
the fleetness of his greyhounds by careful and methodical selection, or by that kind of unconscious selection
which follows from each man trying to keep the best dogs without any thought of modifying the breed. I may add
that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains, in the United
States, one with a light greyhound−like form, which pursues deer, and the other more bulky, with shorter legs,
which more frequently attacks the shepherd's flocks.
Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might
be born with an innate tendency to pursue certain kinds of prey. Nor can this be thought very improbable; for we
often observe great differences in the natural tendencies of our domestic animals; one cat, for instance, taking to
catch rats, another mice; one cat, according to Mr. St. John, bringing home winged game, another hares or rabbits,
and another hunting on marshy ground and almost nightly catching woodcocks or snipes. The tendency to catch
rats rather than mice is known to be inherited. Now, if any slight innate change of habit or of structure benefited
an individual wolf, it would have the best chance of surviving and of leaving offspring. Some of its young would
probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed
which would either supplant or coexist with the parent−form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and
from the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be
formed. These varieties would cross and blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill
Mountains in the United States, one with a light greyhound−like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
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The Origin of Species by means of Natural Selection
It should be observed that in the above illustration, I speak of the slimmest individual wolves, and not of any
single strongly marked variation having been preserved. In former editions of this work I sometimes spoke as if
this latter alternative had frequently occurred. I saw the great importance of individual differences, and this led me
fully to discuss the results of unconscious selection by man, which depends on the preservation of all the more or
less valuable individuals, and on the destruction of the worst. I saw, also, that the preservation in a state of nature
of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if at first
preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until
reading an able and valuable article in the "North British Review" (1867), I did not appreciate how rarely single
variations, whether slight or strongly marked, could be perpetuated. The author takes the case of a pair of animals,
producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on
an average survive to pro−create their kind. This is rather an extreme estimate for most of the higher animals, but
by no means so for many of the lower organisms. He then shows that if a single individual were born, which
varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances
would be strongly against its survival. Supposing it to survive and to breed, and that half its young inherited the
favourable variation; still, as the Reviewer goes onto show, the young would have only a slightly better chance of
surviving and breeding; and this chance would go on decreasing in the succeeding generations. The justice of
these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more easily
by having its beak curved, and if one were born with its beak strongly curved, and which consequently flourished,
nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the
common form; but there can hardly be a doubt, judging by what we see taking place under domestication, that this
result would follow from the preservation during many generations of a large number of individuals with more or
less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks.
It should not, however, be overlooked that certain rather strongly marked variations, which no one would rank
as mere individual differences, frequently recur owing to a similar organisation being similarly acted on−− of
which fact numerous instances could be given with our domestic productions. In such cases, if the varying
individual did not actually transmit to its offspring its newly−acquired character, it would undoubtedly transmit to
them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner.
There can also be little doubt that the tendency to vary in the same manner has often been so strong that all the
individuals of the same species have been similarly modified without the aid of any form of selection. Or only a
third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be
given. Thus Graba estimates that about one−fifth of the guillemots in the Faroe Islands consist of a variety so well
marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans. In cases of this kind,
if the variation were of a beneficial nature, the original form would soon be supplanted by the modified form,
through the survival of the fittest.
To the effects of intercrossing in eliminating variations of all kinds, I shall have to recur; but it may be here
remarked that most animals and plants keep to their proper homes, and do not needlessly wander about; we see
this even with migratory birds, which almost always return to the same spot. Consequently each newly−formed
variety would generally be at first local, as seems to be the common rule with varieties in a state of nature; so that
similarly modified individuals would soon exist in a small body together, and would often breed together. If the
new variety were successful in its battle for life, it would slowly spread from a central district, competing with
and conquering the unchanged individuals on the margins of an ever−increasing circle.
It may be worth while to give another and more complex illustration of the action of natural selection. Certain
plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap: this is
effected, for instance, by glands at the base of the stipules in some Leguminosae, and at the backs of the leaves of
the common laurel. This juice, though small in quantity, is greedily sought by insects; but their visits do not in any
way benefit the plant. Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a
certain number of plants of any species. Insects in seeking the nectar would get dusted with pollen, and would
often transport it from one flower to another. The flowers of two distinct individuals of the same species would
thus get crossed; and the act of crossing, as can be fully proved, gives rise to vigorous seedlings, which
consequently would have the best chance of flourishing and surviving. The plants which produced flowers with
the largest glands or nectaries, excreting most nectar, would oftenest be visited by insects, and would oftenest be
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The Origin of Species by means of Natural Selection
crossed; and so in the long−run would gain the upper hand and form a local variety. The flowers, also, which had
their stamens and pistils placed, in relation to the size and habits of the particular insect which visited them, so as
to favour in any degree the transportal of the pollen, would likewise be favoured. We might have taken the case of
insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formed for the sole
purpose of fertilisation, its destruction appears to be a simple loss to the plant; yet if a little pollen were carried, at
first occasionally and then habitually, by the pollen−devouring insects from flower to flower, and a cross thus
effected, although nine−tenths of the pollen were destroyed it might still be a great gain to the plant to be thus
robbed; and the individuals which produced more and more pollen, and had larger anthers, would be selected.
When our plant, by the above process long continued, had been rendered highly attractive to insects, they
would, unintentionally on their part, regularly carry pollen from flower to flower; and that they do this effectually
I could easily show by many striking facts. I will give only one, as likewise illustrating one step in the separation
of the sexes of plants. Some holly−trees bear only male flowers, which have four stamens producing a rather
small quantity of pollen, and a rudimentary pistil; other holly−trees bear only female flowers; these have a
full−sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there were a few pollen−grains, and on some a
profusion. As the wind had set for several days from the female to the male tree, the pollen could not thus have
been carried. The weather had been cold and boisterous and therefore not favourable to bees, nevertheless every
female flower which I examined had been effectually fertilised by the bees, which had flown from tree to tree in
search of nectar. But to return to our imaginary case; as soon as the plant had been rendered so highly attractive to
insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist
doubts the advantage of what has been called the "physiological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone
in another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes
the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur
in ever so slight a degree under nature, then, as pollen is already carried regularly from flower to flower, and as a
more complete separation of the sexes of our plant would be advantageous on the principle of the division of
labour, individuals with this tendency more and more increased, would be continually favoured or selected, until
at last a complete separation of the sexes might be effected. It would take up too much space to show the various
steps, through dimorphism and other means, by which the separation of the sexes in plants of various kinds is
apparently now in progress; but I may add that some of the species of holly in North America are, according to
Asa Gray, in an exactly intermediate condition, or, as he expresses it, are more or less dioeciously polygamous.
Let us now turn to the nectar−feeding insects; we may suppose the plant of which we have been slowly
increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part
on its nectar for food. I could give many facts showing how anxious bees are to save time: for instance, their habit
of cutting holes and sucking the nectar at the bases of certain flowers, which with a very little more trouble they
can enter by the mouth. Bearing such facts in mind, it may be believed that under certain circumstances individual
differences in the curvature or length of the proboscis, etc., too slight to be appreciated by us, might profit a bee or
other insect, so that certain individuals would be able to obtain their food more quickly than others; and thus the
communities to which they belonged would flourish and throw off many swarms inheriting the same peculiarities.
The tubes of the corolla of the common red or incarnate clovers (Trifolium pratense and incarnatum) do not on a
hasty glance appear to differ in length; yet the hive−bee can easily suck the nectar out of the incarnate clover, but
not out of the common red clover, which is visited by humble−bees alone; so that whole fields of the red clover
offer in vain an abundant supply of precious nectar to the hive−bee. That this nectar is much liked by the hive−bee
is certain; for I have repeatedly seen, but only in the autumn, many hive−bees sucking the flowers through holes
bitten in the base of the tube by humble bees. The difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive−bee, must be very trifling; for I have been assured that when red clover
has been mown, the flowers of the second crop are somewhat smaller, and that these are visited by many
hive−bees. I do not know whether this statement is accurate; nor whether another published statement can be
trusted, namely, that the Ligurian bee, which is generally considered a mere variety of the common hive−bee, and
which freely crosses with it, is able to reach and suck the nectar of the red clover. Thus, in a country where this
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. 45

The Origin of Species by means of Natural Selection
kind of clover abounded, it might be a great advantage to the hive−bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this clover absolutely depends on bees visiting the
flowers, if humble−bees were to become rare in any country, it might be a great advantage to the plant to have a
shorter or more deeply divided corolla, so that the hive−bees should be enabled to suck its flowers. Thus I can
understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified
and adapted to each other in the most perfect manner, by the continued preservation of all the individuals which
presented slight deviations of structure mutually favourable to each other.
I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open
to the same objections which were first urged against Sir Charles Lyell's noble views on "the modern changes of
the earth, as illustrative of geology;" but we now seldom hear the agencies which we see still at work, spoken of
as trifling and insignificant, when used in explaining the excavation of the deepest valleys or the formation of
long lines of inland cliffs. Natural selection acts only by the preservation and accumulation of small inherited
modifications, each profitable to the preserved being; and as modern geology has almost banished such views as
the excavation of a great valley by a single diluvial wave, so will natural selection banish the belief of the
continued creation of new organic beings, or of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course
obvious that two individuals must always (with the exception of the curious and not well understood cases of
parthenogenesis) unite for each birth; but in the case of hermaphrodites this is far from obvious. Nevertheless
there is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur
for the reproduction of their kind. This view was long ago doubtfully suggested by Sprengel, Knight and
Kolreuter. We shall presently see its importance; but I must here treat the subject with extreme brevity, though I
have the materials prepared for an ample discussion. All vertebrate animals, all insects and some other large
groups of animals, pair for each birth. Modern research has much diminished the number of supposed
hermaphrodites and of real hermaphrodites a large number pair; that is, two individuals regularly unite for
reproduction, which is all that concerns us. But still there are many hermaphrodite animals which certainly do not
habitually pair, and a vast majority of plants are hermaphrodites. What reason, it may be asked, is there for
supposing in these cases that two individuals ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts, and made so many experiments, showing, in
accordance with the almost universal belief of breeders, that with animals and plants a cross between different
varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the
offspring; and on the other hand, that CLOSE interbreeding diminishes vigour and fertility; that these facts alone
incline me to believe that it is a general law of nature that no organic being fertilises itself for a perpetuity of
generations; but that a cross with another individual is occasionally−−perhaps at long intervals of
time−−indispensable.
On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the
following, which on any other view are inexplicable. Every hybridizer knows how unfavourable exposure to wet
is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to
the weather! If an occasional cross be indispensable, notwithstanding that the plant's own anthers and pistil stand
so near each other as almost to ensure self− fertilisation, the fullest freedom for the entrance of pollen from
another individual will explain the above state of exposure of the organs. Many flowers, on the other hand, have
their organs of fructification closely enclosed, as in the great papilionaceous or pea−family; but these almost
invariably present beautiful and curious adaptations in relation to the visits of insects. So necessary are the visits
of bees to many papilionaceous flowers, that their fertility is greatly diminished if these visits be prevented. Now,
it is scarcely possible for insects to fly from flower to flower, and not to carry pollen from one to the other, to the
great good of the plant. Insects act like a camel−hair pencil, and it is sufficient, to ensure fertilisation, just to touch
with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that
bees would thus produce a multitude of hybrids between distinct species; for if a plant's own pollen and that from
another species are placed on the same stigma, the former is so prepotent that it invariably and completely
destroys, as has been shown by Gartner, the influence of the foreign pollen.
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The Origin of Species by means of Natural Selection
When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards
it, the contrivance seems adapted solely to ensure self−fertilisation; and no doubt it is useful for this end: but the
agency of insects is often required to cause the stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and in this very genus, which seems to have a special contrivance for self−fertilisation, it is
well known that, if closely−allied forms or varieties are planted near each other, it is hardly possible to raise pure
seedlings, so largely do they naturally cross. In numerous other cases, far from self−fertilisation being favoured,
there are special contrivances which effectually prevent the stigma receiving pollen from its own flower, as I
could show from the works of Sprengel and others, as well as from my own observations: for instance, in Lobelia
fulgens, there is a really beautiful and elaborate contrivance by which all the infinitely numerous pollen−granules
are swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to
receive them; and as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by
placing pollen from one flower on the stigma of another, I raise plenty of seedlings. Another species of Lobelia,
which is visited by bees, seeds freely in my garden. In very many other cases, though there is no special
mechanical contrivance to prevent the stigma receiving pollen from the same flower, yet, as Sprengel, and more
recently Hildebrand and others have shown, and as I can confirm, either the anthers burst before the stigma is
ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these so−named
dichogamous plants have in fact separated sexes, and must habitually be crossed. So it is with the reciprocally
dimorphic and trimorphic plants previously alluded to. How strange are these facts! How strange that the pollen
and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of
self−fertilisation, should be in so many cases mutually useless to each other! How simply are these facts
explained on the view of an occasional cross with a distinct individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other,
a large majority of the seedlings thus raised turn out, as I found, mongrels: for instance, I raised 233 seedling
cabbages from some plants of different varieties growing near each other, and of these only 78 were true to their
kind, and some even of these were not perfectly true. Yet the pistil of each cabbage−flower is surrounded not only
by its own six stamens but by those of the many other flowers on the same plant; and the pollen of each flower
readily gets on its stigma without insect agency; for I have found that plants carefully protected from insects
produce the full number of pods. How, then, comes it that such a vast number of the seedlings are mongrelized? It
must arise from the pollen of a distinct VARIETY having a prepotent effect over the flower's own pollen; and that
this is part of the general law of good being derived from the intercrossing of distinct individuals of the same
species. When distinct SPECIES are crossed the case is reversed, for a plant's own pollen is always prepotent over
foreign pollen; but to this subject we shall return in a future chapter.
In the case of a large tree covered with innumerable flowers, it may be objected that pollen could seldom be
carried from tree to tree, and at most only from flower to flower on the same tree; and flowers on the same tree
can be considered as distinct individuals only in a limited sense. I believe this objection to be valid, but that nature
has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes. When the
sexes are separated, although the male and female flowers may be produced on the same tree, pollen must be
regularly carried from flower to flower; and this will give a better chance of pollen being occasionally carried
from tree to tree. That trees belonging to all orders have their sexes more often separated than other plants, I find
to be the case in this country; and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray
those of the United States, and the result was as I anticipated. On the other hand, Dr. Hooker informs me that the
rule does not hold good in Australia: but if most of the Australian trees are dichogamous, the same result would
follow as if they bore flowers with separated sexes. I have made these few remarks on trees simply to call
attention to the subject.
Turning for a brief space to animals: various terrestrial species are hermaphrodites, such as the land−mollusca
and earth−worms; but these all pair. As yet I have not found a single terrestrial animal which can fertilise itself.
This remarkable fact, which offers so strong a contrast with terrestrial plants, is intelligible on the view of an
occasional cross being indispensable; for owing to the nature of the fertilising element there are no means,
analogous to the action of insects and of the wind with plants, by which an occasional cross could be effected with
terrestrial animals without the concurrence of two individuals. Of aquatic animals, there are many self−fertilising
hermaphrodites; but here the currents of water offer an obvious means for an occasional cross. As in the case of
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The Origin of Species by means of Natural Selection
flowers, I have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to
discover a single hermaphrodite animal with the organs of reproduction so perfectly enclosed that access from
without, and the occasional influence of a distinct individual, can be shown to be physically impossible.
Cirripedes long appeared to me to present, under this point of view, a case of great difficulty; but I have been
enabled, by a fortunate chance, to prove that two individuals, though both are self−fertilising hermaphrodites, do
sometimes cross.
It must have struck most naturalists as a strange anomaly that, both with animals and plants, some species of
the same family and even of the same genus, though agreeing closely with each other in their whole organisation,
are hermaphrodites, and some unisexual. But if, in fact, all hermaphrodites do occasionally intercross, the
difference between them and unisexual species is, as far as function is concerned, very small.
>From these several considerations and from the many special facts which I have collected, but which I am
unable here to give, it appears that with animals and plants an occasional intercross between distinct individuals is
a very general, if not universal, law of nature.
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.
This is an extremely intricate subject. A great amount of variability, under which term individual differences
are always included, will evidently be favourable. A large number of individuals, by giving a better chance within
any given period for the appearance of profitable variations, will compensate for a lesser amount of variability in
each individual, and is, I believe, a highly important element of success. Though nature grants long periods of
time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving
to seize on each place in the economy of nature, if any one species does not become modified and improved in a
corresponding degree with its competitors it will be exterminated. Unless favourable variations be inherited by
some at least of the offspring, nothing can be effected by natural selection. The tendency to reversion may often
check or prevent the work; but as this tendency has not prevented man from forming by selection numerous
domestic races, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite object, and if the individuals be
allowed freely to intercross, his work will completely fail. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals,
improvement surely but slowly follows from this unconscious process of selection, notwithstanding that there is
no separation of selected individuals. Thus it will be under nature; for within a confined area, with some place in
the natural polity not perfectly occupied, all the individuals varying in the right direction, though in different
degrees, will tend to be preserved. But if the area be large, its several districts will almost certainly present
different conditions of life; and then, if the same species undergoes modification in different districts, the newly
formed varieties will intercross on the confines of each. But we shall see in the sixth chapter that intermediate
varieties, inhabiting intermediate districts, will in the long run generally be supplanted by one of the adjoining
varieties. Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which
do not breed at a very quick rate. Hence with animals of this nature, for instance birds, varieties will generally be
confined to separated countries; and this I find to be the case. With hermaphrodite organisms which cross only
occasionally, and likewise with animals which unite for each birth, but which wander little and can increase at a
rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself
in a body and afterward spread, so that the individuals of the new variety would chiefly cross together. On this
principle nurserymen always prefer saving seed from a large body of plants, as the chance of intercrossing is thus
lessened.
Even with animals which unite for each birth, and which do not propagate rapidly, we must not assume that
free intercrossing would always eliminate the effects of natural selection; for I can bring forward a considerable
body of facts showing that within the same area two varieties of the same animal may long remain distinct, from
haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety
preferring to pair together.
Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the
same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals
which unite for each birth; but, as already stated, we have reason to believe that occasional intercrosses take place
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The Origin of Species by means of Natural Selection
with all animals and plants. Even if these take place only at long intervals of time, the young thus produced will
gain so much in vigour and fertility over the offspring from long−continued self−fertilisation, that they will have a
better chance of surviving and propagating their kind; and thus in the long run the influence of crosses, even at
rare intervals, will be great. With respect to organic beings extremely low in the scale, which do not propagate
sexually, nor conjugate, and which cannot possibly intercross, uniformity of character can be retained by them
under the same conditions of life, only through the principle of inheritance, and through natural selection which
will destroy any individuals departing from the proper type. If the conditions of life change and the form
undergoes modification, uniformity of character can be given to the modified offspring, solely by natural selection
preserving similar favourable variations.
Isolation also is an important element in the modification of species through natural selection. In a confined or
isolated area, if not very large, the organic and inorganic conditions of life will generally be almost uniform; so
that natural selection will tend to modify all the varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts, will also be thus prevented. Moritz Wagner has
lately published an interesting essay on this subject, and has shown that the service rendered by isolation in
preventing crosses between newly−formed varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that migration and isolation are necessary elements
for the formation of new species. The importance of isolation is likewise great in preventing, after any physical
change in the conditions, such as of climate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district will be left open to be filled up by the
modification of the old inhabitants. Lastly, isolation will give time for a new variety to be improved at a slow rate;
and this may sometimes be of much importance. If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical conditions, the total number of the inhabitants will
be small; and this will retard the production of new species through natural selection, by decreasing the chances of
favourable variations arising.
The mere lapse of time by itself does nothing, either for or against natural selection. I state this because it has
been erroneously asserted that the element of time has been assumed by me to play an all−important part in
modifying species, as if all the forms of life were necessarily undergoing change through some innate law. Lapse
of time is only so far important, and its importance in this respect is great, that it gives a better chance of
beneficial variations arising and of their being selected, accumulated, and fixed. It likewise tends to increase the
direct action of the physical conditions of life, in relation to the constitution of each organism.
If we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an oceanic
island, although the number of the species inhabiting it is small, as we shall see in our chapter on Geographical
Distribution; yet of these species a very large proportion are endemic,−−that is, have been produced there and
nowhere else in the world. Hence an oceanic island at first sight seems to have been highly favourable for the
production of new species. But we may thus deceive ourselves, for to ascertain whether a small isolated area, or a
large open area like a continent, has been most favourable for the production of new organic forms, we ought to
make the comparison within equal times; and this we are incapable of doing.
Although isolation is of great importance in the production of new species, on the whole I am inclined to
believe that largeness of area is still more important, especially for the production of species which shall prove
capable of enduring for a long period, and of spreading widely. Throughout a great and open area, not only will
there be a better chance of favourable variations, arising from the large number of individuals of the same species
there supported, but the conditions of life are much more complex from the large number of already existing
species; and if some of these many species become modified and improved, others will have to be improved in a
corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been much improved,
will be able to spread over the open and continuous area, and will thus come into competition with many other
forms. Moreover, great areas, though now continuous, will often, owing to former oscillations of level, have
existed in a broken condition, so that the good effects of isolation will generally, to a certain extent, have
concurred. Finally, I conclude that, although small isolated areas have been in some respects highly favourable for
the production of new species, yet that the course of modification will generally have been more rapid on large
areas; and what is more important, that the new forms produced on large areas, which already have been
victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest
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The Origin of Species by means of Natural Selection
number of new varieties and species. They will thus play a more important part in the changing history of the
organic world.
In accordance with this view, we can, perhaps, understand some facts which will be again alluded to in our
chapter on Geographical Distribution; for instance, the fact of the productions of the smaller continent of
Australia now yielding before those of the larger Europaeo−Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised on islands. On a small island, the race for life will
have been less severe, and there will have been less modification and less extermination. Hence, we can
understand how it is that the flora of Madeira, according to Oswald Heer, resembles to a certain extent the extinct
tertiary flora of Europe. All fresh water basins, taken together, make a small area compared with that of the sea or
of the land. Consequently, the competition between fresh water productions will have been less severe than
elsewhere; new forms will have been more slowly produced, and old forms more slowly exterminated. And it is in
fresh water basins that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh
water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and
Lepidosiren, which, like fossils, connect to a certain extent orders at present widely separated in the natural scale.
These anomalous forms may be called living fossils; they have endured to the present day, from having inhabited
a confined area, and from having been exposed to less varied, and therefore less severe, competition.
To sum up, as far as the extreme intricacy of the subject permits, the circumstances favourable and
unfavourable for the production of new species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many oscillations of level, will have been the most
favourable for the production of many new forms of life, fitted to endure for a long time and to spread widely.
While the area existed as a continent the inhabitants will have been numerous in individuals and kinds, and will
have been subjected to severe competition. When converted by subsidence into large separate islands there will
still have existed many individuals of the same species on each island: intercrossing on the confines of the range
of each new species will have been checked: after physical changes of any kind immigration will have been
prevented, so that new places in the polity of each island will have had to be filled up by the modification of the
old inhabitants; and time will have been allowed for the varieties in each to become well modified and perfected.
When, by renewed elevation, the islands were reconverted into a continental area, there will again have been very
severe competition; the most favoured or improved varieties will have been enabled to spread; there will have
been much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants
of the reunited continent will again have been changed; and again there will have been a fair field for natural
selection to improve still further the inhabitants, and thus to produce new species.
That natural selection generally act with extreme slowness I fully admit. It can act only when there are places
in the natural polity of a district which can be better occupied by the modification of some of its existing
inhabitants. The occurrence of such places will often depend on physical changes, which generally take place very
slowly, and on the immigration of better adapted forms being prevented. As some few of the old inhabitants
become modified the mutual relations of others will often be disturbed; and this will create new places, ready to
be filled up by better adapted forms; but all this will take place very slowly. Although all the individuals of the
same species differ in some slight degree from each other, it would often be long before differences of the right
nature in various parts of the organisation might occur. The result would often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply sufficient to neutralise the power of natural
selection. I do not believe so. But I do believe that natural selection will generally act very slowly, only at long
intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow,
intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the
world have changed.
Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no
limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one
with another and with their physical conditions of life, which may have been effected in the long course of time
through nature's power of selection, that is by the survival of the fittest.
EXTINCTION CAUSED BY NATURAL SELECTION.
This subject will be more fully discussed in our chapter on Geology; but it must here be alluded to from being
intimately connected with natural selection. Natural selection acts solely through the preservation of variations in
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The Origin of Species by means of Natural Selection
some way advantageous, which consequently endure. Owing to the high geometrical rate of increase of all
organic beings, each area is already fully stocked with inhabitants, and it follows from this, that as the favoured
forms increase in number, so, generally, will the less favoured decrease and become rare. Rarity, as geology tells
us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a
good chance of utter extinction, during great fluctuations in the nature or the seasons, or from a temporary
increase in the number of its enemies. But we may go further than this; for as new forms are produced, unless we
admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct. That
the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently
attempt to show why it is that the number of species throughout the world has not become immeasurably great.
We have seen that the species which are most numerous in individuals have the best chance of producing
favourable variations within any given period. We have evidence of this, in the facts stated in the second chapter,
showing that it is the common and diffused or dominant species which offer the greatest number of recorded
varieties. Hence, rare species will be less quickly modified or improved within any given period; they will
consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
>From these several considerations I think it inevitably follows, that as new species in the course of time are
formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in
closest competition with those undergoing modification and improvement, will naturally suffer most. And we
have seen in the chapter on the Struggle for Existence that it is the most closely−allied forms,−−varieties of the
same species, and species of the same genus or related genera,−−which, from having nearly the same structure,
constitution and habits, generally come into the severest competition with each other. Consequently, each new
variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and
tend to exterminate them. We see the same process of extermination among our domesticated productions,
through the selection of improved forms by man. Many curious instances could be given showing how quickly
new breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and inferior kinds.
In Yorkshire, it is historically known that the ancient black cattle were displaced by the long−horns, and that these
"were swept away by the short−horns" (I quote the words of an agricultural writer) "as if by some murderous
pestilence."
DIVERGENCE OF CHARACTER.
The principle, which I have designated by this term, is of high importance, and explains, as I believe, several
important facts. In the first place, varieties, even strongly−marked ones, though having somewhat of the character
of species−−as is shown by the hopeless doubts in many cases how to rank them−−yet certainly differ far less
from each other than do good and distinct species. Nevertheless according to my view, varieties are species in the
process of formation, or are, as I have called them, incipient species. How, then, does the lesser difference
between varieties become augmented into the greater difference between species? That this does habitually
happen, we must infer from most of the innumerable species throughout nature presenting well−marked
differences; whereas varieties, the supposed prototypes and parents of future well−marked species, present slight
and ill−defined differences. Mere chance, as we may call it, might cause one variety to differ in some character
from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a
greater degree; but this alone would never account for so habitual and large a degree of difference as that between
the species of the same genus.
As has always been my practice, I have sought light on this head from our domestic productions. We shall
here find something analogous. It will be admitted that the production of races so different as short−horn and
Hereford cattle, race and cart horses, the several breeds of pigeons, etc., could never have been effected by the
mere chance accumulation of similar variations during many successive generations. In practice, a fancier is, for
instance, struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather
longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard, but
like extremes," they both go on (as has actually occurred with the sub−breeds of the tumbler−pigeon) choosing
and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose
that at an early period of history, the men of one nation or district required swifter horses, while those of another
required stronger and bulkier horses. The early differences would be very slight; but, in the course of time, from
the continued selection of swifter horses in the one case, and of stronger ones in the other, the differences would
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The Origin of Species by means of Natural Selection
become greater, and would be noted as forming two sub−breeds. Ultimately after the lapse of centuries, these
sub−breeds would become converted into two well−established and distinct breeds. As the differences became
greater, the inferior animals with intermediate characters, being neither very swift nor very strong, would not have
been used for breeding, and will thus have tended to disappear. Here, then, we see in man's productions the action
of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to
increase, and the breeds to diverge in character, both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most
efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified
the descendants from any one species become in structure, constitution, and habits, by so much will they be better
enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in
numbers.
We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous
quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its
natural power of increase be allowed to act, it can succeed in increasing (the country not undergoing any change
in conditions) only by its varying descendants seizing on places at present occupied by other animals: some of
them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less carnivorous. The more diversified in habits
and structure the descendants of our carnivorous animals become, the more places they will be enabled to occupy.
What applies to one animal will apply throughout all time to all animals−−that is, if they vary−−for otherwise
natural selection can effect nothing. So it will be with plants. It has been experimentally proved, that if a plot of
ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a
greater number of plants and a greater weight of dry herbage can be raised in the latter than in the former case.
The same has been found to hold good when one variety and several mixed varieties of wheat have been sown on
equal spaces of ground. Hence, if any one species of grass were to go on varying, and the varieties were
continually selected which differed from each other in the same manner, though in a very slight degree, as do the
distinct species and genera of grasses, a greater number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground. And we know that each species and each
variety of grass is annually sowing almost countless seeds; and is thus striving, as it may be said, to the utmost to
increase in number. Consequently, in the course of many thousand generations, the most distinct varieties of any
one species of grass would have the best chance of succeeding and of increasing in numbers, and thus of
supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of
species.
The truth of the principle that the greatest amount of life can be supported by great diversification of structure,
is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration,
and where the contest between individual and individual must be very severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many
years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera
and to eight orders, which shows how much these plants differed from each other. So it is with the plants and
insects on small and uniform islets: also in small ponds of fresh water. Farmers find that they can raise more food
by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any small piece of ground, could live on it
(supposing its nature not to be in any way peculiar), and may be said to be striving to the utmost to live there; but,
it is seen, that where they come into the closest competition, the advantages of diversification of structure, with
the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each
other most closely, shall, as a general rule, belong to what we call different genera and orders.
The same principle is seen in the naturalisation of plants through man's agency in foreign lands. It might have
been expected that the plants which would succeed in becoming naturalised in any land would generally have
been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their
own country. It might also, perhaps, have been expected that naturalised plants would have belonged to a few
groups more especially adapted to certain stations in their new homes. But the case is very different; and Alph. de
Candolle has well remarked, in his great and admirable work, that floras gain by naturalisation, proportionally
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The Origin of Species by means of Natural Selection
with the number of the native genera and species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of the Northern United States," 260 naturalised
plants are enumerated, and these belong to 162 genera. We thus see that these naturalised plants are of a highly
diversified nature. They differ, moreover, to a large extent, from the indigenes, for out of the 162 naturalised
genera, no less than 100 genera are not there indigenous, and thus a large proportional addition is made to the
genera now living in the United States.
By considering the nature of the plants or animals which have in any country struggled successfully with the
indigenes, and have there become naturalised, we may gain some crude idea in what manner some of the natives
would have had to be modified in order to gain an advantage over their compatriots; and we may at least infer that
diversification of structure, amounting to new generic differences, would be profitable to them.
The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that
of the physiological division of labour in the organs of the same individual body−−a subject so well elucidated by
Milne Edwards. No physiologist doubts that a stomach by being adapted to digest vegetable matter alone, or flesh
alone, draws most nutriment from these substances. So in the general economy of any land, the more widely and
perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals
be capable of there supporting themselves. A set of animals, with their organisation but little diversified, could
hardly compete with a set more perfectly diversified in structure. It may be doubted, for instance, whether the
Australian marsupials, which are divided into groups differing but little from each other, and feebly representing,
as Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and rodent mammals, could successfully
compete with these well−developed orders. In the Australian mammals, we see the process of diversification in an
early and incomplete stage of development.
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCE
OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR.
After the foregoing discussion, which has been much compressed, we may assume that the modified
descendants of any one species will succeed so much the better as they become more diversified in structure, and
are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of benefit
being derived from divergence of character, combined with the principles of natural selection and of extinction,
tends to act.
The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent
the species of a genus large in its own country; these species are supposed to resemble each other in unequal
degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at
unequal distances. I have said a large genus, because as we saw in the second chapter, on an average more species
vary in large genera than in small genera; and the varying species of the large genera present a greater number of
varieties. We have, also, seen that the species, which are the commonest and most widely−diffused, vary more
than do the rare and restricted species. Let (A) be a common, widely−diffused, and varying species, belonging to
a genus large in its own country. The branching and diverging dotted lines of unequal lengths proceeding from
(A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor
are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be
preserved or naturally selected. And here the importance of the principle of benefit derived from divergence of
character comes in; for this will generally lead to the most different or divergent variations (represented by the
outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the
horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to
have been accumulated to form it into a fairly well−marked variety, such as would be thought worthy of record in
a systematic work.
The intervals between the horizontal lines in the diagram, may represent each a thousand or more generations.
After a thousand generations, species (A) is supposed to have produced two fairly well−marked varieties, namely
a1 and m1. These two varieties will generally still be exposed to the same conditions which made their parents
variable, and the tendency to variability is in itself hereditary; consequently they will likewise tend to vary, and
commonly in nearly the same manner as did their parents. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their parent (A) more numerous than most of
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The Origin of Species by means of Natural Selection
the other inhabitants of the same country; they will also partake of those more general advantages which made the
genus to which the parent−species belonged, a large genus in its own country. And all these circumstances are
favourable to the production of new varieties.
If, then, these two varieties be variable, the most divergent of their variations will generally be preserved
during the next thousand generations. And after this interval, variety a1 is supposed in the diagram to have
produced variety a2, which will, owing to the principle of divergence, differ more from (A) than did variety a1.
Variety m1 is supposed to have produced two varieties, namely m2 and s2, differing from each other, and more
considerably from their common parent (A). We may continue the process by similar steps for any length of time;
some of the varieties, after each thousand generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some failing to produce any. Thus the varieties or
modified descendants of the common parent (A), will generally go on increasing in number and diverging in
character. In the diagram the process is represented up to the ten−thousandth generation, and under a condensed
and simplified form up to the fourteen−thousandth generation.
But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the
diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that
each form remains for long periods unaltered, and then again undergoes modification. Nor do I suppose that the
most divergent varieties are invariably preserved: a medium form may often long endure, and may or may not
produce more than one modified descendant; for natural selection will always act according to the nature of the
places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in structure the descendants from any one species
can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will
increase. In our diagram the line of succession is broken at regular intervals by small numbered letters marking
the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are
imaginary, and might have been inserted anywhere, after intervals long enough to allow the accumulation of a
considerable amount of divergent variation.
As all the modified descendants from a common and widely−diffused species, belonging to a large genus, will
tend to partake of the same advantages which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later and more highly improved branches in the
lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches:
this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In
some cases no doubt the process of modification will be confined to a single line of descent, and the number of
modified descendants will not be increased; although the amount of divergent modification may have been
augmented. This case would be represented in the diagram, if all the lines proceeding from (A) were removed,
excepting that from a1 to a10. In the same way the English racehorse and English pointer have apparently both
gone on slowly diverging in character from their original stocks, without either having given off any fresh
branches or races.
After ten thousand generations, species (A) is supposed to have produced three forms, a10, f10, and m10,
which, from having diverged in character during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we suppose the amount of change between
each horizontal line in our diagram to be excessively small, these three forms may still be only well−marked
varieties; but we have only to suppose the steps in the process of modification to be more numerous or greater in
amount, to convert these three forms into doubtful or at least into well−defined species: thus the diagram
illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater number of generations (as shown in the
diagram in a condensed and simplified manner), we get eight species, marked by the letters between a14 and m14,
all descended from (A). Thus, as I believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a
second species (I) has produced, by analogous steps, after ten thousand generations, either two well−marked
varieties (w10 and z10) or two species, according to the amount of change supposed to be represented between
the horizontal lines. After fourteen thousand generations, six new species, marked by the letters n14 to z14, are
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The Origin of Species by means of Natural Selection
supposed to have been produced. In any genus, the species which are already very different in character from each
other, will generally tend to produce the greatest number of modified descendants; for these will have the best
chance of seizing on new and widely different places in the polity of nature: hence in the diagram I have chosen
the extreme species (A), and the nearly extreme species (I), as those which have largely varied, and have given
rise to new varieties and species. The other nine species (marked by capital letters) of our original genus, may for
long but unequal periods continue to transmit unaltered descendants; and this is shown in the diagram by the
dotted lines unequally prolonged upwards.
But during the process of modification, represented in the diagram, another of our principles, namely that of
extinction, will have played an important part. As in each fully stocked country natural selection necessarily acts
by the selected form having some advantage in the struggle for life over other forms, there will be a constant
tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent
their predecessors and their original progenitor. For it should be remembered that the competition will generally
be most severe between those forms which are most nearly related to each other in habits, constitution and
structure. Hence all the intermediate forms between the earlier and later states, that is between the less and more
improved states of a the same species, as well as the original parent−species itself, will generally tend to become
extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and
improved lines. If, however, the modified offspring of a species get into some distinct country, or become quickly
adapted to some quite new station, in which offspring and progenitor do not come into competition, both may
continue to exist.
If, then, our diagram be assumed to represent a considerable amount of modification, species (A) and all the
earlier varieties will have become extinct, being replaced by eight new species (a14 to m14); and species (I) will
be replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus were supposed to resemble each other in
unequal degrees, as is so generally the case in nature; species (A) being more nearly related to B, C, and D than to
the other species; and species (I) more to G, H, K, L, than to the others. These two species (A and I), were also
supposed to be very common and widely diffused species, so that they must originally have had some advantage
over most of the other species of the genus. Their modified descendants, fourteen in number at the
fourteen−thousandth generation, will probably have inherited some of the same advantages: they have also been
modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many
related places in the natural economy of their country. It seems, therefore, extremely probable that they will have
taken the places of, and thus exterminated, not only their parents (A) and (I), but likewise some of the original
species which were most nearly related to their parents. Hence very few of the original species will have
transmitted offspring to the fourteen−thousandth generation. We may suppose that only one (F) of the two species
(E and F) which were least closely related to the other nine original species, has transmitted descendants to this
late stage of descent.
The new species in our diagram, descended from the original eleven species, will now be fifteen in number.
Owing to the divergent tendency of natural selection, the extreme amount of difference in character between
species a14 and z14 will be much greater than that between the most distinct of the original eleven species. The
new species, moreover, will be allied to each other in a widely different manner. Of the eight descendants from
(A) the three marked a14, q14, p14, will be nearly related from having recently branched off from a10; b14 and
f14, from having diverged at an earlier period from a5, will be in some degree distinct from the three first−named
species; and lastly, o14, e14, and m14, will be nearly related one to the other, but, from having diverged at the
first commencement of the process of modification, will be widely different from the other five species, and may
constitute a sub−genus or a distinct genus.
The six descendants from (I) will form two sub−genera or genera. But as the original species (I) differed
largely from (A), standing nearly at the extreme end of the original genus, the six descendants from (I) will,
owing to inheritance alone, differ considerably from the eight descendants from (A); the two groups, moreover,
are supposed to have gone on diverging in different directions. The intermediate species, also (and this is a very
important consideration), which connected the original species (A) and (I), have all become, except (F), extinct,
and have left no descendants. Hence the six new species descended from (I), and the eight descendants from (A),
will have to be ranked as very distinct genera, or even as distinct sub−families.
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The Origin of Species by means of Natural Selection
Thus it is, as I believe, that two or more genera are produced by descent with modification, from two or more
species of the same genus. And the two or more parent−species are supposed to be descended from some one
species of an earlier genus. In our diagram this is indicated by the broken lines beneath the capital letters,
converging in sub−branches downwards towards a single point; this point represents a species, the supposed
progenitor of our several new sub−genera and genera.
It is worth while to reflect for a moment on the character of the new species F14, which is supposed not to
have diverged much in character, but to have retained the form of (F), either unaltered or altered only in a slight
degree. In this case its affinities to the other fourteen new species will be of a curious and circuitous nature. Being
descended from a form that stood between the parent−species (A) and (I), now supposed to be extinct and
unknown, it will be in some degree intermediate in character between the two groups descended from these two
species. But as these two groups have gone on diverging in character from the type of their parents, the new
species (F14) will not be directly intermediate between them, but rather between types of the two groups; and
every naturalist will be able to call such cases before his mind.
In the diagram each horizontal line has hitherto been supposed to represent a thousand generations, but each
may represent a million or more generations; it may also represent a section of the successive strata of the earth's
crust including extinct remains. We shall, when we come to our chapter on geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light on the affinities of extinct beings, which,
though generally belonging to the same orders, families, or genera, with those now living, yet are often, in some
degree, intermediate in character between existing groups; and we can understand this fact, for the extinct species
lived at various remote epochs when the branching lines of descent had diverged less.
I see no reason to limit the process of modification, as now explained, to the formation of genera alone. If, in
the diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to
be great, the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three
very distinct genera. We shall also have two very distinct genera descended from (I), differing widely from the
descendants of (A). These two groups of genera will thus form two distinct families, or orders, according to the
amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders,
are descended from two species of the original genus; and these are supposed to be descended from some still
more ancient and unknown form.
We have seen that in each country it is the species belonging to the larger genera which oftenest present
varieties or incipient species. This, indeed, might have been expected; for as natural selection acts through one
form having some advantage over other forms in the struggle for existence, it will chiefly act on those which
already have some advantage; and the largeness of any group shows that its species have inherited from a
common ancestor some advantage in common. Hence, the struggle for the production of new and modified
descendants will mainly lie between the larger groups, which are all trying to increase in number. One large group
will slowly conquer another large group, reduce its number, and thus lessen its chance of further variation and
improvement. Within the same large group, the later and more highly perfected sub−groups, from branching out
and seizing on many new places in the polity of nature, will constantly tend to supplant and destroy the earlier and
less improved sub−groups. Small and broken groups and sub−groups will finally disappear. Looking to the future,
we can predict that the groups of organic beings which are now large and triumphant, and which are least broken
up, that is, which have as yet suffered least extinction, will, for a long period, continue to increase. But which
groups will ultimately prevail, no man can predict; for we know that many groups, formerly most extensively
developed, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the
continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and
leave no modified descendants; and consequently that, of the species living at any one period, extremely few will
transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on classification, but I
may add that as, according to this view, extremely few of the more ancient species have transmitted descendants
to the present day, and, as all the descendants of the same species form a class, we can understand how it is that
there exist so few classes in each main division of the animal and vegetable kingdoms. Although few of the most
ancient species have left modified descendants, yet, at remote geological periods, the earth may have been almost
as well peopled with species of many genera, families, orders and classes, as at the present day.
ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.
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The Origin of Species by means of Natural Selection
Natural selection acts exclusively by the preservation and accumulation of variations, which are beneficial
under the organic and inorganic conditions to which each creature is exposed at all periods of life. The ultimate
result is that each creature tends to become more and more improved in relation to its conditions. This
improvement inevitably leads to the gradual advancement of the organisation of the greater number of living
beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each
other's satisfaction what is meant by an advance in organisation. Among the vertebrata the degree of intellect and
an approach in structure to man clearly come into play. It might be thought that the amount of change which the
various parts and organs pass through in their development from embryo to maturity would suffice as a standard
of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure
become less perfect, so that the mature animal cannot be called higher than its larva. Von Baer's standard seems
the most widely applicable and the best, namely, the amount of differentiation of the parts of the same organic
being, in the adult state, as I should be inclined to add, and their specialisation for different functions; or, as Milne
Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure
this subject is if we look, for instance, to fishes, among which some naturalists rank those as highest which, like
the sharks, approach nearest to amphibians; while other naturalists rank the common bony or teleostean fishes as
the highest, inasmuch as they are most strictly fish− like, and differ most from the other vertebrate classes. We see
still more plainly the obscurity of the subject by turning to plants, among which the standard of intellect is of
course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals,
petals, stamens and pistils, fully developed in each flower; whereas other botanists, probably with more truth,
look at the plants which have their several organs much modified and reduced in number as the highest.
If we take as the standard of high organisation, the amount of differentiation and specialisation of the several
organs in each being when adult (and this will include the advancement of the brain for intellectual purposes),
natural selection clearly leads towards this standard: for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is an advantage to each being; and hence the
accumulation of variations tending towards specialisation is within the scope of natural selection. On the other
hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on
every unoccupied or less well occupied place in the economy of nature, that it is quite possible for natural
selection gradually to fit a being to a situation in which several organs would be superfluous or useless: in such
cases there would be retrogression in the scale of organisation. Whether organisation on the whole has actually
advanced from the remotest geological periods to the present day will be more conveniently discussed in our
chapter on Geological Succession.
But it may be objected that if all organic beings thus tend to rise in the scale, how is it that throughout the
world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more
highly developed than others? Why have not the more highly developed forms every where supplanted and
exterminated the lower? Lamarck, who believed in an innate and inevitable tendency towards perfection in all
organic beings, seems to have felt this difficulty so strongly that he was led to suppose that new and simple forms
are continually being produced by spontaneous generation. Science has not as yet proved the truth of this belief,
whatever the future may reveal. On our theory the continued existence of lowly organisms offers no difficulty; for
natural selection, or the survival of the fittest, does not necessarily include progressive development−−it only
takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life.
And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule−−to an
intestinal worm−−or even to an earth−worm, to be highly organised. If it were no advantage, these forms would
be left, by natural selection, unimproved or but little improved, and might remain for indefinite ages in their
present lowly condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have
remained for an enormous period in nearly their present state. But to suppose that most of the many now existing
low forms have not in the least advanced since the first dawn of life would be extremely rash; for every naturalist
who has dissected some of the beings now ranked as very low in the scale, must have been struck with their really
wondrous and beautiful organisation.
Nearly the same remarks are applicable, if we look to the different grades of organisation within the same
great group; for instance, in the vertebrata, to the co−existence of mammals and fish−−among mammalia, to the
co−existence of man and the ornithorhynchus−−among fishes, to the co− existence of the shark and the lancelet
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The Origin of Species by means of Natural Selection
(Amphioxus), which latter fish in the extreme simplicity of its structure approaches the invertebrate classes. But
mammals and fish hardly come into competition with each other; the advancement of the whole class of
mammals, or of certain members in this class, to the highest grade would not lead to their taking the place of
fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires
aerial respiration; so that warm−blooded mammals when inhabiting the water lie under a disadvantage in having
to come continually to the surface to breathe. With fishes, members of the shark family would not tend to supplant
the lancelet; for the lancelet, as I hear from Fritz Muller, has as sole companion and competitor on the barren
sandy shore of South Brazil, an anomalous annelid. The three lowest orders of mammals, namely, marsupials,
edentata, and rodents, co−exist in South America in the same region with numerous monkeys, and probably
interfere little with each other. Although organisation, on the whole, may have advanced and be still advancing
throughout the world, yet the scale will always present many degrees of perfection; for the high advancement of
certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of
those groups with which they do not enter into close competition. In some cases, as we shall hereafter see, lowly
organised forms appear to have been preserved to the present day, from inhabiting confined or peculiar stations,
where they have been subjected to less severe competition, and where their scanty numbers have retarded the
chance of favourable variations arising.
Finally, I believe that many lowly organised forms now exist throughout the world, from various causes. In
some cases variations or individual differences of a favourable nature may never have arisen for natural selection
to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of development.
In some few cases there has been what we must call retrogression or organisation. But the main cause lies in the
fact that under very simple conditions of life a high organisation would be of no service−−possibly would be of
actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure,
how, it has been asked, could the first step in the advancement or differentiation of parts have arisen? Mr. Herbert
Spencer would probably answer that, as soon as simple unicellular organisms came by growth or division to be
compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any
order become differentiated in proportion as their relations to incident forces become different" would come into
action. But as we have no facts to guide us, speculation on the subject is almost useless. It is, however, an error to
suppose that there would be no struggle for existence, and, consequently, no natural selection, until many forms
had been produced: variations in a single species inhabiting an isolated station might be beneficial, and thus the
whole mass of individuals might be modified, or two distinct forms might arise. But, as I remarked towards the
close of the introduction, no one ought to feel surprise at much remaining as yet unexplained on the origin of
species, if we make due allowance for our profound ignorance on the mutual relations of the inhabitants of the
world at the present time, and still more so during past ages.
CONVERGENCE OF CHARACTER.
Mr. H.C. Watson thinks that I have overrated the importance of divergence of character (in which, however,
he apparently believes), and that convergence, as it may be called, has likewise played a part. If two species
belonging to two distinct though allied genera, had both produced a large number of new and divergent forms, it
is conceivable that these might approach each other so closely that they would have all to be classed under the
same genus; and thus the descendants of two distinct genera would converge into one. But it would in most cases
be extremely rash to attribute to convergence a close and general similarity of structure in the modified
descendants of widely distinct forms. The shape of a crystal is determined solely by the molecular forces, and it is
not surprising that dissimilar substances should sometimes assume the same form; but with organic beings we
should bear in mind that the form of each depends on an infinitude of complex relations, namely on the variations
which have arisen, these being due to causes far too intricate to be followed out−−on the nature of the variations
which have been preserved or selected, and this depends on the surrounding physical conditions, and in a still
higher degree on the surrounding organisms with which each being has come into competition−−and lastly, on
inheritance (in itself a fluctuating element) from innumerable progenitors, all of which have had their forms
determined through equally complex relations. It is incredible that the descendants of two organisms, which had
originally differed in a marked manner, should ever afterwards converge so closely as to lead to a near approach
to identity throughout their whole organisation. If this had occurred, we should meet with the same form,
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The Origin of Species by means of Natural Selection
independently of genetic connection, recurring in widely separated geological formations; and the balance of
evidence is opposed to any such an admission.
Mr. Watson has also objected that the continued action of natural selection, together with divergence of
character, would tend to make an indefinite number of specific forms. As far as mere inorganic conditions are
concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable
diversities of heat, moisture, etc.; but I fully admit that the mutual relations of organic beings are more important;
and as the number of species in any country goes on increasing, the organic conditions of life must become more
and more complex. Consequently there seems at first no limit to the amount of profitable diversification of
structure, and therefore no limit to the number of species which might be produced. We do not know that even the
most prolific area is fully stocked with specific forms: at the Cape of Good Hope and in Australia, which support
such an astonishing number of species, many European plants have become naturalised. But geology shows us,
that from an early part of the tertiary period the number of species of shells, and that from the middle part of this
same period, the number of mammals has not greatly or at all increased. What then checks an indefinite increase
in the number of species? The amount of life (I do not mean the number of specific forms) supported on an area
must have a limit, depending so largely as it does on physical conditions; therefore, if an area be inhabited by very
many species, each or nearly each species will be represented by few individuals; and such species will be liable
to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. The
process of extermination in such cases would be rapid, whereas the production of new species must always be
slow. Imagine the extreme case of as many species as individuals in England, and the first severe winter or very
dry summer would exterminate thousands on thousands of species. Rare species, and each species will become
rare if the number of species in any country becomes indefinitely increased, will, on the principal often explained,
present within a given period few favourable variations; consequently, the process of giving birth to new specific
forms would thus be retarded. When any species becomes very rare, close interbreeding will help to exterminate
it; authors have thought that this comes into play in accounting for the deterioration of the aurochs in Lithuania, of
red deer in Scotland and of bears in Norway, etc. Lastly, and this I am inclined to think is the most important
element, a dominant species, which has already beaten many competitors in its own home, will tend to spread and
supplant many others. Alph. de Candolle has shown that those species which spread widely tend generally to
spread VERY widely, consequently they will tend to supplant and exterminate several species in several areas,
and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has recently shown that
in the southeast corner of Australia, where, apparently, there are many invaders from different quarters of the
globe, the endemic Australian species have been greatly reduced in number. How much weight to attribute to
these several considerations I will not pretend to say; but conjointly they must limit in each country the tendency
to an indefinite augmentation of specific forms.
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The Origin of Species by means of Natural Selection
SUMMARY OF CHAPTER.
If under changing conditions of life organic beings present individual differences in almost every part of their
structure, and this cannot be disputed; if there be, owing to their geometrical rate of increase, a severe struggle for
life at some age, season or year, and this certainly cannot be disputed; then, considering the infinite complexity of
the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in
structure, constitution, and habits, to be advantageous to them, it would be a most extraordinary fact if no
variations had ever occurred useful to each being's own welfare, in the same manner as so many variations have
occurred useful to man. But if variations useful to any organic being ever do occur, assuredly individuals thus
characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of
inheritance, these will tend to produce offspring similarly characterised. This principle of preservation, or the
survival of the fittest, I have called natural selection. It leads to the improvement of each creature in relation to its
organic and inorganic conditions of life; and consequently, in most cases, to what must be regarded as an advance
in organisation. Nevertheless, low and simple forms will long endure if well fitted for their simple conditions of
life.
Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed,
or young as easily as the adult. Among many animals sexual selection will have given its aid to ordinary selection
by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will
also give characters useful to the males alone in their struggles or rivalry with other males; and these characters
will be transmitted to one sex or to both sexes, according to the form of inheritance which prevails.
Whether natural selection has really thus acted in adapting the various forms of life to their several conditions
and stations, must be judged by the general tenour and balance of evidence given in the following chapters. But
we have already seen how it entails extinction; and how largely extinction has acted in the world's history,
geology plainly declares. Natural selection, also, leads to divergence of character; for the more organic beings
diverge in structure, habits and constitution, by so much the more can a large number be supported on the area, of
which we see proof by looking to the inhabitants of any small spot, and to the productions naturalised in foreign
lands. Therefore, during the modification of the descendants of any one species, and during the incessant struggle
of all species to increase in numbers, the more diversified the descendants become, the better will be their chance
of success in the battle for life. Thus the small differences distinguishing varieties of the same species, steadily
tend to increase, till they equal the greater differences between species of the same genus, or even of distinct
genera.
We have seen that it is the common, the widely diffused, and widely ranging species, belonging to the larger
genera within each class, which vary most; and these tend to transmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to
divergence of character and to much extinction of the less improved and intermediate forms of life. On these
principles, the nature of the affinities, and the generally well defined distinctions between the innumerable organic
beings in each class throughout the world, may be explained. It is a truly wonderful fact−−the wonder of which
we are apt to overlook from familiarity−−that all animals and all plants throughout all time and space should be
related to each other in groups, subordinate to groups, in the manner which we everywhere behold−−namely,
varieties of the same species most closely related, species of the same genus less closely and unequally related,
forming sections and sub−genera, species of distinct genera much less closely related, and genera related in
different degrees, forming sub−families, families, orders, sub−classes, and classes. The several subordinate
groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other
points, and so on in almost endless cycles. If species had been independently created, no explanation would have
been possible of this kind of classification; but it is explained through inheritance and the complex action of
natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe
this simile largely speaks the truth. The green and budding twigs may represent existing species; and those
produced during former years may represent the long succession of extinct species. At each period of growth all
SUMMARY OF CHAPTER. 60

The Origin of Species by means of Natural Selection
the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and
branches, in the same manner as species and groups of species have at all times overmastered other species in the
great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were
themselves once, when the tree was young, budding twigs; and this connexion of the former and present buds by
ramifying branches may well represent the classification of all extinct and living species in groups subordinate to
groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into
great branches, yet survive and bear the other branches; so with the species which lived during long−past
geological periods, very few have left living and modified descendants. From the first growth of the tree, many a
limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those
whole orders, families, and genera which have now no living representatives, and which are known to us only in a
fossil state. As we here and there see a thin, straggling branch springing from a fork low down in a tree, and
which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like
the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of
life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds
give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler
branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken
branches the crust of the earth, and covers the surface with its ever−branching and beautiful ramifications.
SUMMARY OF CHAPTER. 61

The Origin of Species by means of Natural Selection
CHAPTER V. LAWS OF VARIATION.
Effects of changed conditions −− Use and disuse, combined with natural selection; organs of flight and of
vision −− Acclimatisation −− Correlated variation −− Compensation and economy of growth −− False
correlations −− Multiple, rudimentary, and lowly organised structures variable −− Parts developed in an unusual
manner are highly variable: specific characters more variable than generic: secondary sexual characters variable
−− Species of the same genus vary in an analogous manner −− Reversions to long−lost characters −− Summary.
I have hitherto sometimes spoken as if the variations−−so common and multiform with organic beings under
domestication, and in a lesser degree with those under nature−−were due to chance. This, of course is a wholly
incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation.
Some authors believe it to be as much the function of the reproductive system to produce individual differences,
or slight deviations of structure, as to make the child like its parents. But the fact of variations and monstrosities
occurring much more frequently under domestication than under nature, and the greater variability of species
having wide ranges than of those with restricted ranges, lead to the conclusion that variability is generally related
to the conditions of life to which each species has been exposed during several successive generations. In the first
chapter I attempted to show that changed conditions act in two ways, directly on the whole organisation or on
certain parts alone, and indirectly through the reproductive system. In all cases there are two factors, the nature of
the organism, which is much the most important of the two, and the nature of the conditions. The direct action of
changed conditions leads to definite or indefinite results. In the latter case the organisation seems to become
plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it
yields readily, when subjected to certain conditions, and all, or nearly all, the individuals become modified in the
same way.
It is very difficult to decide how far changed conditions, such as of climate, food, etc., have acted in a definite
manner. There is reason to believe that in the course of time the effects have been greater than can be proved by
clear evidence. But we may safely conclude that the innumerable complex co−adaptations of structure, which we
see throughout nature between various organic beings, cannot be attributed simply to such action. In the following
cases the conditions seem to have produced some slight definite effect: E. Forbes asserts that shells at their
southern limit, and when living in shallow water, are more brightly coloured than those of the same species from
further north or from a greater depth; but this certainly does not always hold good. Mr. Gould believes that birds
of the same species are more brightly coloured under a clear atmosphere, than when living near the coast or on
islands; and Wollaston is convinced that residence near the sea affects the colours of insects. Moquin−Tandon
gives a list of plants which, when growing near the sea−shore, have their leaves in some degree fleshy, though not
elsewhere fleshy. These slightly varying organisms are interesting in as far as they present characters analogous to
those possessed by the species which are confined to similar conditions.
When a variation is of the slightest use to any being, we cannot tell how much to attribute to the accumulative
action of natural selection, and how much to the definite action of the conditions of life. Thus, it is well known to
furriers that animals of the same species have thicker and better fur the further north they live; but who can tell
how much of this difference may be due to the warmest−clad individuals having been favoured and preserved
during many generations, and how much to the action of the severe climate? For it would appear that climate has
some direct action on the hair of our domestic quadrupeds.
Instances could be given of similar varieties being produced from the same species under external conditions
of life as different as can well be conceived; and, on the other hand, of dissimilar varieties being produced under
apparently the same external conditions. Again, innumerable instances are known to every naturalist, of species
keeping true, or not varying at all, although living under the most opposite climates. Such considerations as these
incline me to lay less weight on the direct action of the surrounding conditions, than on a tendency to vary, due to
causes of which we are quite ignorant.
In one sense the conditions of life may be said, not only to cause variability, either directly or indirectly, but
likewise to include natural selection, for the conditions determine whether this or that variety shall survive. But
when man is the selecting agent, we clearly see that the two elements of change are distinct; variability is in some
CHAPTER V. LAWS OF VARIATION. 62

The Origin of Species by means of Natural Selection
manner excited, but it is the will of man which accumulates the variations in certain direction; and it is this latter
agency which answers to the survival of the fittest under nature.
EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY NATURAL
SELECTION.
>From the facts alluded to in the first chapter, I think there can be no doubt that use in our domestic animals
has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are
inherited. Under free nature we have no standard of comparison by which to judge of the effects of
long−continued use or disuse, for we know not the parent−forms; but many animals possess structures which can
be best explained by the effects of disuse. As Professor Owen has remarked, there is no greater anomaly in nature
than a bird that cannot fly; yet there are several in this state. The logger−headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same condition as the domestic Aylesbury
duck: it is a remarkable fact that the young birds, according to Mr. Cunningham, can fly, while the adults have
lost this power. As the larger ground−feeding birds seldom take flight except to escape danger, it is probable that
the nearly wingless condition of several birds, now inhabiting or which lately inhabited several oceanic islands,
tenanted by no beasts of prey, has been caused by disuse. The ostrich indeed inhabits continents, and is exposed to
danger from which it cannot escape by flight, but it can defend itself, by kicking its enemies, as efficiently as
many quadrupeds. We may believe that the progenitor of the ostrich genus had habits like those of the bustard,
and that, as the size and weight of its body were increased during successive generations, its legs were used more
and its wings less, until they became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the anterior tarsi, or feet, of many male
dung−feeding beetles are often broken off; he examined seventeen specimens in his own collection, and not one
had even a relic left. In the Onites apelles the tarsi are so habitually lost that the insect has been described as not
having them. In some other genera they are present, but in a rudimentary condition. In the Ateuchus or sacred
beetle of the Egyptians, they are totally deficient. The evidence that accidental mutilations can be inherited is at
present not decisive; but the remarkable cases observed by Brown−Sequard in guinea−pigs, of the inherited
effects of operations, should make us cautious in denying this tendency. Hence, it will perhaps be safest to look at
the entire absence of the anterior tarsi in Ateuchus, and their rudimentary condition in some other genera, not as
cases of inherited mutilations, but as due to the effects of long−continued disuse; for as many dung−feeding
beetles are generally found with their tarsi lost, this must happen early in life; therefore the tarsi cannot be of
much importance or be much used by these insects.
In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly due
to natural selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the 550 species (but
more are now known) inhabiting Madeira, are so far deficient in wings that they cannot fly; and that, of the
twenty−nine endemic genera, no less than twenty−three have all their species in this condition! Several facts,
namely, that beetles in many parts of the world are very frequently blown to sea and perish; that the beetles in
Madeira, as observed by Mr. Wollaston, lie much concealed, until the wind lulls and the sun shines; that the
proportion of wingless beetles is larger on the exposed Desertas than in Madeira itself; and especially the
extraordinary fact, so strongly insisted on by Mr. Wollaston, that certain large groups of beetles, elsewhere
excessively numerous, which absolutely require the use of their wings, are here almost entirely absent. These
several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to
the action of natural selection, combined probably with disuse. For during many successive generations each
individual beetle which flew least, either from its wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand,
those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed.
The insects in Madeira which are not ground−feeders, and which, as certain flower−feeding coleoptera and
lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite compatible with the action of natural selection. For when a new
insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings, would depend
on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up
the attempt and rarely or never flying. As with mariners shipwrecked near a coast, it would have been better for
the good swimmers if they had been able to swim still further, whereas it would have been better for the bad
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The Origin of Species by means of Natural Selection
swimmers if they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite
covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps
by natural selection. In South America, a burrowing rodent, the tuco−tuco, or Ctenomys, is even more
subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they
were frequently blind. One which I kept alive was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to
any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their
size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so,
natural selection would aid the effects of disuse.
It is well known that several animals, belonging to the most different classes, which inhabit the caves of
Carniola and Kentucky, are blind. In some of the crabs the foot−stalk for the eye remains, though the eye is gone;
the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine
that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed
to disuse. In one of the blind animals, namely, the cave−rat (Neotoma), two of which were captured by Professor
Silliman at above half a mile distance from the mouth of the cave, and therefore not in the profoundest depths, the
eyes were lustrous and of large size; and these animals, as I am informed by Professor Silliman, after having been
exposed for about a month to a graduated light, acquired a dim perception of objects.
It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar
climate; so that, in accordance with the old view of the blind animals having been separately created for the
American and European caverns, very close similarity in their organisation and affinities might have been
expected. This is certainly not the case if we look at the two whole faunas; with respect to the insects alone,
Schiodte has remarked: "We are accordingly prevented from considering the entire phenomenon in any other light
than something purely local, and the similarity which is exhibited in a few forms between the Mammoth Cave (in
Kentucky) and the caves in Carniola, otherwise than as a very plain expression of that analogy which subsists
generally between the fauna of Europe and of North America." On my view we must suppose that American
animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the
outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of
Europe. We have some evidence of this gradation of habit; for, as Schiodte remarks: "We accordingly look upon
the subterranean faunas as small ramifications which have penetrated into the earth from the geographically
limited faunas of the adjacent tracts, and which, as they extended themselves into darkness, have been
accommodated to surrounding circumstances. Animals not far remote from ordinary forms, prepare the transition
from light to darkness. Next follow those that are constructed for twilight; and, last of all, those destined for total
darkness, and whose formation is quite peculiar." These remarks of Schiodte's it should be understood, apply not
to the same, but to distinct species. By the time that an animal had reached, after numberless generations, the
deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection
will often have effected other changes, such as an increase in the length of the antennae or palpi, as a
compensation for blindness. Notwithstanding such modifications, we might expect still to see in the cave−animals
of America, affinities to the other inhabitants of that continent, and in those of Europe to the inhabitants of the
European continent. And this is the case with some of the American cave−animals, as I hear from Professor Dana;
and some of the European cave−insects are very closely allied to those of the surrounding country. It would be
difficult to give any rational explanation of the affinities of the blind cave−animals to the other inhabitants of the
two continents on the ordinary view of their independent creation. That several of the inhabitants of the caves of
the Old and New Worlds should be closely related, we might expect from the well−known relationship of most of
their other productions. As a blind species of Bathyscia is found in abundance on shady rocks far from caves, the
loss of vision in the cave species of this one genus has probably had no relation to its dark habitation; for it is
natural that an insect already deprived of vision should readily become adapted to dark caverns. Another blind
genus (Anophthalmus) offers this remarkable peculiarity, that the species, as Mr. Murray observes, have not as
yet been found anywhere except in caves; yet those which inhabit the several caves of Europe and America are
distinct; but it is possible that the progenitors of these several species, while they were furnished with eyes, may
formerly have ranged over both continents, and then have become extinct, excepting in their present secluded
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The Origin of Species by means of Natural Selection
abodes. Far from feeling surprise that some of the cave−animals should be very anomalous, as Agassiz has
remarked in regard to the blind fish, the Amblyopsis, and as is the case with the blind Proteus, with reference to
the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the
less severe competition to which the scanty inhabitants of these dark abodes will have been exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in the time of sleep, in the amount of rain
requisite for seeds to germinate, etc., and this leads me to say a few words on acclimatisation. As it is extremely
common for distinct species belonging to the same genus to inhabit hot and cold countries, if it be true that all the
species of the same genus are descended from a single parent−form, acclimatisation must be readily effected
during a long course of descent. It is notorious that each species is adapted to the climate of its own home: species
from an arctic or even from a temperate region cannot endure a tropical climate, or conversely. So again, many
succulent plants cannot endure a damp climate. But the degree of adaptation of species to the climates under
which they live is often overrated. We may infer this from our frequent inability to predict whether or not an
imported plant will endure our climate, and from the number of plants and animals brought from different
countries which are here perfectly healthy. We have reason to believe that species in a state of nature are closely
limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to
particular climates. But whether or not this adaptation is in most cases very close, we have evidence with some
few plants, of their becoming, to a certain extent, naturally habituated to different temperatures; that is, they
become acclimatised: thus the pines and rhododendrons, raised from seed collected by Dr. Hooker from the same
species growing at different heights on the Himalayas, were found to possess in this country different
constitutional powers of resisting cold. Mr. Thwaites informs me that he has observed similar facts in Ceylon;
analogous observations have been made by Mr. H.C. Watson on European species of plants brought from the
Azores to England; and I could give other cases. In regard to animals, several authentic instances could be
adduced of species having largely extended, within historical times, their range from warmer to colder latitudes,
and conversely; but we do not positively know that these animals were strictly adapted to their native climate,
though in all ordinary cases we assume such to be the case; nor do we know that they have subsequently become
specially acclimatised to their new homes, so as to be better fitted for them than they were at first.
As we may infer that our domestic animals were originally chosen by uncivilised man because they were
useful, and because they bred readily under confinement, and not because they were subsequently found capable
of far−extended transportation, the common and extraordinary capacity in our domestic animals of not only
withstanding the most different climates, but of being perfectly fertile (a far severer test) under them, may be used
as an argument that a large proportion of other animals now in a state of nature could easily be brought to bear
widely different climates. We must not, however, push the foregoing argument too far, on account of the probable
origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropical and arctic
wolf may perhaps be mingled in our domestic breeds. The rat and mouse cannot be considered as domestic
animals, but they have been transported by man to many parts of the world, and now have a far wider range than
any other rodent; for they live under the cold climate of Faroe in the north and of the Falklands in the south, and
on many an island in the torrid zones. Hence adaptation to any special climate may be looked at as a quality
readily grafted on an innate wide flexibility of constitution, common to most animals. On this view, the capacity
of enduring the most different climates by man himself and by his domestic animals, and the fact of the extinct
elephant and rhinoceros having formerly endured a glacial climate, whereas the living species are now all tropical
or sub−tropical in their habits, ought not to be looked at as anomalies, but as examples of a very common
flexibility of constitution, brought, under peculiar circumstances, into action.
How much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much to the
natural selection of varieties having different innate constitutions, and how much to both means combined, is an
obscure question. That habit or custom has some influence, I must believe, both from analogy and from the
incessant advice given in agricultural works, even in the ancient Encyclopaedias of China, to be very cautious in
transporting animals from one district to another. And as it is not likely that man should have succeeded in
selecting so many breeds and sub−breeds with constitutions specially fitted for their own districts, the result must,
I think, be due to habit. On the other hand, natural selection would inevitably tend to preserve those individuals
which were born with constitutions best adapted to any country which they inhabited. In treatises on many kinds
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The Origin of Species by means of Natural Selection
of cultivated plants, certain varieties are said to withstand certain climates better than others; this is strikingly
shown in works on fruit−trees published in the United States, in which certain varieties are habitually
recommended for the northern and others for the southern states; and as most of these varieties are of recent
origin, they cannot owe their constitutional differences to habit. The case of the Jerusalem artichoke, which is
never propagated in England by seed, and of which, consequently, new varieties have not been produced, has
even been advanced, as proving that acclimatisation cannot be effected, for it is now as tender as ever it was! The
case, also, of the kidney−bean has been often cited for a similar purpose, and with much greater weight; but until
some one will sow, during a score of generations, his kidney−beans so early that a very large proportion are
destroyed by frost, and then collect seed from the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the experiment cannot be said to have been even
tried. Nor let it be supposed that differences in the constitution of seedling kidney−beans never appear, for an
account has been published how much more hardy some seedlings are than others; and of this fact I have myself
observed striking instances.
On the whole, we may conclude that habit, or use and disuse, have, in some cases, played a considerable part
in the modification of the constitution and structure; but that the effects have often been largely combined with,
and sometimes overmastered by, the natural selection of innate variations.
CORRELATED VARIATION.
I mean by this expression that the whole organisation is so tied together, during its growth and development,
that when slight variations in any one part occur and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly understood, and no doubt wholly different
classes of facts may be here easily confounded together. We shall presently see that simple inheritance often gives
the false appearance of correlation. One of the most obvious real cases is, that variations of structure arising in the
young or larvae naturally tend to affect the structure of the mature animal. The several parts which are
homologous, and which, at an early embryonic period, are identical in structure, and which are necessarily
exposed to similar conditions, seem eminently liable to vary in a like manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying
together, for the lower jaw is believed by some anatomists to be homologous with the limbs. These tendencies, I
do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed
with an antler only on one side; and if this had been of any great use to the breed, it might probably have been
rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous
plants: and nothing is more common than the union of homologous parts in normal structures, as in the union of
the petals into a tube. Hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that
with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of the kidneys.
Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the
child. In snakes, according to Schlegel, the shape of the body and the manner of swallowing determine the
position and form of several of the most important viscera.
The nature of the bond is frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked that
certain malconformations frequently, and that others rarely, coexist without our being able to assign any reason.
What can be more singular than the relation in cats between complete whiteness and blue eyes with deafness, or
between the tortoise−shell colour and the female sex; or in pigeons, between their feathered feet and skin betwixt
the outer toes, or between the presence of more or less down on the young pigeon when first hatched, with the
future colour of its plumage; or, again, the relation between the hair and the teeth in the naked Turkish dog,
though here no doubt homology comes into play? With respect to this latter case of correlation, I think it can
hardly be accidental that the two orders of mammals which are most abnormal in their dermal covering, viz.,
Cetacea (whales) and Edentata (armadilloes, scaly ant−eaters, etc.), are likewise on the whole the most abnormal
in their teeth, but there are so many exceptions to this rule, as Mr. Mivart has remarked, that it has little value.
I know of no case better adapted to show the importance of the laws of correlation and variation,
independently of utility, and therefore of natural selection, than that of the difference between the outer and inner
flowers in some Compositous and Umbelliferous plants. Everyone is familiar with the difference between the ray
and central florets of, for instance, the daisy, and this difference is often accompanied with the partial or complete
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The Origin of Species by means of Natural Selection
abortion of the reproductive organs. But in some of these plants the seeds also differ in shape and sculpture. These
differences have sometimes been attributed to the pressure of the involucra on the florets, or to their mutual
pressure, and the shape of the seeds in the ray−florets of some Compositae countenances this idea; but with the
Umbelliferae it is by no means, as Dr. Hooker informs me, the species with the densest heads which most
frequently differ in their inner and outer flowers. It might have been thought that the development of the
ray−petals, by drawing nourishment from the reproductive organs causes their abortion; but this can hardly be the
sole case, for in some Compositae the seeds of the outer and inner florets differ, without any difference in the
corolla. Possibly these several differences may be connected with the different flow of nutriment towards the
central and external flowers. We know, at least, that with irregular flowers those nearest to the axis are most
subject to peloria, that is to become abnormally symmetrical. I may add, as an instance of this fact, and as a
striking case of correlation, that in many pelargoniums the two upper petals in the central flower of the truss often
lose their patches of darker colour; and when this occurs, the adherent nectary is quite aborted, the central flower
thus becoming peloric or regular. When the colour is absent from only one of the two upper petals, the nectary is
not quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel's idea that the ray−florets serve to attract insects,
whose agency is highly advantageous, or necessary for the fertilisation of these plants, is highly probable; and if
so, natural selection may have come into play. But with respect to the seeds, it seems impossible that their
differences in shape, which are not always correlated with any difference in the corolla, can be in any way
beneficial; yet in the Umbelliferae these differences are of such apparent importance−−the seeds being sometimes
orthospermous in the exterior flowers and coelospermous in the central flowers−−that the elder De Candolle
founded his main divisions in the order on such characters. Hence modifications of structure, viewed by
systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as
we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures which are common to whole groups of
species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after thousands of generations, some other and
independent modification; and these two modifications, having been transmitted to a whole group of descendants
with diverse habits, would naturally be thought to be in some necessary manner correlated. Some other
correlations are apparently due to the manner in which natural selection can alone act. For instance, Alph. De
Candolle has remarked that winged seeds are never found in fruits which do not open; I should explain this rule
by the impossibility of seeds gradually becoming winged through natural selection, unless the capsules were
open; for in this case alone could the seeds, which were a little better adapted to be wafted by the wind, gain an
advantage over others less well fitted for wide dispersal.
COMPENSATION AND ECONOMY OF GROWTH.
The elder Geoffroy and Goethe propounded, at about the same time, their law of compensation or
balancement of growth; or, as Goethe expressed it, "in order to spend on one side, nature is forced to economise
on the other side." I think this holds true to a certain extent with our domestic productions: if nourishment flows
to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to
give much milk and to fatten readily. The same varieties of the cabbage do not yield abundant and nutritious
foliage and a copious supply of oil−bearing seeds. When the seeds in our fruits become atrophied, the fruit itself
gains largely in size and quality. In our poultry, a large tuft of feathers on the head is generally accompanied by a
diminished comb, and a large beard by diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good observers, more especially botanists, believe in
its truth. I will not, however, here give any instances, for I see hardly any way of distinguishing between the
effects, on the one hand, of a part being largely developed through natural selection and another and adjoining
part being reduced by the same process or by disuse, and, on the other hand, the actual withdrawal of nutriment
from one part owing to the excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been advanced, and likewise some other
facts, may be merged under a more general principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions of life a structure, before useful,
becomes less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment
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The Origin of Species by means of Natural Selection
wasted in building up a useless structure. I can thus only understand a fact with which I was much struck when
examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is
parasitic within another cirripede and is thus protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas: for the carapace in
all other cirripedes consists of the three highly important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic and protected Proteolepas, the whole anterior
part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennae. Now the
saving of a large and complex structure, when rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life to which every animal is exposed, each would have
a better chance of supporting itself, by less nutriment being wasted.
Thus, as I believe, natural selection will tend in the long run to reduce any part of the organisation, as soon as
it becomes, through changed habits, superfluous, without by any means causing some other part to be largely
developed in a corresponding degree. And conversely, that natural selection may perfectly well succeed in largely
developing an organ without requiring as a necessary compensation the reduction of some adjoining part.
MULTIPLE, RUDIMENTARY, AND LOWLY−ORGANISED STRUCTURES ARE VARIABLE.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with varieties and species, that when any
part or organ is repeated many times in the same individual (as the vertebrae in snakes, and the stamens in
polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in
lesser numbers, is constant. The same author as well as some botanists, have further remarked that multiple parts
are extremely liable to vary in structure. As "vegetative repetition," to use Professor Owen's expression, is a sign
of low organisation; the foregoing statements accord with the common opinion of naturalists, that beings which
stand low in the scale of nature are more variable than those which are higher. I presume that lowness here means
that the several parts of the organisation have been but little specialised for particular functions; and as long as the
same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural
selection should not have preserved or rejected each little deviation of form so carefully as when the part has to
serve for some one special purpose. In the same way that a knife which has to cut all sorts of things may be of
almost any shape; whilst a tool for some particular purpose must be of some particular shape. Natural selection, it
should never be forgotten, can act solely through and for the advantage of each being.
Rudimentary parts, as is generally admitted, are apt to be highly variable. We shall have to recur to this
subject; and I will here only add that their variability seems to result from their uselessness, and consequently
from natural selection having had no power to check deviations in their structure.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY VARIABLE.
Several years ago I was much struck by a remark to the above effect made by Mr. Waterhouse. Professor
Owen, also, seems to have come to a nearly similar conclusion. It is hopeless to attempt to convince any one of
the truth of the above proposition without giving the long array of facts which I have collected, and which cannot
possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowances for them. It should be understood that the rule
by no means applies to any part, however unusually developed, unless it be unusually developed in one species or
in a few species in comparison with the same part in many closely allied species. Thus, the wing of the bat is a
most abnormal structure in the class of mammals; but the rule would not apply here, because the whole group of
bats possesses wings; it would apply only if some one species had wings developed in a remarkable manner in
comparison with the other species of the same genus. The rule applies very strongly in the case of secondary
sexual characters, when displayed in any unusual manner. The term, secondary sexual characters, used by Hunter,
relates to characters which are attached to one sex, but are not directly connected with the act of reproduction. The
rule applies to males and females; but more rarely to females, as they seldom offer remarkable secondary sexual
characters. The rule being so plainly applicable in the case of secondary sexual characters, may be due to the great
variability of these characters, whether or not displayed in any unusual manner−−of which fact I think there can
be little doubt. But that our rule is not confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; I particularly attended to Mr. Waterhouse's remark, whilst investigating this order, and I
am fully convinced that the rule almost always holds good. I shall, in a future work, give a list of all the more
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The Origin of Species by means of Natural Selection
remarkable cases. I will here give only one, as it illustrates the rule in its largest application. The opercular valves
of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species of one genus, Pyrgoma, these valves present a
marvellous amount of diversification; the homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of the same species is so great that it is no
exaggeration to state that the varieties of the same species differ more from each other in the characters derived
from these important organs, than do the species belonging to other distinct genera.
As with birds the individuals of the same species, inhabiting the same country, vary extremely little, I have
particularly attended to them; and the rule certainly seems to hold good in this class. I cannot make out that it
applies to plants, and this would have seriously shaken my belief in its truth, had not the great variability in plants
made it particularly difficult to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner in a species, the fair presumption
is that it is of high importance to that species: nevertheless it is in this case eminently liable to variation. Why
should this be so? On the view that each species has been independently created, with all its parts as we now see
them, I can see no explanation. But on the view that groups of species are descended from some other species, and
have been modified through natural selection, I think we can obtain some light. First let me make some
preliminary remarks. If, in our domestic animals, any part or the whole animal be neglected, and no selection be
applied, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a uniform
character: and the breed may be said to be degenerating. In rudimentary organs, and in those which have been but
little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel case; for
in such cases natural selection either has not or cannot come into full play, and thus the organisation is left in a
fluctuating condition. But what here more particularly concerns us is, that those points in our domestic animals,
which at the present time are undergoing rapid change by continued selection, are also eminently liable to
variation. Look at the individuals of the same breed of the pigeon; and see what a prodigious amount of difference
there is in the beak of tumblers, in the beak and wattle of carriers, in the carriage and tail of fantails, etc., these
being the points now mainly attended to by English fanciers. Even in the same sub−breed, as in that of the
short−faced tumbler, it is notoriously difficult to breed nearly perfect birds, many departing widely from the
standard. There may truly be said to be a constant struggle going on between, on the one hand, the tendency to
reversion to a less perfect state, as well as an innate tendency to new variations, and, on the other hand, the power
of steady selection to keep the breed true. In the long run selection gains the day, and we do not expect to fail so
completely as to breed a bird as coarse as a common tumbler pigeon from a good short−faced strain. But as long
as selection is rapidly going on, much variability in the parts undergoing modification may always be expected.
Now let us turn to nature. When a part has been developed in an extraordinary manner in any one species,
compared with the other species of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several species branched off from the common
progenitor of the genus. This period will seldom be remote in any extreme degree, as species rarely endure for
more than one geological period. An extraordinary amount of modification implies an unusually large and
long−continued amount of variability, which has continually been accumulated by natural selection for the benefit
of the species. But as the variability of the extraordinarily developed part or organ has been so great and
long−continued within a period not excessively remote, we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organisation which have remained for a much longer period
nearly constant. And this, I am convinced, is the case. That the struggle between natural selection on the one
hand, and the tendency to reversion and variability on the other hand, will in the course of time cease; and that the
most abnormally developed organs may be made constant, I see no reason to doubt. Hence, when an organ,
however abnormal it may be, has been transmitted in approximately the same condition to many modified
descendants, as in the case of the wing of the bat, it must have existed, according to our theory, for an immense
period in nearly the same state; and thus it has come not to be more variable than any other structure. It is only in
those cases in which the modification has been comparatively recent and extraordinarily great that we ought to
find the GENERATIVE VARIABILITY, as it may be called, still present in a high degree. For in this case the
variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required
manner and degree, and by the continued rejection of those tending to revert to a former and less modified
CHAPTER V. LAWS OF VARIATION. 69

The Origin of Species by means of Natural Selection
condition.
SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.
The principle discussed under the last heading may be applied to our present subject. It is notorious that
specific characters are more variable than generic. To explain by a simple example what is meant: if in a large
genus of plants some species had blue flowers and some had red, the colour would be only a specific character,
and no one would be surprised at one of the blue species varying into red, or conversely; but if all the species had
blue flowers, the colour would become a generic character, and its variation would be a more unusual
circumstance. I have chosen this example because the explanation which most naturalists would advance is not
here applicable, namely, that specific characters are more variable than generic, because they are taken from parts
of less physiological importance than those commonly used for classing genera. I believe this explanation is
partly, yet only indirectly, true; I shall, however, have to return to this point in the chapter on Classification. It
would be almost superfluous to adduce evidence in support of the statement, that ordinary specific characters are
more variable than generic; but with respect to important characters, I have repeatedly noticed in works on natural
history, that when an author remarks with surprise that some important organ or part, which is generally very
constant throughout a large group of species, DIFFERS considerably in closely−allied species, it is often
VARIABLE in the individuals of the same species. And this fact shows that a character, which is generally of
generic value, when it sinks in value and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind applies to monstrosities: at least Is.
Geoffroy St. Hilaire apparently entertains no doubt, that the more an organ normally differs in the different
species of the same group, the more subject it is to anomalies in the individuals.
On the ordinary view of each species having been independently created, why should that part of the structure,
which differs from the same part in other independently created species of the same genus, be more variable than
those parts which are closely alike in the several species? I do not see that any explanation can be given. But on
the view that species are only strongly marked and fixed varieties, we might expect often to find them still
continuing to vary in those parts of their structure which have varied within a moderately recent period, and which
have thus come to differ. Or to state the case in another manner: the points in which all the species of a genus
resemble each other, and in which they differ from allied genera, are called generic characters; and these
characters may be attributed to inheritance from a common progenitor, for it can rarely have happened that natural
selection will have modified several distinct species, fitted to more or less widely different habits, in exactly the
same manner: and as these so−called generic characters have been inherited from before the period when the
several species first branched off from their common progenitor, and subsequently have not varied or come to
differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. On the
other hand, the points in which species differ from other species of the same genus are called specific characters;
and as these specific characters have varied and come to differ since the period when the species branched off
from a common progenitor, it is probable that they should still often be in some degree variable−−at least more
variable than those parts of the organisation which have for a very long period remained constant.
SECONDARY SEXUAL CHARACTERS VARIABLE.
I think it will be admitted by naturalists, without my entering on details, that secondary sexual characters are
highly variable. It will also be admitted that species of the same group differ from each other more widely in their
secondary sexual characters, than in other parts of their organisation; compare, for instance, the amount of
difference between the males of gallinaceous birds, in which secondary sexual characters are strongly displayed,
with the amount of difference between the females. The cause of the original variability of these characters is not
manifest; but we can see why they should not have been rendered as constant and uniform as others, for they are
accumulated by sexual selection, which is less rigid in its action than ordinary selection, as it does not entail
death, but only gives fewer offspring to the less favoured males. Whatever the cause may be of the variability of
secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the same group a greater amount of difference in these
than in other respects.
It is a remarkable fact, that the secondary differences between the two sexes of the same species are generally
displayed in the very same parts of the organisation in which the species of the same genus differ from each other.
Of this fact I will give in illustration the first two instances which happen to stand on my list; and as the
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The Origin of Species by means of Natural Selection
differences in these cases are of a very unusual nature, the relation can hardly be accidental. The same number of
joints in the tarsi is a character common to very large groups of beetles, but in the Engidae, as Westwood has
remarked, the number varies greatly and the number likewise differs in the two sexes of the same species. Again
in the fossorial hymenoptera, the neuration of the wings is a character of the highest importance, because common
to large groups; but in certain genera the neuration differs in the different species, and likewise in the two sexes of
the same species. Sir J. Lubbock has recently remarked, that several minute crustaceans offer excellent
illustrations of this law. "In Pontella, for instance, the sexual characters are afforded mainly by the anterior
antennae and by the fifth pair of legs: the specific differences also are principally given by these organs." This
relation has a clear meaning on my view: I look at all the species of the same genus as having as certainly
descended from the same progenitor, as have the two sexes of any one species. Consequently, whatever part of the
structure of the common progenitor, or of its early descendants, became variable; variations of this part would, it
is highly probable, be taken advantage of by natural and sexual selection, in order to fit the several places in the
economy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males to
struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species
from species, than of generic characters, or those which are possessed by all the species; that the frequent extreme
variability of any part which is developed in a species in an extraordinary manner in comparison with the same
part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if
it be common to a whole group of species; that the great variability of secondary sexual characters and their great
difference in closely allied species; that secondary sexual and ordinary specific differences are generally
displayed in the same parts of the organisation, are all principles closely connected together. All being mainly due
to the species of the same group being the descendants of a common progenitor, from whom they have inherited
much in common, to parts which have recently and largely varied being more likely still to go on varying than
parts which have long been inherited and have not varied, to natural selection having more or less completely,
according to the lapse of time, overmastered the tendency to reversion and to further variability, to sexual
selection being less rigid than ordinary selection, and to variations in the same parts having been accumulated by
natural and sexual selection, and thus having been adapted for secondary sexual, and for ordinary purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF ONE
SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR REVERTS TO
SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our domestic races. The most distinct breeds
of the pigeon, in countries widely apart, present sub−varieties with reversed feathers on the head, and with
feathers on the feet, characters not possessed by the aboriginal rock−pigeon; these then are analogous variations in
two or more distinct races. The frequent presence of fourteen or even sixteen tail−feathers in the pouter may be
considered as a variation representing the normal structure of another race, the fantail. I presume that no one will
doubt that all such analogous variations are due to the several races of the pigeon having inherited from a
common parent the same constitution and tendency to variation, when acted on by similar unknown influences. In
the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or as commonly called roots,
of the Swedish turnip and ruta−baga, plants which several botanists rank as varieties produced by cultivation from
a common parent: if this be not so, the case will then be one of analogous variation in two so−called distinct
species; and to these a third may be added, namely, the common turnip. According to the ordinary view of each
species having been independently created, we should have to attribute this similarity in the enlarged stems of
these three plants, not to the vera causa of community of descent, and a consequent tendency to vary in a like
manner, but to three separate yet closely related acts of creation. Many similar cases of analogous variation have
been observed by Naudin in the great gourd family, and by various authors in our cereals. Similar cases occurring
with insects under natural conditions have lately been discussed with much ability by Mr. Walsh, who has
grouped them under his law of equable variability.
With pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of
slaty−blue birds with two black bars on the wings, white loins, a bar at the end of the tail, with the outer feathers
externally edged near their bases with white. As all these marks are characteristic of the parent rock−pigeon, I
presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing
CHAPTER V. LAWS OF VARIATION. 71

The Origin of Species by means of Natural Selection
in the several breeds. We may, I think, confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring of two distinct and differently coloured
breeds; and in this case there is nothing in the external conditions of life to cause the reappearance of the
slaty−blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after having been lost for many, probably
for hundreds of generations. But when a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character to the foreign breed−−some say, for a
dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common
expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this remnant of foreign blood. In a breed which has not been crossed, but in which BOTH
parents have lost some character which their progenitor possessed, the tendency, whether strong or weak, to
reproduce the lost character might, as was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a character which has been lost in a breed, reappears
after a great number of generations, the most probable hypothesis is, not that one individual suddenly takes after
an ancestor removed by some hundred generations, but that in each successive generation the character in
question has been lying latent, and at last, under unknown favourable conditions, is developed. With the
barb−pigeon, for instance, which very rarely produces a blue bird, it is probable that there is a latent tendency in
each generation to produce blue plumage. The abstract improbability of such a tendency being transmitted
through a vast number of generations, is not greater than that of quite useless or rudimentary organs being
similarly transmitted. A mere tendency to produce a rudiment is indeed sometimes thus inherited.
As all the species of the same genus are supposed to be descended from a common progenitor, it might be
expected that they would occasionally vary in an analogous manner; so that the varieties of two or more species
would resemble each other, or that a variety of one species would resemble in certain characters another and
distinct species, this other species being, according to our view, only a well−marked and permanent variety. But
characters exclusively due to analogous variation would probably be of an unimportant nature, for the
preservation of all functionally important characters will have been determined through natural selection, in
accordance with the different habits of the species. It might further be expected that the species of the same genus
would occasionally exhibit reversions to long−lost characters. As, however, we do not know the common ancestor
of any natural group, we cannot distinguish between reversionary and analogous characters. If, for instance, we
did not know that the parent rock−pigeon was not feather−footed or turn−crowned, we could not have told,
whether such characters in our domestic breeds were reversions or only analogous variations; but we might have
inferred that the blue colour was a case of reversion from the number of the markings, which are correlated with
this tint, and which would not probably have all appeared together from simple variation. More especially we
might have inferred this from the blue colour and the several marks so often appearing when differently coloured
breeds are crossed. Hence, although under nature it must generally be left doubtful, what cases are reversions to
formerly existing characters, and what are new but analogous variations, yet we ought, on our theory, sometimes
to find the varying offspring of a species assuming characters which are already present in other members of the
same group. And this undoubtedly is the case.
The difficulty in distinguishing variable species is largely due to the varieties mocking, as it were, other
species of the same genus. A considerable catalogue, also, could be given of forms intermediate between two
other forms, which themselves can only doubtfully be ranked as species; and this shows, unless all these closely
allied forms be considered as independently created species, that they have in varying assumed some of the
characters of the others. But the best evidence of analogous variations is afforded by parts or organs which are
generally constant in character, but which occasionally vary so as to resemble, in some degree, the same part or
organ in an allied species. I have collected a long list of such cases; but here, as before, I lie under the great
disadvantage of not being able to give them. I can only repeat that such cases certainly occur, and seem to me
very remarkable.
I will, however, give one curious and complex case, not indeed as affecting any important character, but from
occurring in several species of the same genus, partly under domestication and partly under nature. It is a case
almost certainly of reversion. The ass sometimes has very distinct transverse bars on its legs, like those on the legs
of a zebra. It has been asserted that these are plainest in the foal, and from inquiries which I have made, I believe
CHAPTER V. LAWS OF VARIATION. 72

The Origin of Species by means of Natural Selection
this to be true. The stripe on the shoulder is sometimes double, and is very variable in length and outline. A white
ass, but NOT an albino, has been described without either spinal or shoulder stripe; and these stripes are
sometimes very obscure, or actually quite lost, in dark−coloured asses. The koulan of Pallas is said to have been
seen with a double shoulder−stripe. Mr. Blyth has seen a specimen of the hemionus with a distinct
shoulder−stripe, though it properly has none; and I have been informed by Colonel Poole that foals of this species
are generally striped on the legs and faintly on the shoulder. The quagga, though so plainly barred like a zebra
over the body, is without bars on the legs; but Dr. Gray has figured one specimen with very distinct zebra−like
bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct
breeds, and of ALL colours; transverse bars on the legs are not rare in duns, mouse−duns, and in one instance in a
chestnut; a faint shoulder−stripe may sometimes be seen in duns, and I have seen a trace in a bay horse. My son
made a careful examination and sketch for me of a dun Belgian cart−horse with a double stripe on each shoulder
and with leg−stripes. I have myself seen a dun Devonshire pony, and a small dun Welsh pony has been carefully
described to me, both with THREE parallel stripes on each shoulder.
In the northwest part of India the Kattywar breed of horses is so generally striped, that, as I hear from Colonel
Poole, who examined this breed for the Indian Government, a horse without stripes is not considered as purely
bred. The spine is always striped; the legs are generally barred; and the shoulder−stripe, which is sometimes
double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. The stripes are
often plainest in the foal; and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay
Kattywar horses striped when first foaled. I have also reason to suspect, from information given me by Mr. W.W.
Edwards, that with the English race−horse the spinal stripe is much commoner in the foal than in the full−grown
animal. I have myself recently bred a foal from a bay mare (offspring of a Turkoman horse and a Flemish mare)
by a bay English race−horse. This foal, when a week old, was marked on its hinder quarters and on its forehead
with numerous very narrow, dark, zebra−like bars, and its legs were feebly striped. All the stripes soon
disappeared completely. Without here entering on further details I may state that I have collected cases of leg and
shoulder stripes in horses of very different breeds in various countries from Britain to Eastern China; and from
Norway in the north to the Malay Archipelago in the south. In all parts of the world these stripes occur far
oftenest in duns and mouse−duns; by the term dun a large range of colour is included, from one between brown
and black to a close approach to cream colour.
I am aware that Colonel Hamilton Smith, who has written on this subject, believes that the several breeds of
the horse are descended from several aboriginal species, one of which, the dun, was striped; and that the
above−described appearances are all due to ancient crosses with the dun stock. But this view may be safely
rejected, for it is highly improbable that the heavy Belgian cart−horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world, should have all have been crossed with one
supposed aboriginal stock.
Now let us turn to the effects of crossing the several species of the horse genus. Rollin asserts that the
common mule from the ass and horse is particularly apt to have bars on its legs; according to Mr. Gosse, in certain
parts of the United States, about nine out of ten mules have striped legs. I once saw a mule with its legs so much
striped that any one might have thought that it was a hybrid zebra; and Mr. W.C. Martin, in his excellent treatise
on the horse, has given a figure of a similar mule. In four coloured drawings, which I have seen, of hybrids
between the ass and zebra, the legs were much more plainly barred than the rest of the body; and in one of them
there was a double shoulder−stripe. In Lord Morton's famous hybrid, from a chestnut mare and male quagga, the
hybrid and even the pure offspring subsequently produced from the same mare by a black Arabian sire, were
much more plainly barred across the legs than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs me that he knows of a second case) from
the ass and the hemionus; and this hybrid, though the ass only occasionally has stripes on his legs and the
hemionus has none and has not even a shoulder−stripe, nevertheless had all four legs barred, and had three short
shoulder−stripes, like those on the dun Devonshire and Welsh ponies, and even had some zebra−like stripes on
the sides of its face. With respect to this last fact, I was so convinced that not even a stripe of colour appears from
what is commonly called chance, that I was led solely from the occurrence of the face−stripes on this hybrid from
the ass and hemionus to ask Colonel Poole whether such face−stripes ever occurred in the eminently striped
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The Origin of Species by means of Natural Selection
Kattywar breed of horses, and was, as we have seen, answered in the affirmative.
What now are we to say to these several facts? We see several distinct species of the horse genus becoming,
by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. In the horse we see this
tendency strong whenever a dun tint appears−−a tint which approaches to that of the general colouring of the
other species of the genus. The appearance of the stripes is not accompanied by any change of form, or by any
other new character. We see this tendency to become striped most strongly displayed in hybrids from between
several of the most distinct species. Now observe the case of the several breeds of pigeons: they are descended
from a pigeon (including two or three sub−species or geographical races) of a bluish colour, with certain bars and
other marks; and when any breed assumes by simple variation a bluish tint, these bars and other marks invariably
reappear; but without any other change of form or character. When the oldest and truest breeds of various colours
are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels. I have
stated that the most probable hypothesis to account for the reappearance of very ancient characters, is−−that there
is a TENDENCY in the young of each successive generation to produce the long−lost character, and that this
tendency, from unknown causes, sometimes prevails. And we have just seen that in several species of the horse
genus the stripes are either plainer or appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how exactly parallel is the case with that of the
species of the horse genus! For myself, I venture confidently to look back thousands on thousands of generations,
and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of
our domestic horse (whether or not it be descended from one or more wild stocks) of the ass, the hemionus,
quagga, and zebra.
He who believes that each equine species was independently created, will, I presume, assert that each species
has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so
as often to become striped like the other species of the genus; and that each has been created with a strong
tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in
their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to
reject a real for an unreal, or at least for an unknown cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never
lived, but had been created in stone so as to mock the shells now living on the sea−shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign
any reason why this or that part has varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between varieties of the same species, and the
greater differences between species of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the
course of time, though we have not sufficient evidence on this head. Habit in producing constitutional
peculiarities, and use in strengthening, and disuse in weakening and diminishing organs, appear in many cases to
have been potent in their effects. Homologous parts tend to vary in the same manner, and homologous parts tend
to cohere. Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one
part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the
structure which can be saved without detriment will be saved. Changes of structure at an early age may affect
parts subsequently developed; and many cases of correlated variation, the nature of which we are unable to
understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps arising from such
parts not having been closely specialised for any particular function, so that their modifications have not been
closely checked by natural selection. It follows probably from this same cause, that organic beings low in the
scale are more variable than those standing higher in the scale, and which have their whole organisation more
specialised. Rudimentary organs, from being useless, are not regulated by natural selection, and hence are
variable. Specific characters−−that is, the characters which have come to differ since the several species of the
same genus branched off from a common parent−−are more variable than generic characters, or those which have
long been inherited, and have not differed within this same period. In these remarks we have referred to special
parts or organs being still variable, because they have recently varied and thus come to differ; but we have also
seen in the second chapter that the same principle applies to the whole individual; for in a district where many
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The Origin of Species by means of Natural Selection
species of a genus are found−−that is, where there has been much former variation and differentiation, or where
the manufactory of new specific forms has been actively at work−−in that district and among these species, we
now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters
differ much in the species of the same group. Variability in the same parts of the organisation has generally been
taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an
extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we can understand why it should often still
be variable in a much higher degree than other parts; for variation is a long−continued and slow process, and
natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with an extraordinarily developed organ has become the
parent of many modified descendants−−which on our view must be a very slow process, requiring a long lapse of
time−−in this case, natural selection has succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may have been developed. Species inheriting nearly the same constitution from a
common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same
species may occasionally revert to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous variation, such modifications will add to the
beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference between the offspring and their parents−−and a cause for
each must exist−−we have reason to believe that it is the steady accumulation of beneficial differences which has
given rise to all the more important modifications of structure in relation to the habits of each species.
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The Origin of Species by means of Natural Selection
CHAPTER VI. DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification −− Absence or rarity of transitional varieties −−
Transitions in habits of life −− Diversified habits in the same species −− Species with habits widely different from
those of their allies −− Organs of extreme perfection −− Modes of transition −− Cases of difficulty −− Natura non
facit saltum −− Organs of small importance −− Organs not in all cases absolutely perfect −− The law of Unity of
Type and of the Conditions of Existence embraced by the theory of Natural Selection.
Long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him.
Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered;
but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal
to the theory.
These difficulties and objections may be classed under the following heads: First, why, if species have
descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why
is not all nature in confusion, instead of the species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been
formed by the modification of some other animal with widely different habits and structure? Can we believe that
natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe,
which serves as a fly−flapper, and, on the other hand, an organ so wonderful as the eye?
Thirdly, can instincts be acquired and modified through natural selection? What shall we say to the instinct
which leads the bee to make cells, and which has practically anticipated the discoveries of profound
mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring,
whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads will be here discussed; some miscellaneous objections in the following chapter; Instinct
and Hybridism in the two succeeding chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable modifications, each new form will tend in a
fully−stocked country to take the place of, and finally to exterminate, its own less improved parent−form and
other less−favoured forms with which it comes into competition. Thus extinction and natural selection go hand in
hand. Hence, if we look at each species as descended from some unknown form, both the parent and all the
transitional varieties will generally have been exterminated by the very process of the formation and perfection of
the new form.
But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded
in countless numbers in the crust of the earth? It will be more convenient to discuss this question in the chapter on
the imperfection of the geological record; and I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed. The crust of the earth is a vast museum; but
the natural collections have been imperfectly made, and only at long intervals of time.
But it may be urged that when several closely allied species inhabit the same territory, we surely ought to find
at the present time many transitional forms. Let us take a simple case: in travelling from north to south over a
continent, we generally meet at successive intervals with closely allied or representative species, evidently filling
nearly the same place in the natural economy of the land. These representative species often meet and interlock;
and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other.
But if we compare these species where they intermingle, they are generally as absolutely distinct from each other
in every detail of structure as are specimens taken from the metropolis inhabited by each. By my theory these
allied species are descended from a common parent; and during the process of modification, each has become
adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent−form
and all the transitional varieties between its past and present states. Hence we ought not to expect at the present
time to meet with numerous transitional varieties in each region, though they must have existed there, and may be
embedded there in a fossil condition. But in the intermediate region, having intermediate conditions of life, why
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The Origin of Species by means of Natural Selection
do we not now find closely−linking intermediate varieties? This difficulty for a long time quite confounded me.
But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an area is now continuous, that it has
been continuous during a long period. Geology would lead us to believe that most continents have been broken up
into islands even during the later tertiary periods; and in such islands distinct species might have been separately
formed without the possibility of intermediate varieties existing in the intermediate zones. By changes in the form
of the land and of climate, marine areas now continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over this way of escaping from the difficulty; for
I believe that many perfectly defined species have been formed on strictly continuous areas; though I do not doubt
that the formerly broken condition of areas now continuous, has played an important part in the formation of new
species, more especially with freely−crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we generally find them tolerably numerous
over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally disappearing.
Hence the neutral territory between two representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains, and sometimes it is quite remarkable how
abruptly, as Alph. De Candolle has observed, a common alpine species disappears. The same fact has been
noticed by E. Forbes in sounding the depths of the sea with the dredge. To those who look at climate and the
physical conditions of life as the all−important elements of distribution, these facts ought to cause surprise, as
climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all
either prey on or serve as prey for others; in short, that each organic being is either directly or indirectly related in
the most important manner to other organic beings−−we see that the range of the inhabitants of any country by no
means exclusively depends on insensibly changing physical conditions, but in large part on the presence of other
species, on which it lives, or by which it is destroyed, or with which it comes into competition; and as these
species are already defined objects, not blending one into another by insensible gradations, the range of any one
species, depending as it does on the range of others, will tend to be sharply defined. Moreover, each species on
the confines of its range, where it exists in lessened numbers, will, during fluctuations in the number of its
enemies or of its prey, or in the nature of the seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
As allied or representative species, when inhabiting a continuous area, are generally distributed in such a
manner that each has a wide range, with a comparatively narrow neutral territory between them, in which they
become rather suddenly rarer and rarer; then, as varieties do not essentially differ from species, the same rule will
probably apply to both; and if we take a varying species inhabiting a very large area, we shall have to adapt two
varieties to two large areas, and a third variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as I can
make out, this rule holds good with varieties in a state of nature. I have met with striking instances of the rule in
the case of varieties intermediate between well−marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when varieties
intermediate between two other forms occur, they are much rarer numerically than the forms which they connect.
Now, if we may trust these facts and inferences, and conclude that varieties linking two other varieties together
generally have existed in lesser numbers than the forms which they connect, then we can understand why
intermediate varieties should not endure for very long periods: why, as a general rule, they should be exterminated
and disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a greater chance of being
exterminated than one existing in large numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on both sides of it. But it is a far more important
consideration, that during the process of further modification, by which two varieties are supposed to be
converted and perfected into two distinct species, the two which exist in larger numbers, from inhabiting larger
areas, will have a great advantage over the intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have a better chance, within any given period, of
presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in
CHAPTER VI. DIFFICULTIES OF THE THEORY. 77

The Origin of Species by means of Natural Selection
lesser numbers. Hence, the more common forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved. It is the same principle which, as I believe,
accounts for the common species in each country, as shown in the second chapter, presenting on an average a
greater number of well−marked varieties than do the rarer species. I may illustrate what I mean by supposing
three varieties of sheep to be kept, one adapted to an extensive mountainous region; a second to a comparatively
narrow, hilly tract; and a third to the wide plains at the base; and that the inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances in this case will be strongly in favour of the
great holders on the mountains or on the plains improving their breeds more quickly than the small holders on the
intermediate narrow, hilly tract; and consequently the improved mountain or plain breed will soon take the place
of the less improved hill breed; and thus the two breeds, which originally existed in greater numbers, will come
into close contact with each other, without the interposition of the supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably well−defined objects, and do not at any one period
present an inextricable chaos of varying and intermediate links: first, because new varieties are very slowly
formed, for variation is a slow process, and natural selection can do nothing until favourable individual
differences or variations occur, and until a place in the natural polity of the country can be better filled by some
modification of some one or more of its inhabitants. And such new places will depend on slow changes of
climate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree, on
some of the old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting
and reacting on each other. So that, in any one region and at any one time, we ought to see only a few species
presenting slight modifications of structure in some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent period as isolated portions, in
which many forms, more especially among the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative species. In this case, intermediate varieties
between the several representative species and their common parent, must formerly have existed within each
isolated portion of the land, but these links during the process of natural selection will have been supplanted and
exterminated, so that they will no longer be found in a living state.
Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area,
intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to
connect. From this cause alone the intermediate varieties will be liable to accidental extermination; and during the
process of further modification through natural selection, they will almost certainly be beaten and supplanted by
the forms which they connect; for these, from existing in greater numbers will, in the aggregate, present more
varieties, and thus be further improved through natural selection and gain further advantages.
Lastly, looking not to any one time, but at all time, if my theory be true, numberless intermediate varieties,
linking closely together all the species of the same group, must assuredly have existed; but the very process of
natural selection constantly tends, as has been so often remarked, to exterminate the parent forms and the
intermediate links. Consequently evidence of their former existence could be found only among fossil remains,
which are preserved, as we shall attempt to show in a future chapter, in an extremely imperfect and intermittent
record.
ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for instance, could a land carnivorous animal
have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?
It would be easy to show that there now exist carnivorous animals presenting close intermediate grades from
strictly terrestrial to aquatic habits; and as each exists by a struggle for life, it is clear that each must be well
adapted to its place in nature. Look at the Mustela vison of North America, which has webbed feet, and which
resembles an otter in its fur, short legs, and form of tail; during summer this animal dives for and preys on fish,
but during the long winter it leaves the frozen waters, and preys, like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an insectivorous quadruped could possibly have been
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The Origin of Species by means of Natural Selection
converted into a flying bat, the question would have been far more difficult to answer. Yet I think such difficulties
have little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for, out of the many striking cases which I have
collected, I can give only one or two instances of transitional habits and structures in allied species; and of
diversified habits, either constant or occasional, in the same species. And it seems to me that nothing less than a
long list of such cases is sufficient to lessen the difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly
flattened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide
and with the skin on their flanks rather full, to the so−called flying squirrels; and flying squirrels have their limbs
and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to
glide through the air to an astonishing distance from tree to tree. We cannot doubt that each structure is of use to
each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to collect food more
quickly, or, as there is reason to believe, to lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is possible to conceive under all possible conditions.
Let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones
become modified, and all analogy would lead us to believe that some, at least, of the squirrels would decrease in
numbers or become exterminated, unless they also become modified and improved in structure in a corresponding
manner. Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank−membranes, each modification being useful, each being
propagated, until, by the accumulated effects of this process of natural selection, a perfect so−called flying
squirrel was produced.
Now look at the Galeopithecus or so−called flying lemur, which was formerly ranked among bats, but is now
believed to belong to the Insectivora. An extremely wide flank−membrane stretches from the corners of the jaw to
the tail, and includes the limbs with the elongated fingers. This flank− membrane is furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Insectivora, yet there is no difficulty in supposing that such links formerly existed,
and that each was developed in the same manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any insuperable difficulty in further believing it
possible that the membrane−connected fingers and fore−arm of the Galeopithecus might have been greatly
lengthened by natural selection; and this, as far as the organs of flight are concerned, would have converted the
animal into a bat. In certain bats in which the wing−membrane extends from the top of the shoulder to the tail and
includes the hind−legs, we perhaps see traces of an apparatus originally fitted for gliding through the air rather
than for flight.
If about a dozen genera of birds were to become extinct, who would have ventured to surmise that birds might
have existed which used their wings solely as flappers, like the logger headed duck (Micropterus of Eyton); as
fins in the water and as front legs on the land, like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the apteryx? Yet the structure of each of these birds is good for it, under the conditions of life to
which it is exposed, for each has to live by a struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the grades of wing−structure here alluded to,
which perhaps may all be the result of disuse, indicate the steps by which birds actually acquired their perfect
power of flight; but they serve to show what diversified means of transition are at least possible.
Seeing that a few members of such water−breathing classes as the Crustacea and Mollusca are adapted to live
on the land; and seeing that we have flying birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying−fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. If this had
been effected, who would have ever imagined that in an early transitional state they had been inhabitants of the
open ocean, and had used their incipient organs of flight exclusively, so far as we know, to escape being devoured
by other fish?
When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we
should bear in mind that animals displaying early transitional grades of the structure will seldom have survived to
the present day, for they will have been supplanted by their successors, which were gradually rendered more
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The Origin of Species by means of Natural Selection
perfect through natural selection. Furthermore, we may conclude that transitional states between structures fitted
for very different habits of life will rarely have been developed at an early period in great numbers and under
many subordinate forms. Thus, to return to our imaginary illustration of the flying−fish, it does not seem probable
that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of
many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other animals in the battle for life. Hence the
chance of discovering species with transitional grades of structure in a fossil condition will always be less, from
their having existed in lesser numbers, than in the case of species with fully developed structures.
I will now give two or three instances, both of diversified and of changed habits, in the individuals of the same
species. In either case it would be easy for natural selection to adapt the structure of the animal to its changed
habits, or exclusively to one of its several habits. It is, however, difficult to decide and immaterial for us, whether
habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed
habits; both probably often occurring almost simultaneously. Of cases of changed habits it will suffice merely to
allude to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances.
Of diversified habits innumerable instances could be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot and then proceeding to another, like a kestrel, and at other
times standing stationary on the margin of water, and then dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper; it sometimes, like
a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the seeds of
the yew on a branch, and thus breaking them like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water.
As we sometimes see individuals following habits different from those proper to their species and to the other
species of the same genus, we might expect that such individuals would occasionally give rise to new species,
having anomalous habits, and with their structure either slightly or considerably modified from that of their type.
And such instances occur in nature. Can a more striking instance of adaptation be given than that of a woodpecker
for climbing trees and seizing insects in the chinks of the bark? Yet in North America there are woodpeckers
which feed largely on fruit, and others with elongated wings which chase insects on the wing. On the plains of La
Plata, where hardly a tree grows, there is a woodpecker (Colaptes campestris) which has two toes before and two
behind, a long− pointed tongue, pointed tail−feathers, sufficiently stiff to support the bird in a vertical position on
a post, but not so stiff as in the typical wood−peckers, and a straight, strong beak. The beak, however, is not so
straight or so strong as in the typical woodpeckers but it is strong enough to bore into wood. Hence this Colaptes,
in all the essential parts of its structure, is a woodpecker. Even in such trifling characters as the colouring, the
harsh tone of the voice, and undulatory flight, its close blood−relationship to our common woodpecker is plainly
declared; yet, as I can assert, not only from my own observations, but from those of the accurate Azara, in certain
large districts it does not climb trees, and it makes its nest in holes in banks! In certain other districts, however,
this same woodpecker, as Mr. Hudson states, frequents trees, and bores holes in the trunk for its nest. I may
mention as another illustration of the varied habits of this genus, that a Mexican Colaptes has been described by
De Saussure as boring holes into hard wood in order to lay up a store of acorns.
Petrels are the most aerial and oceanic of birds, but, in the quiet sounds of Tierra del Fuego, the Puffinuria
berardi, in its general habits, in its astonishing power of diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for an auk or a grebe; nevertheless, it is essentially a petrel, but
with many parts of its organisation profoundly modified in relation to its new habits of life; whereas the
woodpecker of La Plata has had its structure only slightly modified. In the case of the water−ouzel, the acutest
observer, by examining its dead body, would never have suspected its sub−aquatic habits; yet this bird, which is
allied to the thrush family, subsists by diving,−−using its wings under water and grasping stones with its feet. All
the members of the great order of Hymenopterous insects are terrestrial, excepting the genus Proctotrupes, which
Sir John Lubbock has discovered to be aquatic in its habits; it often enters the water and dives about by the use
not of its legs but of its wings, and remains as long as four hours beneath the surface; yet it exhibits no
modification in structure in accordance with its abnormal habits.
He who believes that each being has been created as we now see it, must occasionally have felt surprise when
he has met with an animal having habits and structure not in agreement. What can be plainer than that the webbed
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The Origin of Species by means of Natural Selection
feet of ducks and geese are formed for swimming? Yet there are upland geese with webbed feet which rarely go
near the water; and no one except Audubon, has seen the frigate−bird, which has all its four toes webbed, alight
on the surface of the ocean. On the other hand, grebes and coots are eminently aquatic, although their toes are
only bordered by membrane. What seems plainer than that the long toes, not furnished with membrane, of the
Grallatores, are formed for walking over swamps and floating plants. The water−hen and landrail are members of
this order, yet the first is nearly as aquatic as the coot, and the second is nearly as terrestrial as the quail or
partridge. In such cases, and many others could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become almost rudimentary in function,
though not in structure. In the frigate−bird, the deeply scooped membrane between the toes shows that structure
has begun to change.
He who believes in separate and innumerable acts of creation may say, that in these cases it has pleased the
Creator to cause a being of one type to take the place of one belonging to another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers;
and that if any one being varies ever so little, either in habits or structure, and thus gains an advantage over some
other inhabitant of the same country, it will seize on the place of that inhabitant, however different that may be
from its own place. Hence it will cause him no surprise that there should be geese and frigate−birds with webbed
feet, living on the dry land and rarely alighting on the water, that there should be long−toed corncrakes, living in
meadows instead of in swamps; that there should be woodpeckers where hardly a tree grows; that there should be
diving thrushes and diving Hymenoptera, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for
admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been
formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the
sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old
saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if
numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each
grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be
inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing
conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a
nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark
that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does
not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into
nerves, endowed with this special sensibility.
In searching for the gradations through which an organ in any species has been perfected, we ought to look
exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to look to other species
and genera of the same group, that is to the collateral descendants from the same parent−form, in order to see
what gradations are possible, and for the chance of some gradations having been transmitted in an unaltered or
little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps
by which it has been perfected.
The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment−cells and
covered by translucent skin, but without any lens or other refractive body. We may, however, according to M.
Jourdain, descend even a step lower and find aggregates of pigment−cells, apparently serving as organs of vision,
without any nerves, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of
distinct vision, and serve only to distinguish light from darkness. In certain star−fishes, small depressions in the
layer of pigment which surrounds the nerve are filled, as described by the author just quoted, with transparent
gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this
serves not to form an image, but only to concentrate the luminous rays and render their perception more easy. In
this concentration of the rays we gain the first and by far the most important step towards the formation of a true,
picture−forming eye; for we have only to place the naked extremity of the optic nerve, which in some of the lower
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The Origin of Species by means of Natural Selection
animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating
apparatus, and an image will be formed on it.
In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter
sometimes forming a sort of pupil, but destitute of lens or other optical contrivance. With insects it is now known
that the numerous facets on the cornea of their great compound eyes form true lenses, and that the cones include
curiously modified nervous filaments. But these organs in the Articulata are so much diversified that Muller
formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and
graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of
all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very
great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with
pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any
member of the Articulata class.
He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that
large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural
selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in
this case he does not know the transitional states. It has been objected that in order to modify the eye and still
preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed,
could not be done through natural selection; but as I have attempted to show in my work on the variation of
domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if they were
extremely slight and gradual. Different kinds of modification would, also, serve for the same general purpose: as
Mr. Wallace has remarked, "If a lens has too short or too long a focus, it may be amended either by an alteration
of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then
any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular
movements of the eye are neither of them essential to vision, but only improvements which might have been
added and perfected at any stage of the construction of the instrument." Within the highest division of the animal
kingdom, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little
sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus. In
fishes and reptiles, as Owen has remarked, "The range of gradation of dioptric structures is very great." It is a
significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is
formed in the embryo by an accumulation of epidermic cells, lying in a sack−like fold of the skin; and the vitreous
body is formed from embryonic subcutaneous tissue. To arrive, however, at a just conclusion regarding the
formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the
reason should conquer the imagination; but I have felt the difficulty far to keenly to be surprised at others
hesitating to extend the principle of natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been
perfected by the long−continued efforts of the highest human intellects; and we naturally infer that the eye has
been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right
to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with
fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually
changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different
distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose
that there is a power, represented by natural selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any
way or degree, tends to produce a distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is produced, and then the old ones to be all destroyed.
In living bodies, variation will cause the slight alteration, generation will multiply them almost infinitely, and
natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years;
and during each year on millions of individuals of many kinds; and may we not believe that a living optical
instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
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The Origin of Species by means of Natural Selection
MODES Of TRANSITION.
If it could be demonstrated that any complex organ existed, which could not possibly have been formed by
numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such
case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to
much−isolated species, around which, according to the theory, there has been much extinction. Or again, if we
take an organ common to all the members of a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed, we should have to look to very ancient
ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have been formed by transitional
gradations of some kind. Numerous cases could be given among the lower animals of the same organ performing
at the same time wholly distinct functions; thus in the larva of the dragon−fly and in the fish Cobites the
alimentary canal respires, digests, and excretes. In the Hydra, the animal may be turned inside out, and the
exterior surface will then digest and the stomach respire. In such cases natural selection might specialise, if any
advantage were thus gained, the whole or part of an organ, which had previously performed two functions, for one
function alone, and thus by insensible steps greatly change its nature. Many plants are known which regularly
produce at the same time differently constructed flowers; and if such plants were to produce one kind alone, a
great change would be effected with comparative suddenness in the character of the species. It is, however,
probable that the two sorts of flowers borne by the same plant were originally differentiated by finely graduated
steps, which may still be followed in some few cases.
Again, two distinct organs, or the same organ under two very different forms, may simultaneously perform in
the same individual the same function, and this is an extremely important means of transition: to give one
instance−−there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that
they breathe free air in their swim−bladders, this latter organ being divided by highly vascular partitions and
having a ductus pneumaticus for the supply of air. To give another instance from the vegetable kingdom: plants
climb by three distinct means, by spirally twining, by clasping a support with their sensitive tendrils, and by the
emission of aerial rootlets; these three means are usually found in distinct groups, but some few species exhibit
two of the means, or even all three, combined in the same individual. In all such cases one of the two organs
might readily be modified and perfected so as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be modified for some other and quite distinct
purpose, or be wholly obliterated.
The illustration of the swim−bladder in fishes is a good one, because it shows us clearly the highly important
fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a widely
different purpose, namely respiration. The swim−bladder has, also, been worked in as an accessory to the auditory
organs of certain fishes. All physiologists admit that the swim−bladder is homologous, or "ideally similar" in
position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the
swim− bladder has actually been converted into lungs, or an organ used exclusively for respiration.
According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary
generation from an ancient and unknown prototype which was furnished with a floating apparatus or
swim−bladder. We can thus, as I infer from Professor Owen's interesting description of these parts, understand the
strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea,
with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed.
In the higher Vertebrata the branchiae have wholly disappeared−−but in the embryo the slits on the sides of the
neck and the loop−like course of the arteries still mark their former position. But it is conceivable that the now
utterly lost branchiae might have been gradually worked in by natural selection for some distinct purpose: for
instance, Landois has shown that the wings of insects are developed from the trachea; it is therefore highly
probable that in this great class organs which once served for respiration have been actually converted into organs
for flight.
In considering transitions of organs, it is so important to bear in mind the probability of conversion from one
function to another, that I will give another instance. Pedunculated cirripedes have two minute folds of skin,
called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until
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they are hatched within the sack. These cirripedes have no branchiae, the whole surface of the body and of the
sack, together with the small frena, serving for respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the sack, within the well−enclosed shell; but they
have, in the same relative position with the frena, large, much−folded membranes, which freely communicate
with the circulatory lacunae of the sack and body, and which have been considered by all naturalists to act as
branchiae. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with
the branchiae of the other family; indeed, they graduate into each other. Therefore it need not be doubted that the
two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the
act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase
in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and
they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the
branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the
sack?
There is another possible mode of transition, namely, through the acceleration or retardation of the period of
reproduction. This has lately been insisted on by Professor Cope and others in the United States. It is now known
that some animals are capable of reproduction at a very early age, before they have acquired their perfect
characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage
of development would sooner or later be lost; and in this case, especially if the larva differed much from the
mature form, the character of the species would be greatly changed and degraded. Again, not a few animals, after
arriving at maturity, go on changing in character during nearly their whole lives. With mammals, for instance, the
form of the skull is often much altered with age, of which Dr. Murie has given some striking instances with seals.
Every one knows how the horns of stags become more and more branched, and the plumes of some birds become
more finely developed, as they grow older. Professor Cope states that the teeth of certain lizards change much in
shape with advancing years. With crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases− −and many could be given−−if the age for
reproduction were retarded, the character of the species, at least in its adult state, would be modified; nor is it
improbable that the previous and earlier stages of development would in some cases be hurried through and
finally lost. Whether species have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is probable that the differences between the young
and the mature, and between the mature and the old, were primordially acquired by graduated steps.
SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.
Although we must be extremely cautious in concluding that any organ could not have been produced by
successive, small, transitional gradations, yet undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often differently constructed from either the males
or fertile females; but this case will be treated of in the next chapter. The electric organs of fishes offer another
case of special difficulty; for it is impossible to conceive by what steps these wondrous organs have been
produced. But this is not surprising, for we do not even know of what use they are. In the gymnotus and torpedo
they no doubt serve as powerful means of defence, and perhaps for securing prey; yet in the ray, as observed by
Matteucci, an analogous organ in the tail manifests but little electricity, even when the animal is greatly irritated;
so little that it can hardly be of any use for the above purposes. Moreover, in the ray, besides the organ just
referred to, there is, as Dr. R. McDonnell has shown, another organ near the head, not known to be electrical, but
which appears to be the real homologue of the electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the
nerves, and in the manner in which they are acted on by various reagents. It should, also, be especially observed
that muscular contraction is accompanied by an electrical discharge; and, as Dr. Radcliffe insists, "in the electrical
apparatus of the torpedo during rest, there would seem to be a charge in every respect like that which is met with
in muscle and nerve during the rest, and the discharge of the torpedo, instead of being peculiar, may be only
another form of the discharge which attends upon the action of muscle and motor nerve." Beyond this we cannot
at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know
nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold
to maintain that no serviceable transitions are possible by which these organs might have been gradually
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The Origin of Species by means of Natural Selection
developed.
These organs appear at first to offer another and far more serious difficulty; for they occur in about a dozen
kinds of fish, of which several are widely remote in their affinities. When the same organ is found in several
members of the same class, especially if in members having very different habits of life, we may generally
attribute its presence to inheritance from a common ancestor; and its absence in some of the members to loss
through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient
progenitor, we might have expected that all electric fishes would have been specially related to each other; but
this is far from the case. Nor does geology at all lead to the belief that most fishes formerly possessed electric
organs, which their modified descendants have now lost. But when we look at the subject more closely, we find in
the several fishes provided with electric organs, that these are situated in different parts of the body, that they
differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by
which the electricity is excited−−and lastly, in being supplied with nerves proceeding from different sources, and
this is perhaps the most important of all the differences. Hence in the several fishes furnished with electric organs,
these cannot be considered as homologous, but only as analogous in function. Consequently there is no reason to
suppose that they have been inherited from a common progenitor; for had this been the case they would have
closely resembled each other in all respects. Thus the difficulty of an organ, apparently the same, arising in
several remotely allied species, disappears, leaving only the lesser yet still great difficulty: namely, by what
graduated steps these organs have been developed in each separate group of fishes.
The luminous organs which occur in a few insects, belonging to widely different families, and which are
situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be given; for instance in plants, the very curious
contrivance of a mass of pollen−grains, borne on a foot−stalk with an adhesive gland, is apparently the same in
Orchis and Asclepias, genera almost as remote as is possible among flowering plants; but here again the parts are
not homologous. In all cases of beings, far removed from each other in the scale of organisation, which are
furnished with similar and peculiar organs, it will be found that although the general appearance and function of
the organs may be the same, yet fundamental differences between them can always be detected. For instance, the
eyes of Cephalopods or cuttle−fish and of vertebrate animals appear wonderfully alike; and in such widely
sundered groups no part of this resemblance can be due to inheritance from a common progenitor. Mr. Mivart has
advanced this case as one of special difficulty, but I am unable to see the force of his argument. An organ for
vision must be formed of transparent tissue, and must include some sort of lens for throwing an image at the back
of a darkened chamber. Beyond this superficial resemblance, there is hardly any real similarity between the eyes
of cuttle−fish and vertebrates, as may be seen by consulting Hensen's admirable memoir on these organs in the
Cephalopoda. It is impossible for me here to enter on details, but I may specify a few of the points of difference.
The crystalline lens in the higher cuttle−fish consists of two parts, placed one behind the other like two lenses,
both having a very different structure and disposition to what occurs in the vertebrata. The retina is wholly
different, with an actual inversion of the elemental parts, and with a large nervous ganglion included within the
membranes of the eye. The relations of the muscles are as different as it is possible to conceive, and so in other
points. Hence it is not a little difficult to decide how far even the same terms ought to be employed in describing
the eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one to deny that the eye in either case
could have been developed through the natural selection of successive slight variations; but if this be admitted in
the one case it is clearly possible in the other; and fundamental differences of structure in the visual organs of two
groups might have been anticipated, in accordance with this view of their manner of formation. As two men have
sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural
selection, working for the good of each being, and taking advantage of all favourable variations, has produced
similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in
common to inheritance from a common progenitor.
Fritz Muller, in order to test the conclusions arrived at in this volume, has followed out with much care a
nearly similar line of argument. Several families of crustaceans include a few species, possessing an air−breathing
apparatus and fitted to live out of the water. In two of these families, which were more especially examined by
Muller, and which are nearly related to each other, the species agree most closely in all important characters:
namely in their sense organs, circulating systems, in the position of the tufts of hair within their complex
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stomachs, and lastly in the whole structure of the water−breathing branchiae, even to the microscopical hooks by
which they are cleansed. Hence it might have been expected that in the few species belonging to both families
which live on the land, the equally important air−breathing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have been made to differ, while all the other important organs
were closely similar, or rather, identical.
Fritz Muller argues that this close similarity in so many points of structure must, in accordance with the views
advanced by me, be accounted for by inheritance from a common progenitor. But as the vast majority of the
species in the above two families, as well as most other crustaceans, are aquatic in their habits, it is improbable in
the highest degree that their common progenitor should have been adapted for breathing air. Muller was thus led
carefully to examine the apparatus in the air−breathing species; and he found it to differ in each in several
important points, as in the position of the orifices, in the manner in which they are opened and closed, and in
some accessory details. Now such differences are intelligible, and might even have been expected, on the
supposition that species belonging to distinct families had slowly become adapted to live more and more out of
water, and to breathe the air. For these species, from belonging to distinct families, would have differed to a
certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz.,
the nature of the organism and that of the surrounding conditions, their variability assuredly would not have been
exactly the same. Consequently natural selection would have had different materials or variations to work on, in
order to arrive at the same functional result; and the structures thus acquired would almost necessarily have
differed. On the hypothesis of separate acts of creation the whole case remains unintelligible. This line of
argument seems to have had great weight in leading Fritz Muller to accept the views maintained by me in this
volume.
Another distinguished zoologist, the late Professor Claparede, has argued in the same manner, and has arrived
at the same result. He shows that there are parasitic mites (Acaridae), belonging to distinct sub−families and
families, which are furnished with hair−claspers. These organs must have been independently developed, as they
could not have been inherited from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips, and of appendages on the under side of the
hind part of the body.
In the foregoing cases, we see the same end gained and the same function performed, in beings not at all or
only remotely allied, by organs in appearance, though not in development, closely similar. On the other hand, it is
a common rule throughout nature that the same end should be gained, even sometimes in the case of closely
related beings, by the most diversified means. How differently constructed is the feathered wing of a bird and the
membrane−covered wing of a bat; and still more so the four wings of a butterfly, the two wings of a fly, and the
two wings with the elytra of a beetle. Bivalve shells are made to open and shut, but on what a number of patterns
is the hinge constructed, from the long row of neatly interlocking teeth in a Nucula to the simple ligament of a
Mussel! Seeds are disseminated by their minuteness, by their capsule being converted into a light balloon−like
envelope, by being embedded in pulp or flesh, formed of the most diverse parts, and rendered nutritious, as well
as conspicuously coloured, so as to attract and be devoured by birds, by having hooks and grapnels of many kinds
and serrated awns, so as to adhere to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by every breeze. I will give one other
instance: for this subject of the same end being gained by the most diversified means well deserves attention.
Some authors maintain that organic beings have been formed in many ways for the sake of mere variety, almost
like toys in a shop, but such a view of nature is incredible. With plants having separated sexes, and with those in
which, though hermaphrodites, the pollen does not spontaneously fall on the stigma, some aid is necessary for
their fertilisation. With several kinds this is effected by the pollen−grains, which are light and incoherent, being
blown by the wind through mere chance on to the stigma; and this is the simplest plan which can well be
conceived. An almost equally simple, though very different plan occurs in many plants in which a symmetrical
flower secretes a few drops of nectar, and is consequently visited by insects; and these carry the pollen from the
anthers to the stigma.
>From this simple stage we may pass through an inexhaustible number of contrivances, all for the same
purpose and effected in essentially the same manner, but entailing changes in every part of the flower. The nectar
may be stored in variously shaped receptacles, with the stamens and pistils modified in many ways, sometimes
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forming trap−like contrivances, and sometimes capable of neatly adapted movements through irritability or
elasticity. From such structures we may advance till we come to such a case of extraordinary adaptation as that
lately described by Dr. Cruger in the Coryanthes. This orchid has part of its labellum or lower lip hollowed out
into a great bucket, into which drops of almost pure water continually fall from two secreting horns which stand
above it; and when the bucket is half−full, the water overflows by a spout on one side. The basal part of the
labellum stands over the bucket, and is itself hollowed out into a sort of chamber with two lateral entrances;
within this chamber there are curious fleshy ridges. The most ingenious man, if he had not witnessed what takes
place, could never have imagined what purpose all these parts serve. But Dr. Cruger saw crowds of large
humble−bees visiting the gigantic flowers of this orchid, not in order to suck nectar, but to gnaw off the ridges
within the chamber above the bucket; in doing this they frequently pushed each other into the bucket, and their
wings being thus wetted they could not fly away, but were compelled to crawl out through the passage formed by
the spout or overflow. Dr. Cruger saw a "continual procession" of bees thus crawling out of their involuntary bath.
The passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back
against the viscid stigma and then against the viscid glands of the pollen−masses. The pollen−masses are thus
glued to the back of the bee which first happens to crawl out through the passage of a lately expanded flower, and
are thus carried away. Dr. Cruger sent me a flower in spirits of wine, with a bee which he had killed before it had
quite crawled out, with a pollen−mass still fastened to its back. When the bee, thus provided, flies to another
flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by
the passage, the pollen−mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the
flower is fertilised. Now at last we see the full use of every part of the flower, of the water−secreting horns of the
bucket half−full of water, which prevents the bees from flying away, and forces them to crawl out through the
spout, and rub against the properly placed viscid pollen−masses and the viscid stigma.
The construction of the flower in another closely allied orchid, namely, the Catasetum, is widely different,
though serving the same end; and is equally curious. Bees visit these flowers, like those of the Coryanthes, in
order to gnaw the labellum; in doing this they inevitably touch a long, tapering, sensitive projection, or, as I have
called it, the antenna. This antenna, when touched, transmits a sensation or vibration to a certain membrane which
is instantly ruptured; this sets free a spring by which the pollen−mass is shot forth, like an arrow, in the right
direction, and adheres by its viscid extremity to the back of the bee. The pollen−mass of the male plant (for the
sexes are separate in this orchid) is thus carried to the flower of the female plant, where it is brought into contact
with the stigma, which is viscid enough to break certain elastic threads, and retain the pollen, thus effecting
fertilisation.
How, it may be asked, in the foregoing and in innumerable other instances, can we understand the graduated
scale of complexity and the multifarious means for gaining the same end. The answer no doubt is, as already
remarked, that when two forms vary, which already differ from each other in some slight degree, the variability
will not be of the same exact nature, and consequently the results obtained through natural selection for the same
general purpose will not be the same. We should also bear in mind that every highly developed organism has
passed through many changes; and that each modified structure tends to be inherited, so that each modification
will not readily be quite lost, but may be again and again further altered. Hence, the structure of each part of each
species, for whatever purpose it may serve, is the sum of many inherited changes, through which the species has
passed during its successive adaptations to changed habits and conditions of life.
Finally, then, although in many cases it is most difficult even to conjecture by what transitions organs could
have arrived at their present state; yet, considering how small the proportion of living and known forms is to the
extinct and unknown, I have been astonished how rarely an organ can be named, towards which no transitional
grade is known to lead. It is certainly true, that new organs appearing as if created for some special purpose rarely
or never appear in any being; as indeed is shown by that old, but somewhat exaggerated, canon in natural history
of "Natura non facit saltum." We meet with this admission in the writings of almost every experienced naturalist;
or, as Milne Edwards has well expressed it, "Nature is prodigal in variety, but niggard in innovation." Why, on the
theory of Creation, should there be so much variety and so little real novelty? Why should all the parts and organs
of many independent beings, each supposed to have been separately created for its own proper place in nature, be
so commonly linked together by graduated steps? Why should not Nature take a sudden leap from structure to
structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection
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The Origin of Species by means of Natural Selection
acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must
advance by the short and sure, though slow steps.
ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.
As natural selection acts by life and death, by the survival of the fittest, and by the destruction of the less
well−fitted individuals, I have sometimes felt great difficulty in understanding the origin or formation of parts of
little importance; almost as great, though of a very different kind, as in the case of the most perfect and complex
organs.
In the first place, we are much too ignorant in regard to the whole economy of any one organic being to say
what slight modifications would be of importance or not. In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the colour of its flesh, the colour of the skin and hair of
quadrupeds, which, from being correlated with constitutional differences, or from determining the attacks of
insects, might assuredly be acted on by natural selection. The tail of the giraffe looks like an artificially
constructed fly−flapper; and it seems at first incredible that this could have been adapted for its present purpose
by successive slight modifications, each better and better fitted, for so trifling an object as to drive away flies; yet
we should pause before being too positive even in this case, for we know that the distribution and existence of
cattle and other animals in South America absolutely depend on their power of resisting the attacks of insects: so
that individuals which could by any means defend themselves from these small enemies, would be able to range
into new pastures and thus gain a great advantage. It is not that the larger quadrupeds are actually destroyed
(except in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are
more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of
prey.
Organs now of trifling importance have probably in some cases been of high importance to an early
progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in
nearly the same state, although now of very slight use; but any actually injurious deviations in their structure
would of course have been checked by natural selection. Seeing how important an organ of locomotion the tail is
in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their
lungs or modified swim−bladders betray their aquatic origin, may perhaps be thus accounted for. A
well−developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all
sorts of purposes, as a fly−flapper, an organ of prehension, or as an aid in turning, as in the case of the dog,
though the aid in this latter respect must be slight, for the hare, with hardly any tail, can double still more quickly.
In the second place, we may easily err in attributing importance to characters, and in believing that they have
been developed through natural selection. We must by no means overlook the effects of the definite action of
changed conditions of life, of so−called spontaneous variations, which seem to depend in a quite subordinate
degree on the nature of the conditions, of the tendency to reversion to long−lost characters, of the complex laws of
growth, such as of correlation, comprehension, of the pressure of one part on another, etc., and finally of sexual
selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the
other sex, though of no use to the sex. But structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified descendants, under new conditions of
life and newly acquired habits.
If green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, I
dare say that we should have thought that the green colour was a beautiful adaptation to conceal this
tree−frequenting bird from its enemies; and consequently that it was a character of importance, and had been
acquired through natural selection; as it is, the colour is probably in chief part due to sexual selection. A trailing
palm in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely constructed hooks clustered
around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant; but as we see
nearly similar hooks on many trees which are not climbers, and which, as there is reason to believe from the
distribution of the thorn− bearing species in Africa and South America, serve as a defence against browsing
quadrupeds, so the spikes on the palm may at first have been developed for this object, and subsequently have
been improved and taken advantage of by the plant, as it underwent further modification and became a climber.
The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity;
and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in
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The Origin of Species by means of Natural Selection
drawing any such inference, when we see that the skin on the head of the clean−feeding male turkey is likewise
naked. The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of
young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen
from the laws of growth, and has been taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the cause of each slight variation or individual difference; and we are
immediately made conscious of this by reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized countries, where there has been but little
methodical selection. Animals kept by savages in different countries often have to struggle for their own
subsistence, and are exposed to a certain extent to natural selection, and individuals with slightly different
constitutions would succeed best under different climates. With cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain plants; so that even colour would be thus
subjected to the action of natural selection. Some observers are convinced that a damp climate affects the growth
of the hair, and that with the hair the horns are correlated. Mountain breeds always differ from lowland breeds;
and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even
the form of the pelvis; and then by the law of homologous variation, the front limbs and the head would probably
be affected. The shape, also, of the pelvis might affect by pressure the shape of certain parts of the young in the
womb. The laborious breathing necessary in high regions tends, as we have good reason to believe, to increase the
size of the chest; and again correlation would come into play. The effects of lessened exercise, together with
abundant food, on the whole organisation is probably still more important, and this, as H. von Nathusius has lately
shown in his excellent Treatise, is apparently one chief cause of the great modification which the breeds of swine
have undergone. But we are far too ignorant to speculate on the relative importance of the several known and
unknown causes of variation; and I have made these remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which nevertheless are generally admitted to have arisen
through ordinary generation from one or a few parent−stocks, we ought not to lay too much stress on our
ignorance of the precise cause of the slight analogous differences between true species.
UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED.
The foregoing remarks lead me to say a few words on the protest lately made by some naturalists against the
utilitarian doctrine that every detail of structure has been produced for the good of its possessor. They believe that
many structures have been created for the sake of beauty, to delight man or the Creator (but this latter point is
beyond the scope of scientific discussion), or for the sake of mere variety, a view already discussed. Such
doctrines, if true, would be absolutely fatal to my theory. I fully admit that many structures are now of no direct
use to their possessors, and may never have been of any use to their progenitors; but this does not prove that they
were formed solely for beauty or variety. No doubt the definite action of changed conditions, and the various
causes of modifications, lately specified, have all produced an effect, probably a great effect, independently of
any advantage thus gained. But a still more important consideration is that the chief part of the organisation of
every living creature is due to inheritance; and consequently, though each being assuredly is well fitted for its
place in nature, many structures have now no very close and direct relation to present habits of life. Thus, we can
hardly believe that the webbed feet of the upland goose, or of the frigate− bird, are of special use to these birds;
we cannot believe that the similar bones in the arm of the monkey, in the fore leg of the horse, in the wing of the
bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to
inheritance. But webbed feet no doubt were as useful to the progenitor of the upland goose and of the frigate−bird,
as they now are to the most aquatic of living birds. So we may believe that the progenitor of the seal did not
possess a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe
that the several bones in the limbs of the monkey, horse and bat, were originally developed, on the principle of
utility, probably through the reduction of more numerous bones in the fin of some ancient fish−like progenitor of
the whole class. It is scarcely possible to decide how much allowance ought to be made for such causes of change,
as the definite action of external conditions, so−called spontaneous variations, and the complex laws of growth;
but with these important exceptions, we may conclude that the structure of every living creature either now is, or
was formerly, of some direct or indirect use to its possessor.
With respect to the belief that organic beings have been created beautiful for the delight of man−−a belief
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which it has been pronounced is subversive of my whole theory−−I may first remark that the sense of beauty
obviously depends on the nature of the mind, irrespective of any real quality in the admired object; and that the
idea of what is beautiful, is not innate or unalterable. We see this, for instance, in the men of different races
admiring an entirely different standard of beauty in their women. If beautiful objects had been created solely for
man's gratification, it ought to be shown that before man appeared there was less beauty on the face of the earth
than since he came on the stage. Were the beautiful volute and cone shells of the Eocene epoch, and the gracefully
sculptured ammonites of the Secondary period, created that man might ages afterwards admire them in his
cabinet? Few objects are more beautiful than the minute siliceous cases of the diatomaceae: were these created
that they might be examined and admired under the higher powers of the microscope? The beauty in this latter
case, and in many others, is apparently wholly due to symmetry of growth. Flowers rank among the most
beautiful productions of nature; but they have been rendered conspicuous in contrast with the green leaves, and in
consequence at the same time beautiful, so that they may be easily observed by insects. I have come to this
conclusion from finding it an invariable rule that when a flower is fertilised by the wind it never has a
gaily−coloured corolla. Several plants habitually produce two kinds of flowers; one kind open and coloured so as
to attract insects; the other closed, not coloured, destitute of nectar, and never visited by insects. Hence, we may
conclude that, if insects had not been developed on the face of the earth, our plants would not have been decked
with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut and ash
trees, on grasses, spinach, docks and nettles, which are all fertilised through the agency of the wind. A similar line
of argument holds good with fruits; that a ripe strawberry or cherry is as pleasing to the eye as to the palate−−that
the gaily−coloured fruit of the spindle−wood tree and the scarlet berries of the holly are beautiful objects−−will
be admitted by everyone. But this beauty serves merely as a guide to birds and beasts, in order that the fruit may
be devoured and the matured seeds disseminated. I infer that this is the case from having as yet found no
exception to the rule that seeds are always thus disseminated when embedded within a fruit of any kind (that is
within a fleshy or pulpy envelope), if it be coloured of any brilliant tint, or rendered conspicuous by being white
or black.
On the other hand, I willingly admit that a great number of male animals, as all our most gorgeous birds, some
fishes, reptiles, and mammals, and a host of magnificently coloured butterflies, have been rendered beautiful for
beauty's sake. But this has been effected through sexual selection, that is, by the more beautiful males having been
continually preferred by the females, and not for the delight of man. So it is with the music of birds. We may infer
from all this that a nearly similar taste for beautiful colours and for musical sounds runs through a large part of the
animal kingdom. When the female is as beautifully coloured as the male, which is not rarely the case with birds
and butterflies, the cause apparently lies in the colours acquired through sexual selection having been transmitted
to both sexes, instead of to the males alone. How the sense of beauty in its simplest form−−that is, the reception of
a peculiar kind of pleasure from certain colours, forms and sounds−−was first developed in the mind of man and
of the lower animals, is a very obscure subject. The same sort of difficulty is presented if we enquire how it is that
certain flavours and odours give pleasure, and others displeasure. Habit in all these cases appears to have come to
a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in
each species.
Natural selection cannot possibly produce any modification in a species exclusively for the good of another
species; though throughout nature one species incessantly takes advantage of, and profits by the structures of
others. But natural selection can and does often produce structures for the direct injury of other animals, as we see
in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living
bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for
the exclusive good of another species, it would annihilate my theory, for such could not have been produced
through natural selection. Although many statements may be found in works on natural history to this effect, I
cannot find even one which seems to me of any weight. It is admitted that the rattlesnake has a poison−fang for its
own defence and for the destruction of its prey; but some authors suppose that at the same time it is furnished with
a rattle for its own injury, namely, to warn its prey. I would almost as soon believe that the cat curls the end of its
tail when preparing to spring, in order to warn the doomed mouse. It is a much more probable view that the
rattlesnake uses its rattle, the cobra expands its frill and the puff−adder swells while hissing so loudly and harshly,
in order to alarm the many birds and beasts which are known to attack even the most venomous species. Snakes
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The Origin of Species by means of Natural Selection
act on the same principle which makes the hen ruffle her feathers and expand her wings when a dog approaches
her chickens. But I have not space here to enlarge on the many ways by which animals endeavour to frighten
away their enemies.
Natural selection will never produce in a being any structure more injurious than beneficial to that being, for
natural selection acts solely by and for the good of each. No organ will be formed, as Paley has remarked, for the
purpose of causing pain or for doing an injury to its possessor. If a fair balance be struck between the good and
evil caused by each part, each will be found on the whole advantageous. After the lapse of time, under changing
conditions of life, if any part comes to be injurious, it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as, or slightly more perfect than the other
inhabitants of the same country with which it comes into competition. And we see that this is the standard of
perfection attained under nature. The endemic productions of New Zealand, for instance, are perfect, one
compared with another; but they are now rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute perfection, nor do we always meet, as far as
we can judge, with this high standard under nature. The correction for the aberration of light is said by Muller not
to be perfect even in that most perfect organ, the human eye. Helmholtz, whose judgment no one will dispute,
after describing in the strongest terms the wonderful powers of the human eye, adds these remarkable words:
"That which we have discovered in the way of inexactness and imperfection in the optical machine and in the
image on the retina, is as nothing in comparison with the incongruities which we have just come across in the
domain of the sensations. One might say that nature has taken delight in accumulating contradictions in order to
remove all foundation from the theory of a pre−existing harmony between the external and internal worlds." If our
reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells
us, though we may easily err on both sides, that some other contrivances are less perfect. Can we consider the
sting of the bee as perfect, which, when used against many kinds of enemies, cannot be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having existed in a remote progenitor, as a boring and serrated instrument,
like that in so many members of the same great order, and that it has since been modified but not perfected for its
present purpose, with the poison originally adapted for some other object, such as to produce galls, since
intensified, we can perhaps understand how it is that the use of the sting should so often cause the insect's own
death: for if on the whole the power of stinging be useful to the social community, it will fulfil all the
requirements of natural selection, though it may cause the death of some few members. If we admire the truly
wonderful power of scent by which the males of many insects find their females, can we admire the production
for this single purpose of thousands of drones, which are utterly useless to the community for any other purpose,
and which are ultimately slaughtered by their industrious and sterile sisters? It may be difficult, but we ought to
admire the savage instinctive hatred of the queen−bee, which urges her to destroy the young queens, her
daughters, as soon as they are born, or to perish herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter fortunately is most rare, is all the same to the
inexorable principles of natural selection. If we admire the several ingenious contrivances by which orchids and
many other plants are fertilised through insect agency, can we consider as equally perfect the elaboration of dense
clouds of pollen by our fir−trees, so that a few granules may be wafted by chance on to the ovules?
SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE
EMBRACED BY THE THEORY OF NATURAL SELECTION.
We have in this chapter discussed some of the difficulties and objections which may be urged against the
theory. Many of them are serious; but I think that in the discussion light has been thrown on several facts, which
on the belief of independent acts of creation are utterly obscure. We have seen that species at any one period are
not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the
process of natural selection is always very slow, and at any one time acts only on a few forms; and partly because
the very process of natural selection implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a continuous area, must often have been formed
when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part
to another. When two varieties are formed in two districts of a continuous area, an intermediate variety will often
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The Origin of Species by means of Natural Selection
be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in
lesser numbers than the two forms which it connects; consequently the two latter, during the course of further
modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate
variety, and will thus generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding that the most different habits of life
could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from
an animal which at first only glided through the air.
We have seen that a species under new conditions of life may change its habits, or it may have diversified
habits, with some very unlike those of its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet,
ground woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been formed by natural selection, is enough
to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity, each
good for its possessor, then under changing conditions of life, there is no logical impossibility in the acquirement
of any conceivable degree of perfection through natural selection. In the cases in which we know of no
intermediate or transitional states, we should be extremely cautious in concluding that none can have existed, for
the metamorphoses of many organs show what wonderful changes in function are at least possible. For instance, a
swim−bladder has apparently been converted into an air−breathing lung. The same organ having performed
simultaneously very different functions, and then having been in part or in whole specialised for one function; and
two distinct organs having performed at the same time the same function, the one having been perfected whilst
aided by the other, must often have largely facilitated transitions.
We have seen that in two beings widely remote from each other in the natural scale, organs serving for the
same purpose and in external appearance closely similar may have been separately and independently formed; but
when such organs are closely examined, essential differences in their structure can almost always be detected; and
this naturally follows from the principle of natural selection. On the other hand, the common rule throughout
nature is infinite diversity of structure for gaining the same end; and this again naturally follows from the same
great principle.
In many cases we are far too ignorant to be enabled to assert that a part or organ is so unimportant for the
welfare of a species, that modifications in its structure could not have been slowly accumulated by means of
natural selection. In many other cases, modifications are probably the direct result of the laws of variation or of
growth, independently of any good having been thus gained. But even such structures have often, as we may feel
assured, been subsequently taken advantage of, and still further modified, for the good of species under new
conditions of life. We may, also, believe that a part formerly of high importance has frequently been retained (as
the tail of an aquatic animal by its terrestrial descendants), though it has become of such small importance that it
could not, in its present state, have been acquired by means of natural selection.
Natural selection can produce nothing in one species for the exclusive good or injury of another; though it
may well produce parts, organs, and excretions highly useful or even indispensable, or highly injurious to another
species, but in all cases at the same time useful to the possessor. In each well−stocked country natural selection
acts through the competition of the inhabitants and consequently leads to success in the battle for life, only in
accordance with the standard of that particular country. Hence the inhabitants of one country, generally the
smaller one, often yield to the inhabitants of another and generally the larger country. For in the larger country
there will have existed more individuals, and more diversified forms, and the competition will have been severer,
and thus the standard of perfection will have been rendered higher. Natural selection will not necessarily lead to
absolute perfection; nor, as far as we can judge by our limited faculties, can absolute perfection be everywhere
predicated.
On the theory of natural selection we can clearly understand the full meaning of that old canon in natural
history, "Natura non facit saltum." This canon, if we look to the present inhabitants alone of the world, is not
strictly correct; but if we include all those of past times, whether known or unknown, it must on this theory be
strictly true.
It is generally acknowledged that all organic beings have been formed on two great laws−−Unity of Type, and
the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in
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The Origin of Species by means of Natural Selection
organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of
type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the
illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now
adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them
during past periods of time: the adaptations being aided in many cases by the increased use or disuse of parts,
being affected by the direct action of external conditions of life, and subjected in all cases to the several laws of
growth and variation. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes,
through the inheritance of former variations and adaptations, that of Unity of Type.
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The Origin of Species by means of Natural Selection
CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF
NATURAL SELECTION.
Longevity −− Modifications not necessarily simultaneous −− Modifications apparently of no direct service −−
Progressive development −− Characters of small functional importance, the most constant −− Supposed
incompetence of natural selection to account for the incipient stages of useful structures −− Causes which
interfere with the acquisition through natural selection of useful structures −− Gradations of structure with
changed functions −− Widely different organs in members of the same class, developed from one and the same
source −− Reasons for disbelieving in great and abrupt modifications.
I will devote this chapter to the consideration of various miscellaneous objections which have been advanced
against my views, as some of the previous discussions may thus be made clearer; but it would be useless to
discuss all of them, as many have been made by writers who have not taken the trouble to understand the subject.
Thus a distinguished German naturalist has asserted that the weakest part of my theory is, that I consider all
organic beings as imperfect: what I have really said is, that all are not as perfect as they might have been in
relation to their conditions; and this is shown to be the case by so many native forms in many quarters of the
world having yielded their places to intruding foreigners. Nor can organic beings, even if they were at any one
time perfectly adapted to their conditions of life, have remained so, when their conditions changed, unless they
themselves likewise changed; and no one will dispute that the physical conditions of each country, as well as the
number and kinds of its inhabitants, have undergone many mutations.
A critic has lately insisted, with some parade of mathematical accuracy, that longevity is a great advantage to
all species, so that he who believes in natural selection "must arrange his genealogical tree" in such a manner that
all the descendants have longer lives than their progenitors! Cannot our critics conceive that a biennial plant or
one of the lower animals might range into a cold climate and perish there every winter; and yet, owing to
advantages gained through natural selection, survive from year to year by means of its seeds or ova? Mr. E. Ray
Lankester has recently discussed this subject, and he concludes, as far as its extreme complexity allows him to
form a judgment, that longevity is generally related to the standard of each species in the scale of organisation, as
well as to the amount of expenditure in reproduction and in general activity. And these conditions have, it is
probable, been largely determined through natural selection.
It has been argued that, as none of the animals and plants of Egypt, of which we know anything, have changed
during the last three or four thousand years, so probably have none in any part of the world. But, as Mr. G.H.
Lewes has remarked, this line of argument proves too much, for the ancient domestic races figured on the
Egyptian monuments, or embalmed, are closely similar or even identical with those now living; yet all naturalists
admit that such races have been produced through the modification of their original types. The many animals
which have remained unchanged since the commencement of the glacial period, would have been an
incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great
distances; whereas, in Egypt, during the last several thousand years, the conditions of life, as far as we know, have
remained absolutely uniform. The fact of little or no modification having been effected since the glacial period,
would have been of some avail against those who believe in an innate and necessary law of development, but is
powerless against the doctrine of natural selection or the survival of the fittest, which implies that when variations
or individual differences of a beneficial nature happen to arise, these will be preserved; but this will be effected
only under certain favourable circumstances.
The celebrated palaeontologist, Bronn, at the close of his German translation of this work, asks how, on the
principle of natural selection, can a variety live side by side with the parent species? If both have become fitted
for slightly different habits of life or conditions, they might live together; and if we lay on one side polymorphic
species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size,
albinism, etc., the more permanent varieties are generally found, as far as I can discover, inhabiting distinct
stations, such as high land or low land, dry or moist districts. Moreover, in the case of animals which wander
much about and cross freely, their varieties seem to be generally confined to distinct regions.
Bronn also insists that distinct species never differ from each other in single characters, but in many parts; and
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The Origin of Species by means of Natural Selection
he asks, how it always comes that many parts of the organisation should have been modified at the same time
through variation and natural selection? But there is no necessity for supposing that all the parts of any being have
been simultaneously modified. The most striking modifications, excellently adapted for some purpose, might, as
was formerly remarked, be acquired by successive variations, if slight, first in one part and then in another; and as
they would be transmitted all together, they would appear to us as if they had been simultaneously developed. The
best answer, however, to the above objection is afforded by those domestic races which have been modified,
chiefly through man's power of selection, for some special purpose. Look at the race and dray−horse, or at the
greyhound and mastiff. Their whole frames, and even their mental characteristics, have been modified; but if we
could trace each step in the history of their transformation−−and the latter steps can be traced−−we should not see
great and simultaneous changes, but first one part and then another slightly modified and improved. Even when
selection has been applied by man to some one character alone−−of which our cultivated plants offer the best
instances−− it will invariably be found that although this one part, whether it be the flower, fruit, or leaves, has
been greatly changed, almost all the other parts have been slightly modified. This may be attributed partly to the
principle of correlated growth, and partly to so−called spontaneous variation.
A much more serious objection has been urged by Bronn, and recently by Broca, namely, that many
characters appear to be of no service whatever to their possessors, and therefore cannot have been influenced
through natural selection. Bronn adduces the length of the ears and tails in the different species of hares and
mice−−the complex folds of enamel in the teeth of many animals, and a multitude of analogous cases. With
respect to plants, this subject has been discussed by Nageli in an admirable essay. He admits that natural selection
has effected much, but he insists that the families of plants differ chiefly from each other in morphological
characters, which appear to be quite unimportant for the welfare of the species. He consequently believes in an
innate tendency towards progressive and more perfect development. He specifies the arrangement of the cells in
the tissues, and of the leaves on the axis, as cases in which natural selection could not have acted. To these may be
added the numerical divisions in the parts of the flower, the position of the ovules, the shape of the seed, when not
of any use for dissemination, etc.
There is much force in the above objection. Nevertheless, we ought, in the first place, to be extremely cautious
in pretending to decide what structures now are, or have formerly been, of use to each species. In the second
place, it should always be borne in mind that when one part is modified, so will be other parts, through certain
dimly seen causes, such as an increased or diminished flow of nutriment to a part, mutual pressure, an early
developed part affecting one subsequently developed, and so forth −−as well as through other causes which lead
to the many mysterious cases of correlation, which we do not in the least understand. These agencies may be all
grouped together, for the sake of brevity, under the expression of the laws of growth. In the third place, we have
to allow for the direct and definite action of changed conditions of life, and for so−called spontaneous variations,
in which the nature of the conditions apparently plays a quite subordinate part. Bud−variations, such as the
appearance of a moss−rose on a common rose, or of a nectarine on a peach−tree, offer good instances of
spontaneous variations; but even in these cases, if we bear in mind the power of a minute drop of poison in
producing complex galls, we ought not to feel too sure that the above variations are not the effect of some local
change in the nature of the sap, due to some change in the conditions. There must be some efficient cause for each
slight individual difference, as well as for more strongly marked variations which occasionally arise; and if the
unknown cause were to act persistently, it is almost certain that all the individuals of the species would be
similarly modified.
In the earlier editions of this work I underrated, as it now seems probable, the frequency and importance of
modifications due to spontaneous variability. But it is impossible to attribute to this cause the innumerable
structures which are so well adapted to the habits of life of each species. I can no more believe in this than that the
well−adapted form of a race−horse or greyhound, which before the principle of selection by man was well
understood, excited so much surprise in the minds of the older naturalists, can thus be explained.
It may be worth while to illustrate some of the foregoing remarks. With respect to the assumed inutility of
various parts and organs, it is hardly necessary to observe that even in the higher and best−known animals many
structures exist, which are so highly developed that no one doubts that they are of importance, yet their use has
not been, or has only recently been, ascertained. As Bronn gives the length of the ears and tail in the several
species of mice as instances, though trifling ones, of differences in structure which can be of no special use, I may
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The Origin of Species by means of Natural Selection
mention that, according to Dr. Schobl, the external ears of the common mouse are supplied in an extraordinary
manner with nerves, so that they no doubt serve as tactile organs; hence the length of the ears can hardly be quite
unimportant. We shall, also, presently see that the tail is a highly useful prehensile organ to some of the species;
and its use would be much influence by its length.
With respect to plants, to which on account of Nageli's essay I shall confine myself in the following remarks,
it will be admitted that the flowers of the orchids present a multitude of curious structures, which a few years ago
would have been considered as mere morphological differences without any special function; but they are now
known to be of the highest importance for the fertilisation of the species through the aid of insects, and have
probably been gained through natural selection. No one until lately would have imagined that in dimorphic and
trimorphic plants the different lengths of the stamens and pistils, and their arrangement, could have been of any
service, but now we know this to be the case.
In certain whole groups of plants the ovules stand erect, and in others they are suspended; and within the same
ovarium of some few plants, one ovule holds the former and a second ovule the latter position. These positions
seem at first purely morphological, or of no physiological signification; but Dr. Hooker informs me that within the
same ovarium the upper ovules alone in some cases, and in others the lower ones alone are fertilised; and he
suggests that this probably depends on the direction in which the pollen−tubes enter the ovarium. If so, the
position of the ovules, even when one is erect and the other suspended within the same ovarium, would follow the
selection of any slight deviations in position which favoured their fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce flowers of two kinds−−the one open, of the
ordinary structure, the other closed and imperfect. These two kinds of flowers sometimes differ wonderfully in
structure, yet may be seen to graduate into each other on the same plant. The ordinary and open flowers can be
intercrossed; and the benefits which certainly are derived from this process are thus secured. The closed and
imperfect flowers are, however, manifestly of high importance, as they yield with the utmost safety a large stock
of seed, with the expenditure of wonderfully little pollen. The two kinds of flowers often differ much, as just
stated, in structure. The petals in the imperfect flowers almost always consist of mere rudiments, and the
pollen−grains are reduced in diameter. In Ononis columnae five of the alternate stamens are rudimentary; and in
some species of Viola three stamens are in this state, two retaining their proper function, but being of very small
size. In six out of thirty of the closed flowers in an Indian violet (name unknown, for the plants have never
produced with me perfect flowers), the sepals are reduced from the normal number of five to three. In one section
of the Malpighiaceae the closed flowers, according to A. de Jussieu, are still further modified, for the five stamens
which stand opposite to the sepals are all aborted, a sixth stamen standing opposite to a petal being alone
developed; and this stamen is not present in the ordinary flowers of this species; the style is aborted; and the
ovaria are reduced from three to two. Now although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of pollen, when rendered by the closure of the
flowers superfluous, yet hardly any of the above special modifications can have been thus determined, but must
have followed from the laws of growth, including the functional inactivity of parts, during the progress of the
reduction of the pollen and the closure of the flowers.
It is so necessary to appreciate the important effects of the laws of growth, that I will give some additional
cases of another kind, namely of differences in the same part or organ, due to differences in relative position on
the same plant. In the Spanish chestnut, and in certain fir− trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright branches. In the common rue and some other
plants, one flower, usually the central or terminal one, opens first, and has five sepals and petals, and five
divisions to the ovarium; while all the other flowers on the plant are tetramerous. In the British Adoxa the
uppermost flower generally has two calyx−lobes with the other organs tetramerous, while the surrounding flowers
generally have three calyx−lobes with the other organs pentamerous. In many Compositae and Umbelliferae (and
in some other plants) the circumferential flowers have their corollas much more developed than those of the
centre; and this seems often connected with the abortion of the reproductive organs. It is a more curious fact,
previously referred to, that the achenes or seeds of the circumference and centre sometimes differ greatly in form,
colour and other characters. In Carthamus and some other Compositae the central achenes alone are furnished
with a pappus; and in Hyoseris the same head yields achenes of three different forms. In certain Umbelliferae the
exterior seeds, according to Tausch, are orthospermous, and the central one coelospermous, and this is a character
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The Origin of Species by means of Natural Selection
which was considered by De Candolle to be in other species of the highest systematic importance. Professor
Braun mentions a Fumariaceous genus, in which the flowers in the lower part of the spike bear oval, ribbed,
one−seeded nutlets; and in the upper part of the spike, lanceolate, two−valved and two−seeded siliques. In these
several cases, with the exception of that of the well−developed ray− florets, which are of service in making the
flowers conspicuous to insects, natural selection cannot, as far as we can judge, have come into play, or only in a
quite subordinate manner. All these modifications follow from the relative position and inter−action of the parts;
and it can hardly be doubted that if all the flowers and leaves on the same plant had been subjected to the same
external and internal condition, as are the flowers and leaves in certain positions, all would have been modified in
the same manner.
In numerous other cases we find modifications of structure, which are considered by botanists to be generally
of a highly important nature, affecting only some of the flowers on the same plant, or occurring on distinct plants,
which grow close together under the same conditions. As these variations seem of no special use to the plants,
they cannot have been influenced by natural selection. Of their cause we are quite ignorant; we cannot even
attribute them, as in the last class of cases, to any proximate agency, such as relative position. I will give only a
few instances. It is so common to observe on the same plant, flowers indifferently tetramerous, pentamerous, etc.,
that I need not give examples; but as numerical variations are comparatively rare when the parts are few, I may
mention that, according to De Candolle, the flowers of Papaver bracteatum offer either two sepals with four petals
(which is the common type with poppies), or three sepals with six petals. The manner in which the petals are
folded in the bud is in most groups a very constant morphological character; but Professor Asa Gray states that
with some species of Mimulus, the aestivation is almost as frequently that of the Rhinanthideae as of the
Antirrhinideae, to which latter tribe the genus belongs. Aug. St. Hilaire gives the following cases: the genus
Zanthoxylon belongs to a division of the Rutaceae with a single ovary, but in some species flowers may be found
on the same plant, and even in the same panicle, with either one or two ovaries. In Helianthemum the capsule has
been described as unilocular or tri−locular; and in H. mutabile, "Une lame PLUS OU MOINS LARGE, s'etend
entre le pericarpe et le placenta." In the flowers of Saponaria officinalis Dr. Masters has observed instances of
both marginal and free central placentation. Lastly, St. Hilaire found towards the southern extreme of the range of
Gomphia oleaeformis two forms which he did not at first doubt were distinct species, but he subsequently saw
them growing on the same bush; and he then adds, "Voila donc dans un meme individu des loges et un style qui
se rattachent tantot a un axe verticale et tantot a un gynobase."
We thus see that with plants many morphological changes may be attributed to the laws of growth and the
inter−action of parts, independently of natural selection. But with respect to Nageli's doctrine of an innate
tendency towards perfection or progressive development, can it be said in the case of these strongly pronounced
variations, that the plants have been caught in the act of progressing towards a higher state of development? On
the contrary, I should infer from the mere fact of the parts in question differing or varying greatly on the same
plant, that such modifications were of extremely small importance to the plants themselves, of whatever
importance they may generally be to us for our classifications. The acquisition of a useless part can hardly be said
to raise an organism in the natural scale; and in the case of the imperfect, closed flowers, above described, if any
new principle has to be invoked, it must be one of retrogression rather than of progression; and so it must be with
many parasitic and degraded animals. We are ignorant of the exciting cause of the above specified modifications;
but if the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be
almost uniform; and in this case all the individuals of the species would be modified in the same manner.
>From the fact of the above characters being unimportant for the welfare of the species, any slight variations
which occurred in them would not have been accumulated and augmented through natural selection. A structure
which has been developed through long−continued selection, when it ceases to be of service to a species,
generally becomes variable, as we see with rudimentary organs; for it will no longer be regulated by this same
power of selection. But when, from the nature of the organism and of the conditions, modifications have been
induced which are unimportant for the welfare of the species, they may be, and apparently often have been,
transmitted in nearly the same state to numerous, otherwise modified, descendants. It cannot have been of much
importance to the greater number of mammals, birds, or reptiles, whether they were clothed with hair, feathers or
scales; yet hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles. A
structure, whatever it may be, which is common to many allied forms, is ranked by us as of high systematic
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importance, and consequently is often assumed to be of high vital importance to the species. Thus, as I am
inclined to believe, morphological differences, which we consider as important−−such as the arrangement of the
leaves, the divisions of the flower or of the ovarium, the position of the ovules, etc., first appeared in many cases
as fluctuating variations, which sooner or later became constant through the nature of the organism and of the
surrounding conditions, as well as through the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the welfare of the species, any slight deviations in
them could not have been governed or accumulated through this latter agency. It is a strange result which we thus
arrive at, namely, that characters of slight vital importance to the species, are the most important to the
systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no
means so paradoxical as it may at first appear.
Although we have no good evidence of the existence in organic beings of an innate tendency towards
progressive development, yet this necessarily follows, as I have attempted to show in the fourth chapter, through
the continued action of natural selection. For the best definition which has ever been given of a high standard of
organisation, is the degree to which the parts have been specialised or differentiated; and natural selection tends
towards this end, inasmuch as the parts are thus enabled to perform their functions more efficiently.
A distinguished zoologist, Mr. St. George Mivart, has recently collected all the objections which have ever
been advanced by myself and others against the theory of natural selection, as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art and force. When thus marshalled, they make a formidable
array; and as it forms no part of Mr. Mivart's plan to give the various facts and considerations opposed to his
conclusions, no slight effort of reason and memory is left to the reader, who may wish to weigh the evidence on
both sides. When discussing special cases, Mr. Mivart passes over the effects of the increased use and disuse of
parts, which I have always maintained to be highly important, and have treated in my "Variation under
Domestication" at greater length than, as I believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in the work just referred to I have collected a
greater number of well−established cases than can be found in any other work known to me. My judgment may
not be trustworthy, but after reading with care Mr. Mivart's book, and comparing each section with what I have
said on the same head, I never before felt so strongly convinced of the general truth of the conclusions here
arrived at, subject, of course, in so intricate a subject, to much partial error.
All Mr. Mivart's objections will be, or have been, considered in the present volume. The one new point which
appears to have struck many readers is, "That natural selection is incompetent to account for the incipient stages
of useful structures." This subject is intimately connected with that of the gradation of the characters, often
accompanied by a change of function, for instance, the conversion of a swim−bladder into lungs, points which
were discussed in the last chapter under two headings. Nevertheless, I will here consider in some detail several of
the cases advanced by Mr. Mivart, selecting those which are the most illustrative, as want of space prevents me
from considering all.
The giraffe, by its lofty stature, much elongated neck, fore legs, head and tongue, has its whole frame
beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the
other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during
dearths. The Niata cattle in South America show us how small a difference in structure may make, during such
periods, a great difference in preserving an animal's life. These cattle can browse as well as others on grass, but
from the projection of the lower jaw they cannot, during the often recurrent droughts, browse on the twigs of
trees, reeds, etc., to which food the common cattle and horses are then driven; so that at these times the Niatas
perish, if not fed by their owners. Before coming to Mr. Mivart's objections, it may be well to explain once again
how natural selection will act in all ordinary cases. Man has modified some of his animals, without necessarily
having attended to special points of structure, by simply preserving and breeding from the fleetest individuals, as
with the race−horse and greyhound, or as with the game−cock, by breeding from the victorious birds. So under
nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to
reach even an inch or two above the others, will often have been preserved; for they will have roamed over the
whole country in search of food. That the individuals of the same species often differ slightly in the relative
lengths of all their parts may be seen in many works of natural history, in which careful measurements are given.
These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or
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importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable
habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated
than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the
same bodily peculiarities, or with a tendency to vary again in the same manner; while the individuals less
favoured in the same respects will have been the most liable to perish.
We here see that there is no need to separate single pairs, as man does, when he methodically improves a
breed: natural selection will preserve and thus separate all the superior individuals, allowing them freely to
intercross, and will destroy all the inferior individuals. By this process long−continued, which exactly corresponds
with what I have called unconscious selection by man, combined, no doubt, in a most important manner with the
inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped
might be converted into a giraffe.
To this conclusion Mr. Mivart brings forward two objections. One is that the increased size of the body would
obviously require an increased supply of food, and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages." But as the
giraffe does actually exist in large numbers in Africa, and as some of the largest antelopes in the world, taller than
an ox, abound there, why should we doubt that, as far as size is concerned, intermediate gradations could formerly
have existed there, subjected as now to severe dearths. Assuredly the being able to reach, at each stage of
increased size, to a supply of food, left untouched by the other hoofed quadrupeds of the country, would have
been of some advantage to the nascent giraffe. Nor must we overlook the fact, that increased bulk would act as a
protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck−−and the taller
the better−− would, as Mr. Chauncey Wright has remarked, serve as a watch−tower. It is from this cause, as Sir S.
Baker remarks, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a
means of offence or defence, by violently swinging its head armed with stump−like horns. The preservation of
each species can rarely be determined by any one advantage, but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objection), if natural selection be so potent, and if high browsing
be so great an advantage, why has not any other hoofed quadruped acquired a long neck and lofty stature, besides
the giraffe, and, in a lesser degree, the camel, guanaco and macrauchenia? Or, again, why has not any member of
the group acquired a long proboscis? With respect to South Africa, which was formerly inhabited by numerous
herds of the giraffe, the answer is not difficult, and can best be given by an illustration. In every meadow in
England, in which trees grow, we see the lower branches trimmed or planed to an exact level by the browsing of
the horses or cattle; and what advantage would it be, for instance, to sheep, if kept there, to acquire slightly longer
necks? In every district some one kind of animal will almost certainly be able to browse higher than the others;
and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through
natural selection and the effects of increased use. In South Africa the competition for browsing on the higher
branches of the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate
animals.
Why, in other quarters of the world, various animals belonging to this same order have not acquired either an
elongated neck or a proboscis, cannot be distinctly answered; but it is as unreasonable to expect a distinct answer
to such a question as why some event in the history of mankind did not occur in one country while it did in
another. We are ignorant with respect to the conditions which determine the numbers and range of each species,
and we cannot even conjecture what changes of structure would be favourable to its increase in some new
country. We can, however, see in a general manner that various causes might have interfered with the
development of a long neck or proboscis. To reach the foliage at a considerable height (without climbing, for
which hoofed animals are singularly ill−constructed) implies greatly increased bulk of body; and we know that
some areas support singularly few large quadrupeds, for instance South America, though it is so luxuriant, while
South Africa abounds with them to an unparalleled degree. Why this should be so we do not know; nor why the
later tertiary periods should have been much more favourable for their existence than the present time. Whatever
the causes may have been, we can see that certain districts and times would have been much more favourable than
others for the development of so large a quadruped as the giraffe.
In order that an animal should acquire some structure specially and largely developed, it is almost
indispensable that several other parts should be modified and coadapted. Although every part of the body varies
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slightly, it does not follow that the necessary parts should always vary in the right direction and to the right
degree. With the different species of our domesticated animals we know that the parts vary in a different manner
and degree, and that some species are much more variable than others. Even if the fitting variations did arise, it
does not follow that natural selection would be able to act on them and produce a structure which apparently
would be beneficial to the species. For instance, if the number of individuals existing in a country is determined
chiefly through destruction by beasts of prey−−by external or internal parasites, etc.−−as seems often to be the
case, then natural selection will be able to do little, or will be greatly retarded, in modifying any particular
structure for obtaining food. Lastly, natural selection is a slow process, and the same favourable conditions must
long endure in order that any marked effect should thus be produced. Except by assigning such general and vague
reasons, we cannot explain why, in many quarters of the world, hoofed quadrupeds have not acquired much
elongated necks or other means for browsing on the higher branches of trees.
Objections of the same nature as the foregoing have been advanced by many writers. In each case various
causes, besides the general ones just indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to certain species. One writer asks, why has not the
ostrich acquired the power of flight? But a moment's reflection will show what an enormous supply of food would
be necessary to give to this bird of the desert force to move its huge body through the air. Oceanic islands are
inhabited by bats and seals, but by no terrestrial mammals; yet as some of these bats are peculiar species, they
must have long inhabited their present homes. Therefore Sir C. Lyell asks, and assigns certain reasons in answer,
why have not seals and bats given birth on such islands to forms fitted to live on the land? But seals would
necessarily be first converted into terrestrial carnivorous animals of considerable size, and bats into terrestrial
insectivorous animals; for the former there would be no prey; for the bats ground−insects would serve as food, but
these would already be largely preyed on by the reptiles or birds, which first colonise and abound on most oceanic
islands. Gradations of structure, with each stage beneficial to a changing species, will be favoured only under
certain peculiar conditions. A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in
streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean. But
seals would not find on oceanic islands the conditions favourable to their gradual reconversion into a terrestrial
form. Bats, as formerly shown, probably acquired their wings by at first gliding through the air from tree to tree,
like the so−called flying squirrels, for the sake of escaping from their enemies, or for avoiding falls; but when the
power of true flight had once been acquired, it would never be reconverted back, at least for the above purposes,
into the less efficient power of gliding through the air. Bats, might, indeed, like many birds, have had their wings
greatly reduced in size, or completely lost, through disuse; but in this case it would be necessary that they should
first have acquired the power of running quickly on the ground, by the aid of their hind legs alone, so as to
compete with birds or other ground animals; and for such a change a bat seems singularly ill−fitted. These
conjectural remarks have been made merely to show that a transition of structure, with each step beneficial, is a
highly complex affair; and that there is nothing strange in a transition not having occurred in any particular case.
Lastly, more than one writer has asked why have some animals had their mental powers more highly
developed than others, as such development would be advantageous to all? Why have not apes acquired the
intellectual powers of man? Various causes could be assigned; but as they are conjectural, and their relative
probability cannot be weighed, it would be useless to give them. A definite answer to the latter question ought not
to be expected, seeing that no one can solve the simpler problem, why, of two races of savages, one has risen
higher in the scale of civilisation than the other; and this apparently implies increased brain power.
We will return to Mr. Mivart's other objections. Insects often resemble for the sake of protection various
objects, such as green or decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds, and living
insects; but to this latter point I shall hereafter recur. The resemblance is often wonderfully close, and is not
confined to colour, but extends to form, and even to the manner in which the insects hold themselves. The
caterpillars which project motionless like dead twigs from the bushes on which they feed, offer an excellent
instance of a resemblance of this kind. The cases of the imitation of such objects as the excrement of birds, are
rare and exceptional. On this head, Mr. Mivart remarks, "As, according to Mr. Darwin's theory, there is a constant
tendency to indefinite variation, and as the minute incipient variations will be in ALL DIRECTIONS, they must
tend to neutralize each other, and at first to form such unstable modifications that it is difficult, if not impossible,
to see how such indefinite oscillations of infinitesimal beginnings can ever build up a sufficiently appreciable
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resemblance to a leaf, bamboo, or other object, for natural selection to seize upon and perpetuate."
But in all the foregoing cases the insects in their original state no doubt presented some rude and accidental
resemblance to an object commonly found in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the diversity in form and colour of the hosts of
insects which exist. As some rude resemblance is necessary for the first start, we can understand how it is that the
larger and higher animals do not (with the exception, as far as I know, of one fish) resemble for the sake of
protection special objects, but only the surface which commonly surrounds them, and this chiefly in colour.
Assuming that an insect originally happened to resemble in some degree a dead twig or a decayed leaf, and that it
varied slightly in many ways, then all the variations which rendered the insect at all more like any such object,
and thus favoured its escape, would be preserved, while other variations would be neglected and ultimately lost;
or, if they rendered the insect at all less like the imitated object, they would be eliminated. There would indeed be
force in Mr. Mivart's objection, if we were to attempt to account for the above resemblances, independently of
natural selection, through mere fluctuating variability; but as the case stands there is none.
Nor can I see any force in Mr. Mivart's difficulty with respect to "the last touches of perfection in the
mimicry;" as in the case given by Mr. Wallace, of a walking−stick insect (Ceroxylus laceratus), which resembles
"a stick grown over by a creeping moss or jungermannia." So close was this resemblance, that a native Dyak
maintained that the foliaceous excrescences were really moss. Insects are preyed on by birds and other enemies
whose sight is probably sharper than ours, and every grade in resemblance which aided an insect to escape notice
or detection, would tend towards its preservation; and the more perfect the resemblance so much the better for the
insect. Considering the nature of the differences between the species in the group which includes the above
Ceroxylus, there is nothing improbable in this insect having varied in the irregularities on its surface, and in these
having become more or less green−coloured; for in every group the characters which differ in the several species
are the most apt to vary, while the generic characters, or those common to all the species, are the most constant.
The Greenland whale is one of the most wonderful animals in the world, and the baleen, or whalebone, one of
its greatest peculiarities. The baleen consists of a row, on each side of the upper jaw, of about 300 plates or
laminae, which stand close together transversely to the longer axis of the mouth. Within the main row there are
some subsidiary rows. The extremities and inner margins of all the plates are frayed into stiff bristles, which
clothe the whole gigantic palate, and serve to strain or sift the water, and thus to secure the minute prey on which
these great animals subsist. The middle and longest lamina in the Greenland whale is ten, twelve, or even fifteen
feet in length; but in the different species of Cetaceans there are gradations in length; the middle lamina being in
one species, according to Scoresby, four feet, in another three, in another eighteen inches, and in the Balaenoptera
rostrata only about nine inches in length. The quality of the whalebone also differs in the different species.
With respect to the baleen, Mr. Mivart remarks that if it "had once attained such a size and development as to
be at all useful, then its preservation and augmentation within serviceable limits would be promoted by natural
selection alone. But how to obtain the beginning of such useful development?" In answer, it may be asked, why
should not the early progenitors of the whales with baleen have possessed a mouth constructed something like the
lamellated beak of a duck? Ducks, like whales, subsist by sifting the mud and water; and the family has
sometimes been called Criblatores, or sifters. I hope that I may not be misconstrued into saying that the
progenitors of whales did actually possess mouths lamellated like the beak of a duck. I wish only to show that this
is not incredible, and that the immense plates of baleen in the Greenland whale might have been developed from
such lamellae by finely graduated steps, each of service to its possessor.
The beak of a shoveller−duck (Spatula clypeata) is a more beautiful and complex structure than the mouth of a
whale. The upper mandible is furnished on each side (in the specimen examined by me) with a row or comb
formed of 188 thin, elastic lamellae, obliquely bevelled so as to be pointed, and placed transversely to the longer
axis of the mouth. They arise from the palate, and are attached by flexible membrane to the sides of the mandible.
Those standing towards the middle are the longest, being about one−third of an inch in length, and they project
fourteen one− hundredths of an inch beneath the edge. At their bases there is a short subsidiary row of obliquely
transverse lamellae. In these several respects they resemble the plates of baleen in the mouth of a whale. But
towards the extremity of the beak they differ much, as they project inward, instead of straight downward. The
entire head of the shoveller, though incomparably less bulky, is about one−eighteenth of the length of the head of
a moderately large Balaenoptera rostrata, in which species the baleen is only nine inches long; so that if we were
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to make the head of the shoveller as long as that of the Balaenoptera, the lamellae would be six inches in length,
that is, two−thirds of the length of the baleen in this species of whale. The lower mandible of the shoveller−duck
is furnished with lamellae of equal length with these above, but finer; and in being thus furnished it differs
conspicuously from the lower jaw of a whale, which is destitute of baleen. On the other hand, the extremities of
these lower lamellae are frayed into fine bristly points, so that they thus curiously resemble the plates of baleen. In
the genus Prion, a member of the distinct family of the Petrels, the upper mandible alone is furnished with
lamellae, which are well developed and project beneath the margin; so that the beak of this bird resembles in this
respect the mouth of a whale.
>From the highly developed structure of the shoveller's beak we may proceed (as I have learned from
information and specimens sent to me by Mr. Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some respects through that of the Aix sponsa, to the
beak of the common duck. In this latter species the lamellae are much coarser than in the shoveller, and are firmly
attached to the sides of the mandible; they are only about fifty in number on each side, and do not project at all
beneath the margin. They are square−topped, and are edged with translucent, hardish tissue, as if for crushing
food. The edges of the lower mandible are crossed by numerous fine ridges, which project very little. Although
the beak is thus very inferior as a sifter to that of a shoveller, yet this bird, as every one knows, constantly uses it
for this purpose. There are other species, as I hear from Mr. Salvin, in which the lamellae are considerably less
developed than in the common duck; but I do not know whether they use their beaks for sifting the water.
Turning to another group of the same family. In the Egyptian goose (Chenalopex) the beak closely resembles
that of the common duck; but the lamellae are not so numerous, nor so distinct from each other, nor do they
project so much inward; yet this goose, as I am informed by Mr. E. Bartlett, "uses its bill like a duck by throwing
the water out at the corners." Its chief food, however, is grass, which it crops like the common goose. In this latter
bird the lamellae of the upper mandible are much coarser than in the common duck, almost confluent, about
twenty−seven in number on each side, and terminating upward in teeth−like knobs. The palate is also covered
with hard rounded knobs. The edges of the lower mandible are serrated with teeth much more prominent, coarser
and sharper than in the duck. The common goose does not sift the water, but uses its beak exclusively for tearing
or cutting herbage, for which purpose it is so well fitted that it can crop grass closer than almost any other animal.
There are other species of geese, as I hear from Mr. Bartlett, in which the lamellae are less developed than in the
common goose.
We thus see that a member of the duck family, with a beak constructed like that of a common goose and
adapted solely for grazing, or even a member with a beak having less well−developed lamellae, might be
converted by small changes into a species like the Egyptian goose−−this into one like the common duck−−and,
lastly, into one like the shoveller, provided with a beak almost exclusively adapted for sifting the water; for this
bird could hardly use any part of its beak, except the hooked tip, for seizing or tearing solid food. The beak of a
goose, as I may add, might also be converted by small changes into one provided with prominent, recurved teeth,
like those of the Merganser (a member of the same family), serving for the widely different purpose of securing
live fish.
Returning to the whales. The Hyperoodon bidens is destitute of true teeth in an efficient condition, but its
palate is roughened, according to Lacepede, with small unequal, hard points of horn. There is, therefore, nothing
improbable in supposing that some early Cetacean form was provided with similar points of horn on the palate,
but rather more regularly placed, and which, like the knobs on the beak of the goose, aided it in seizing or tearing
its food. If so, it will hardly be denied that the points might have been converted through variation and natural
selection into lamellae as well−developed as those of the Egyptian goose, in which case they would have been
used both for seizing objects and for sifting the water; then into lamellae like those of the domestic duck; and so
onward, until they became as well constructed as those of the shoveller, in which case they would have served
exclusively as a sifting apparatus. From this stage, in which the lamellae would be two−thirds of the length of the
plates of baleen in the Balaenoptera rostrata, gradations, which may be observed in still−existing Cetaceans, lead
us onward to the enormous plates of baleen in the Greenland whale. Nor is there the least reason to doubt that
each step in this scale might have been as serviceable to certain ancient Cetaceans, with the functions of the parts
slowly changing during the progress of development, as are the gradations in the beaks of the different existing
members of the duck−family. We should bear in mind that each species of duck is subjected to a severe struggle
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for existence, and that the structure of every part of its frame must be well adapted to its conditions of life.
The Pleuronectidae, or Flat−fish, are remarkable for their asymmetrical bodies. They rest on one side−−in the
greater number of species on the left, but in some on the right side; and occasionally reversed adult specimens
occur. The lower, or resting−surface, resembles at first sight the ventral surface of an ordinary fish; it is of a white
colour, less developed in many ways than the upper side, with the lateral fins often of smaller size. But the eyes
offer the most remarkable peculiarity; for they are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then symmetrical, with both sides equally
coloured. Soon the eye proper to the lower side begins to glide slowly round the head to the upper side; but does
not pass right through the skull, as was formerly thought to be the case. It is obvious that unless the lower eye did
thus travel round, it could not be used by the fish while lying in its habitual position on one side. The lower eye
would, also, have been liable to be abraded by the sandy bottom. That the Pleuronectidae are admirably adapted
by their flattened and asymmetrical structure for their habits of life, is manifest from several species, such as
soles, flounders, etc., being extremely common. The chief advantages thus gained seem to be protection from
their enemies, and facility for feeding on the ground. The different members, however, of the family present, as
Schiodte remarks, "a long series of forms exhibiting a gradual transition from Hippoglossus pinguis, which does
not in any considerable degree alter the shape in which it leaves the ovum, to the soles, which are entirely thrown
to one side."
Mr. Mivart has taken up this case, and remarks that a sudden spontaneous transformation in the position of the
eyes is hardly conceivable, in which I quite agree with him. He then adds: "If the transit was gradual, then how
such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the
individual is, indeed, far from clear. It seems, even, that such an incipient transformation must rather have been
injurious." But he might have found an answer to this objection in the excellent observations published in 1867 by
Malm. The Pleuronectidae, while very young and still symmetrical, with their eyes standing on opposite sides of
the head, cannot long retain a vertical position, owing to the excessive depth of their bodies, the small size of their
lateral fins, and to their being destitute of a swim−bladder. Hence, soon growing tired, they fall to the bottom on
one side. While thus at rest they often twist, as Malm observed, the lower eye upward, to see above them; and
they do this so vigorously that the eye is pressed hard against the upper part of the orbit. The forehead between
the eyes consequently becomes, as could be plainly seen, temporarily contracted in breadth. On one occasion
Malm saw a young fish raise and depress the lower eye through an angular distance of about seventy degrees.
We should remember that the skull at this early age is cartilaginous and flexible, so that it readily yields to
muscular action. It is also known with the higher animals, even after early youth, that the skull yields and is
altered in shape, if the skin or muscles be permanently contracted through disease or some accident. With
long−eared rabbits, if one ear flops forward and downward, its weight drags forward all the bones of the skull on
the same side, of which I have given a figure. Malm states that the newly−hatched young of perches, salmon, and
several other symmetrical fishes, have the habit of occasionally resting on one side at the bottom; and he has
observed that they often then strain their lower eyes so as to look upward; and their skulls are thus rendered rather
crooked. These fishes, however, are soon able to hold themselves in a vertical position, and no permanent effect is
thus produced. With the Pleuronectidae, on the other hand, the older they grow the more habitually they rest on
one side, owing to the increasing flatness of their bodies, and a permanent effect is thus produced on the form of
the head, and on the position of the eyes. Judging from analogy, the tendency to distortion would no doubt be
increased through the principle of inheritance. Schiodte believes, in opposition to some other naturalists, that the
Pleuronectidae are not quite symmetrical even in the embryo; and if this be so, we could understand how it is that
certain species, while young, habitually fall over and rest on the left side, and other species on the right side.
Malm adds, in confirmation of the above view, that the adult Trachypterus arcticus, which is not a member of the
Pleuronectidae, rests on its left side at the bottom, and swims diagonally through the water; and in this fish, the
two sides of the head are said to be somewhat dissimilar. Our great authority on Fishes, Dr. Gunther, concludes
his abstract of Malm's paper, by remarking that "the author gives a very simple explanation of the abnormal
condition of the Pleuronectoids."
We thus see that the first stages of the transit of the eye from one side of the head to the other, which Mr.
Mivart considers would be injurious, may be attributed to the habit, no doubt beneficial to the individual and to
the species, of endeavouring to look upward with both eyes, while resting on one side at the bottom. We may also
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attribute to the inherited effects of use the fact of the mouth in several kinds of flat−fish being bent towards the
lower surface, with the jaw bones stronger and more effective on this, the eyeless side of the head, than on the
other, for the sake, as Dr. Traquair supposes, of feeding with ease on the ground. Disuse, on the other hand, will
account for the less developed condition of the whole inferior half of the body, including the lateral fins; though
Yarrel thinks that the reduced size of these fins is advantageous to the fish, as "there is so much less room for their
action than with the larger fins above." Perhaps the lesser number of teeth in the proportion of four to seven in the
upper halves of the two jaws of the plaice, to twenty−five to thirty in the lower halves, may likewise be accounted
for by disuse. >From the colourless state of the ventral surface of most fishes and of many other animals, we may
reasonably suppose that the absence of colour in flat−fish on the side, whether it be the right or left, which is
under− most, is due to the exclusion of light. But it cannot be supposed that the peculiar speckled appearance of
the upper side of the sole, so like the sandy bed of the sea, or the power in some species, as recently shown by
Pouchet, of changing their colour in accordance with the surrounding surface, or the presence of bony tubercles
on the upper side of the turbot, are due to the action of the light. Here natural selection has probably come into
play, as well as in adapting the general shape of the body of these fishes, and many other peculiarities, to their
habits of life. We should keep in mind, as I have before insisted, that the inherited effects of the increased use of
parts, and perhaps of their disuse, will be strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals which inherit in the highest degree the effects of
the increased and beneficial use of any part. How much to attribute in each particular case to the effects of use,
and how much to natural selection, it seems impossible to decide.
I may give another instance of a structure which apparently owes its origin exclusively to use or habit. The
extremity of the tail in some American monkeys has been converted into a wonderfully perfect prehensile organ,
and serves as a fifth hand. A reviewer, who agrees with Mr. Mivart in every detail, remarks on this structure: "It is
impossible to believe that in any number of ages the first slight incipient tendency to grasp could preserve the
lives of the individuals possessing it, or favour their chance of having and of rearing offspring." But there is no
necessity for any such belief. Habit, and this almost implies that some benefit great or small is thus derived,
would in all probability suffice for the work. Brehm saw the young of an African monkey (Cercopithecus)
clinging to the under surface of their mother by their hands, and at the same time they hooked their little tails
round that of their mother. Professor Henslow kept in confinement some harvest mice (Mus messorius) which do
not possess a structurally prehensive tail; but he frequently observed that they curled their tails round the branches
of a bush placed in the cage, and thus aided themselves in climbing. I have received an analogous account from
Dr. Gunther, who has seen a mouse thus suspend itself. If the harvest mouse had been more strictly arboreal, it
would perhaps have had its tail rendered structurally prehensile, as is the case with some members of the same
order. Why Cercopithecus, considering its habits while young, has not become thus provided, it would be difficult
to say. It is, however, possible that the long tail of this monkey may be of more service to it as a balancing organ
in making its prodigious leaps, than as a prehensile organ.
The mammary glands are common to the whole class of mammals, and are indispensable for their existence;
they must, therefore, have been developed at an extremely remote period, and we can know nothing positively
about their manner of development. Mr. Mivart asks: "Is it conceivable that the young of any animal was ever
saved from destruction by accidentally sucking a drop of scarcely nutritious fluid from an accidentally
hypertrophied cutaneous gland of its mother? And even if one was so, what chance was there of the perpetuation
of such a variation?" But the case is not here put fairly. It is admitted by most evolutionists that mammals are
descended from a marsupial form; and if so, the mammary glands will have been at first developed within the
marsupial sack. In the case of the fish (Hippocampus) the eggs are hatched, and the young are reared for a time,
within a sack of this nature; and an American naturalist, Mr. Lockwood, believes from what he has seen of the
development of the young, that they are nourished by a secretion from the cutaneous glands of the sack. Now,
with the early progenitors of mammals, almost before they deserved to be thus designated, is it not at least
possible that the young might have been similarly nourished? And in this case, the individuals which secreted a
fluid, in some degree or manner the most nutritious, so as to partake of the nature of milk, would in the long run
have reared a larger number of well−nourished offspring, than would the individuals which secreted a poorer
fluid; and thus the cutaneous glands, which are the homologues of the mammary glands, would have been
improved or rendered more effective. It accords with the widely extended principle of specialisation, that the
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glands over a certain space of the sack should have become more highly developed than the remainder; and they
would then have formed a breast, but at first without a nipple, as we see in the Ornithorhyncus, at the base of the
mammalian series. Through what agency the glands over a certain space became more highly specialised than the
others, I will not pretend to decide, whether in part through compensation of growth, the effects of use, or of
natural selection.
The development of the mammary glands would have been of no service, and could not have been affected
through natural selection, unless the young at the same time were able to partake of the secretion. There is no
greater difficulty in understanding how young mammals have instinctively learned to suck the breast, than in
understanding how unhatched chickens have learned to break the egg−shell by tapping against it with their
specially adapted beaks; or how a few hours after leaving the shell they have learned to pick up grains of food. In
such cases the most probable solution seems to be, that the habit was at first acquired by practice at a more
advanced age, and afterwards transmitted to the offspring at an earlier age. But the young kangaroo is said not to
suck, only to cling to the nipple of its mother, who has the power of injecting milk into the mouth of her helpless,
half−formed offspring. On this head Mr. Mivart remarks: "Did no special provision exist, the young one must
infallibly be choked by the intrusion of the milk into the wind−pipe. But there IS a special provision. The larynx
is so elongated that it rises up into the posterior end of the nasal passage, and is thus enabled to give free entrance
to the air for the lungs, while the milk passes harmlessly on each side of this elongated larynx, and so safely
attains the gullet behind it." Mr. Mivart then asks how did natural selection remove in the adult kangaroo (and in
most other mammals, on the assumption that they are descended from a marsupial form), "this at least perfectly
innocent and harmless structure?" It may be suggested in answer that the voice, which is certainly of high
importance to many animals, could hardly have been used with full force as long as the larynx entered the nasal
passage; and Professor Flower has suggested to me that this structure would have greatly interfered with an
animal swallowing solid food.
We will now turn for a short space to the lower divisions of the animal kingdom. The Echinodermata
(star−fishes, sea−urchins, etc.) are furnished with remarkable organs, called pedicellariae, which consist, when
well developed, of a tridactyle forceps−−that is, of one formed of three serrated arms, neatly fitting together and
placed on the summit of a flexible stem, moved by muscles. These forceps can seize firmly hold of any object;
and Alexander Agassiz has seen an Echinus or sea−urchin rapidly passing particles of excrement from forceps to
forceps down certain lines of its body, in order that its shell should not be fouled. But there is no doubt that
besides removing dirt of all kinds, they subserve other functions; and one of these apparently is defence.
With respect to these organs, Mr. Mivart, as on so many previous occasions, asks: "What would be the utility
of the FIRST RUDIMENTARY BEGINNINGS of such structures, and how could such insipient buddings have
ever preserved the life of a single Echinus?" He adds, "not even the SUDDEN development of the snapping action
would have been beneficial without the freely movable stalk, nor could the latter have been efficient without the
snapping jaws, yet no minute, nearly indefinite variations could simultaneously evolve these complex
co−ordinations of structure; to deny this seems to do no less than to affirm a startling paradox." Paradoxical as
this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but capable of a snapping
action, certainly exist on some star− fishes; and this is intelligible if they serve, at least in part, as a means of
defence. Mr. Agassiz, to whose great kindness I am indebted for much information on the subject, informs me
that there are other star− fishes, in which one of the three arms of the forceps is reduced to a support for the other
two; and again, other genera in which the third arm is completely lost. In Echinoneus, the shell is described by M.
Perrier as bearing two kinds of pedicellariae, one resembling those of Echinus, and the other those of Spatangus;
and such cases are always interesting as affording the means of apparently sudden transitions, through the
abortion of one of the two states of an organ.
With respect to the steps by which these curious organs have been evolved, Mr. Agassiz infers from his own
researches and those of Mr. Muller, that both in star−fishes and sea−urchins the pedicellariae must undoubtedly
be looked at as modified spines. This may be inferred from their manner of development in the individual, as well
as from a long and perfect series of gradations in different species and genera, from simple granules to ordinary
spines, to perfect tridactyle pedicellariae. The gradation extends even to the manner in which ordinary spines and
the pedicellariae, with their supporting calcareous rods, are articulated to the shell. In certain genera of
star−fishes, "the very combinations needed to show that the pedicellariae are only modified branching spines"
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may be found. Thus we have fixed spines, with three equi−distant, serrated, movable branches, articulated to near
their bases; and higher up, on the same spine, three other movable branches. Now when the latter arise from the
summit of a spine they form, in fact, a rude tridactyle pedicellariae, and such may be seen on the same spine
together with the three lower branches. In this case the identity in nature between the arms of the pedicellariae and
the movable branches of a spine, is unmistakable. It is generally admitted that the ordinary spines serve as a
protection; and if so, there can be no reason to doubt that those furnished with serrated and movable branches
likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting
together they acted as a prehensile or snapping apparatus. Thus every gradation, from an ordinary fixed spine to a
fixed pedicellariae, would be of service.
In certain genera of star−fishes these organs, instead of being fixed or borne on an immovable support, are
placed on the summit of a flexible and muscular, though short, stem; and in this case they probably subserve some
additional function besides defence. In the sea−urchins the steps can be followed by which a fixed spine becomes
articulated to the shell, and is thus rendered movable. I wish I had space here to give a fuller abstract of Mr.
Agassiz's interesting observations on the development of the pedicellariae. All possible gradations, as he adds,
may likewise be found between the pedicellariae of the star−fishes and the hooks of the Ophiurians, another group
of the Echinodermata; and again between the pedicellariae of sea−urchins and the anchors of the Holothuriae, also
belonging to the same great class.
Certain compound animals, or zoophytes, as they have been termed, namely the Polyzoa, are provided with
curious organs called avicularia. These differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in miniature, seated on a neck and capable of
movement, as is likewise the lower jaw or mandible. In one species observed by me, all the avicularia on the same
branch often moved simultaneously backwards and forwards, with the lower jaw widely open, through an angle of
about 90 degrees, in the course of five seconds; and their movement caused the whole polyzoary to tremble.
When the jaws are touched with a needle they seize it so firmly that the branch can thus be shaken.
Mr. Mivart adduces this case, chiefly on account of the supposed difficulty of organs, namely the avicularia of
the Polyzoa and the pedicellariae of the Echinodermata, which he considers as "essentially similar," having been
developed through natural selection in widely distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariae and avicularia. The latter resembles somewhat
more closely the chelae or pincers of Crustaceans; and Mr. Mivart might have adduced with equal appropriateness
this resemblance as a special difficulty, or even their resemblance to the head and beak of a bird. The avicularia
are believed by Mr. Busk, Dr. Smitt and Dr. Nitsche−−naturalists who have carefully studied this group−−to be
homologous with the zooids and their cells which compose the zoophyte, the movable lip or lid of the cell
corresponding with the lower and movable mandible of the avicularium. Mr. Busk, however, does not know of
any gradations now existing between a zooid and an avicularium. It is therefore impossible to conjecture by what
serviceable gradations the one could have been converted into the other, but it by no means follows from this that
such gradations have not existed.
As the chelae of Crustaceans resemble in some degree the avicularia of Polyzoa, both serving as pincers, it
may be worth while to show that with the former a long series of serviceable gradations still exists. In the first and
simplest stage, the terminal segment of a limb shuts down either on the square summit of the broad penultimate
segment, or against one whole side, and is thus enabled to catch hold of an object, but the limb still serves as an
organ of locomotion. We next find one corner of the broad penultimate segment slightly prominent, sometimes
furnished with irregular teeth, and against these the terminal segment shuts down. By an increase in the size of
this projection, with its shape, as well as that of the terminal segment, slightly modified and improved, the pincers
are rendered more and more perfect, until we have at last an instrument as efficient as the chelae of a lobster. And
all these gradations can be actually traced.
Besides the avicularia, the polyzoa possess curious organs called vibracula. These generally consist of long
bristles, capable of movement and easily excited. In one species examined by me the vibracula were slightly
curved and serrated along the outer margin, and all of them on the same polyzoary often moved simultaneously;
so that, acting like long oars, they swept a branch rapidly across the object−glass of my microscope. When a
branch was placed on its face, the vibracula became entangled, and they made violent efforts to free themselves.
They are supposed to serve as a defence, and may be seen, as Mr. Busk remarks, "to sweep slowly and carefully
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over the surface of the polyzoary, removing what might be noxious to the delicate inhabitants of the cells when
their tentacula are protruded." The avicularia, like the vibracula, probably serve for defence, but they also catch
and kill small living animals, which, it is believed, are afterwards swept by the currents within reach of the
tentacula of the zooids. Some species are provided with avicularia and vibracula, some with avicularia alone and a
few with vibracula alone.
It is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum, and an
avicularium like the head of a bird; yet they are almost certainly homologous and have been developed from the
same common source, namely a zooid with its cell. Hence, we can understand how it is that these organs graduate
in some cases, as I am informed by Mr. Busk, into each other. Thus, with the avicularia of several species of
Lepralia, the movable mandible is so much produced and is so like a bristle that the presence of the upper or fixed
beak alone serves to determine its avicularian nature. The vibracula may have been directly developed from the
lips of the cells, without having passed through the avicularian stage; but it seems more probable that they have
passed through this stage, as during the early stages of the transformation, the other parts of the cell, with the
included zooid, could hardly have disappeared at once. In many cases the vibracula have a grooved support at the
base, which seems to represent the fixed beak; though this support in some species is quite absent. This view of
the development of the vibracula, if trustworthy, is interesting; for supposing that all the species provided with
avicularia had become extinct, no one with the most vivid imagination would ever have thought that the vibracula
had originally existed as part of an organ, resembling a bird's head, or an irregular box or hood. It is interesting to
see two such widely different organs developed from a common origin; and as the movable lip of the cell serves
as a protection to the zooid, there is no difficulty in believing that all the gradations, by which the lip became
converted first into the lower mandible of an avicularium, and then into an elongated bristle, likewise served as a
protection in different ways and under different circumstances.
In the vegetable kingdom Mr. Mivart only alludes to two cases, namely the structure of the flowers of orchids,
and the movements of climbing plants. With respect to the former, he says: "The explanation of their ORIGIN is
deemed thoroughly unsatisfactory−−utterly insufficient to explain the incipient, infinitesimal beginnings of
structures which are of utility only when they are considerably developed." As I have fully treated this subject in
another work, I will here give only a few details on one alone of the most striking peculiarities of the flowers of
orchids, namely, their pollinia. A pollinium, when highly developed, consists of a mass of pollen−grains, affixed
to an elastic foot−stalk or caudicle, and this to a little mass of extremely viscid matter. The pollinia are by this
means transported by insects from one flower to the stigma of another. In some orchids there is no caudicle to the
pollen−masses, and the grains are merely tied together by fine threads; but as these are not confined to orchids,
they need not here be considered; yet I may mention that at the base of the orchidaceous series, in Cypripedium,
we can see how the threads were probably first developed. In other orchids the threads cohere at one end of the
pollen−masses; and this forms the first or nascent trace of a caudicle. That this is the origin of the caudicle, even
when of considerable length and highly developed, we have good evidence in the aborted pollen−grains which
can sometimes be detected embedded within the central and solid parts.
With respect to the second chief peculiarity, namely, the little mass of viscid matter attached to the end of the
caudicle, a long series of gradations can be specified, each of plain service to the plant. In most flowers belonging
to other orders the stigma secretes a little viscid matter. Now, in certain orchids similar viscid matter is secreted,
but in much larger quantities by one alone of the three stigmas; and this stigma, perhaps in consequence of the
copious secretion, is rendered sterile. When an insect visits a flower of this kind, it rubs off some of the viscid
matter, and thus at the same time drags away some of the pollen−grains. >From this simple condition, which
differs but little from that of a multitude of common flowers, there are endless gradations−−to species in which
the pollen−mass terminates in a very short, free caudicle−−to others in which the caudicle becomes firmly
attached to the viscid matter, with the sterile stigma itself much modified. In this latter case we have a pollinium
in its most highly developed and perfect condition. He who will carefully examine the flowers of orchids for
himself will not deny the existence of the above series of gradations−−from a mass of pollen−grains merely tied
together by threads, with the stigma differing but little from that of the ordinary flowers, to a highly complex
pollinium, admirably adapted for transportal by insects; nor will he deny that all the gradations in the several
species are admirably adapted in relation to the general structure of each flower for its fertilisation by different
insects. In this, and in almost every other case, the enquiry may be pushed further backwards; and it may be asked
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The Origin of Species by means of Natural Selection
how did the stigma of an ordinary flower become viscid, but as we do not know the full history of any one group
of beings, it is as useless to ask, as it is hopeless to attempt answering, such questions.
We will now turn to climbing plants. These can be arranged in a long series, from those which simply twine
round a support, to those which I have called leaf−climbers, and to those provided with tendrils. In these two
latter classes the stems have generally, but not always, lost the power of twining, though they retain the power of
revolving, which the tendrils likewise possess. The gradations from leaf−climbers to tendril bearers are
wonderfully close, and certain plants may be differently placed in either class. But in ascending the series from
simple twiners to leaf−climbers, an important quality is added, namely sensitiveness to a touch, by which means
the foot−stalks of the leaves or flowers, or these modified and converted into tendrils, are excited to bend round
and clasp the touching object. He who will read my memoir on these plants will, I think, admit that all the many
gradations in function and structure between simple twiners and tendril−bearers are in each case beneficial in a
high degree to the species. For instance, it is clearly a great advantage to a twining plant to become a
leaf−climber; and it is probable that every twiner which possessed leaves with long foot−stalks would have been
developed into a leaf−climber, if the foot−stalks had possessed in any slight degree the requisite sensitiveness to a
touch.
As twining is the simplest means of ascending a support, and forms the basis of our series, it may naturally be
asked how did plants acquire this power in an incipient degree, afterwards to be improved and increased through
natural selection. The power of twining depends, firstly, on the stems while young being extremely flexible (but
this is a character common to many plants which are not climbers); and, secondly, on their continually bending to
all points of the compass, one after the other in succession, in the same order. By this movement the stems are
inclined to all sides, and are made to move round and round. As soon as the lower part of a stem strikes against
any object and is stopped, the upper part still goes on bending and revolving, and thus necessarily twines round
and up the support. The revolving movement ceases after the early growth of each shoot. As in many widely
separated families of plants, single species and single genera possess the power of revolving, and have thus
become twiners, they must have independently acquired it, and cannot have inherited it from a common
progenitor. Hence, I was led to predict that some slight tendency to a movement of this kind would be found to be
far from uncommon with plants which did not climb; and that this had afforded the basis for natural selection to
work on and improve. When I made this prediction, I knew of only one imperfect case, namely, of the young
flower−peduncles of a Maurandia which revolved slightly and irregularly, like the stems of twining plants, but
without making any use of this habit. Soon afterwards Fritz Muller discovered that the young stems of an Alisma
and of a Linum−− plants which do not climb and are widely separated in the natural system−− revolved plainly,
though irregularly, and he states that he has reason to suspect that this occurs with some other plants. These slight
movements appear to be of no service to the plants in question; anyhow, they are not of the least use in the way of
climbing, which is the point that concerns us. Nevertheless we can see that if the stems of these plants had been
flexible, and if under the conditions to which they are exposed it had profited them to ascend to a height, then the
habit of slightly and irregularly revolving might have been increased and utilised through natural selection, until
they had become converted into well−developed twining species.
With respect to the sensitiveness of the foot−stalks of the leaves and flowers, and of tendrils, nearly the same
remarks are applicable as in the case of the revolving movements of twining plants. As a vast number of species,
belonging to widely distinct groups, are endowed with this kind of sensitiveness, it ought to be found in a nascent
condition in many plants which have not become climbers. This is the case: I observed that the young
flower−peduncles of the above Maurandia curved themselves a little towards the side which was touched. Morren
found in several species of Oxalis that the leaves and their foot−stalks moved, especially after exposure to a hot
sun, when they were gently and repeatedly touched, or when the plant was shaken. I repeated these observations
on some other species of Oxalis with the same result; in some of them the movement was distinct, but was best
seen in the young leaves; in others it was extremely slight. It is a more important fact that according to the high
authority of Hofmeister, the young shoots and leaves of all plants move after being shaken; and with climbing
plants it is, as we know, only during the early stages of growth that the foot−stalks and tendrils are sensitive.
It is scarcely possible that the above slight movements, due to a touch or shake, in the young and growing
organs of plants, can be of any functional importance to them. But plants possess, in obedience to various stimuli,
powers of movement, which are of manifest importance to them; for instance, towards and more rarely from the
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The Origin of Species by means of Natural Selection
light−−in opposition to, and more rarely in the direction of, the attraction of gravity. When the nerves and muscles
of an animal are excited by galvanism or by the absorption of strychnine, the consequent movements may be
called an incidental result, for the nerves and muscles have not been rendered specially sensitive to these stimuli.
So with plants it appears that, from having the power of movement in obedience to certain stimuli, they are
excited in an incidental manner by a touch, or by being shaken. Hence there is no great difficulty in admitting that
in the case of leaf−climbers and tendril−bearers, it is this tendency which has been taken advantage of and
increased through natural selection. It is, however, probable, from reasons which I have assigned in my memoir,
that this will have occurred only with plants which had already acquired the power of revolving, and had thus
become twiners.
I have already endeavoured to explain how plants became twiners, namely, by the increase of a tendency to
slight and irregular revolving movements, which were at first of no use to them; this movement, as well as that
due to a touch or shake, being the incidental result of the power of moving, gained for other and beneficial
purposes. Whether, during the gradual development of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that certain periodical movements, for instance
the so−called sleep of plants, are governed by habit.
I have now considered enough, perhaps more than enough, of the cases, selected with care by a skilful
naturalist, to prove that natural selection is incompetent to account for the incipient stages of useful structures;
and I have shown, as I hope, that there is no great difficulty on this head. A good opportunity has thus been
afforded for enlarging a little on gradations of structure, often associated with strange functions−−an important
subject, which was not treated at sufficient length in the former editions of this work. I will now briefly
recapitulate the foregoing cases.
With the giraffe, the continued preservation of the individuals of some extinct high−reaching ruminant, which
had the longest necks, legs, etc., and could browse a little above the average height, and the continued destruction
of those which could not browse so high, would have sufficed for the production of this remarkable quadruped;
but the prolonged use of all the parts, together with inheritance, will have aided in an important manner in their
co−ordination. With the many insects which imitate various objects, there is no improbability in the belief that an
accidental resemblance to some common object was in each case the foundation for the work of natural selection,
since perfected through the occasional preservation of slight variations which made the resemblance at all closer;
and this will have been carried on as long as the insect continued to vary, and as long as a more and more perfect
resemblance led to its escape from sharp−sighted enemies. In certain species of whales there is a tendency to the
formation of irregular little points of horn on the palate; and it seems to be quite within the scope of natural
selection to preserve all favourable variations, until the points were converted, first into lamellated knobs or teeth,
like those on the beak of a goose−−then into short lamellae, like those of the domestic ducks−−and then into
lamellae, as perfect as those of the shoveller−duck−−and finally into the gigantic plates of baleen, as in the mouth
of the Greenland whale. In the family of the ducks, the lamellae are first used as teeth, then partly as teeth and
partly as a sifting apparatus, and at last almost exclusively for this latter purpose.
With such structures as the above lamellae of horn or whalebone, habit or use can have done little or nothing,
as far as we can judge, towards their development. On the other hand, the transportal of the lower eye of a
flat−fish to the upper side of the head, and the formation of a prehensile tail, may be attributed almost wholly to
continued use, together with inheritance. With respect to the mammae of the higher animals, the most probable
conjecture is that primordially the cutaneous glands over the whole surface of a marsupial sack secreted a
nutritious fluid; and that these glands were improved in function through natural selection, and concentrated into a
confined area, in which case they would have formed a mamma. There is no more difficulty in understanding how
the branched spines of some ancient Echinoderm, which served as a defence, became developed through natural
selection into tridactyle pedicellariae, than in understanding the development of the pincers of crustaceans,
through slight, serviceable modifications in the ultimate and penultimate segments of a limb, which was at first
used solely for locomotion. In the avicularia and vibracula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with the vibracula we can understand how the successive
gradations might have been of service. With the pollinia of orchids, the threads which originally served to tie
together the pollen−grains, can be traced cohering into caudicles; and the steps can likewise be followed by which
viscid matter, such as that secreted by the stigmas of ordinary flowers, and still subserving nearly but not quite the
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same purpose, became attached to the free ends of the caudicles−−all these gradations being of manifest benefit to
the plants in question. With respect to climbing plants, I need not repeat what has been so lately said.
It has often been asked, if natural selection be so potent, why has not this or that structure been gained by
certain species, to which it would apparently have been advantageous? But it is unreasonable to expect a precise
answer to such questions, considering our ignorance of the past history of each species, and of the conditions
which at the present day determine its numbers and range. In most cases only general reasons, but in some few
cases special reasons, can be assigned. Thus to adapt a species to new habits of life, many co−ordinated
modifications are almost indispensable, and it may often have happened that the requisite parts did not vary in the
right manner or to the right degree. Many species must have been prevented from increasing in numbers through
destructive agencies, which stood in no relation to certain structures, which we imagine would have been gained
through natural selection from appearing to us advantageous to the species. In this case, as the struggle for life did
not depend on such structures, they could not have been acquired through natural selection. In many cases
complex and long−enduring conditions, often of a peculiar nature, are necessary for the development of a
structure; and the requisite conditions may seldom have concurred. The belief that any given structure, which we
think, often erroneously, would have been beneficial to a species, would have been gained under all circumstances
through natural selection, is opposed to what we can understand of its manner of action. Mr. Mivart does not deny
that natural selection has effected something; but he considers it as "demonstrably insufficient" to account for the
phenomena which I explain by its agency. His chief arguments have now been considered, and the others will
hereafter be considered. They seem to me to partake little of the character of demonstration, and to have little
weight in comparison with those in favour of the power of natural selection, aided by the other agencies often
specified. I am bound to add, that some of the facts and arguments here used by me, have been advanced for the
same purpose in an able article lately published in the "Medico−Chirurgical Review."
At the present day almost all naturalists admit evolution under some form. Mr. Mivart believes that species
change through "an internal force or tendency," about which it is not pretended that anything is known. That
species have a capacity for change will be admitted by all evolutionists; but there is no need, as it seems to me, to
invoke any internal force beyond the tendency to ordinary variability, which through the aid of selection, by man
has given rise to many well−adapted domestic races, and which, through the aid of natural selection, would
equally well give rise by graduated steps to natural races or species. The final result will generally have been, as
already explained, an advance, but in some few cases a retrogression, in organisation.
Mr. Mivart is further inclined to believe, and some naturalists agree with him, that new species manifest
themselves "with suddenness and by modifications appearing at once." For instance, he supposes that the
differences between the extinct three−toed Hipparion and the horse arose suddenly. He thinks it difficult to
believe that the wing of a bird "was developed in any other way than by a comparatively sudden modification of a
marked and important kind;" and apparently he would extend the same view to the wings of bats and
pterodactyles. This conclusion, which implies great breaks or discontinuity in the series, appears to me
improbable in the highest degree.
Everyone who believes in slow and gradual evolution, will of course admit that specific changes may have
been as abrupt and as great as any single variation which we meet with under nature, or even under domestication.
But as species are more variable when domesticated or cultivated than under their natural conditions, it is not
probable that such great and abrupt variations have often occurred under nature, as are known occasionally to
arise under domestication. Of these latter variations several may be attributed to reversion; and the characters
which thus reappear were, it is probable, in many cases at first gained in a gradual manner. A still greater number
must be called monstrosities, such as six−fingered men, porcupine men, Ancon sheep, Niata cattle, etc.; and as
they are widely different in character from natural species, they throw very little light on our subject. Excluding
such cases of abrupt variations, the few which remain would at best constitute, if found in a state of nature,
doubtful species, closely related to their parental types.
My reasons for doubting whether natural species have changed as abruptly as have occasionally domestic
races, and for entirely disbelieving that they have changed in the wonderful manner indicated by Mr. Mivart, are
as follows. According to our experience, abrupt and strongly marked variations occur in our domesticated
productions, singly and at rather long intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction and by subsequent intercrossing; and so it is
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The Origin of Species by means of Natural Selection
known to be under domestication, unless abrupt variations of this kind are specially preserved and separated by
the care of man. Hence, in order that a new species should suddenly appear in the manner supposed by Mr.
Mivart, it is almost necessary to believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This difficulty, as in the case of unconscious
selection by man, is avoided on the theory of gradual evolution, through the preservation of a large number of
individuals, which varied more or less in any favourable direction, and of the destruction of a large number which
varied in an opposite manner.
That many species have been evolved in an extremely gradual manner, there can hardly be a doubt. The
species and even the genera of many large natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from north to south, from lowland to upland,
etc., we meet with a host of closely related or representative species; as we likewise do on certain distinct
continents, which we have reason to believe were formerly connected. But in making these and the following
remarks, I am compelled to allude to subjects hereafter to be discussed. Look at the many outlying islands round a
continent, and see how many of their inhabitants can be raised only to the rank of doubtful species. So it is if we
look to past times, and compare the species which have just passed away with those still living within the same
areas; or if we compare the fossil species embedded in the sub−stages of the same geological formation. It is
indeed manifest that multitudes of species are related in the closest manner to other species that still exist, or have
lately existed; and it will hardly be maintained that such species have been developed in an abrupt or sudden
manner. Nor should it be forgotten, when we look to the special parts of allied species, instead of to distinct
species, that numerous and wonderfully fine gradations can be traced, connecting together widely different
structures.
Many large groups of facts are intelligible only on the principle that species have been evolved by very small
steps. For instance, the fact that the species included in the larger genera are more closely related to each other,
and present a greater number of varieties than do the species in the smaller genera. The former are also grouped in
little clusters, like varieties round species; and they present other analogies with varieties, as was shown in our
second chapter. On this same principle we can understand how it is that specific characters are more variable than
generic characters; and how the parts which are developed in an extraordinary degree or manner are more variable
than other parts of the same species. Many analogous facts, all pointing in the same direction, could be added.
Although very many species have almost certainly been produced by steps not greater than those separating
fine varieties; yet it may be maintained that some have been developed in a different and abrupt manner. Such an
admission, however, ought not to be made without strong evidence being assigned. The vague and in some
respects false analogies, as they have been shown to be by Mr. Chauncey Wright, which have been advanced in
favour of this view, such as the sudden crystallisation of inorganic substances, or the falling of a facetted spheroid
from one facet to another, hardly deserve consideration. One class of facts, however, namely, the sudden
appearance of new and distinct forms of life in our geological formations supports at first sight the belief in abrupt
development. But the value of this evidence depends entirely on the perfection of the geological record, in relation
to periods remote in the history of the world. If the record is as fragmentary as many geologists strenuously assert,
there is nothing strange in new forms appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated by Mr. Mivart, such as the sudden
development of the wings of birds or bats, or the sudden conversion of a Hipparion into a horse, hardly any light
is thrown by the belief in abrupt modifications on the deficiency of connecting links in our geological formations.
But against the belief in such abrupt changes, embryology enters a strong protest. It is notorious that the wings of
birds and bats, and the legs of horses or other quadrupeds, are undistinguishable at an early embryonic period, and
that they become differentiated by insensibly fine steps. Embryological resemblances of all kinds can be
accounted for, as we shall hereafter see, by the progenitors of our existing species having varied after early youth,
and having transmitted their newly−acquired characters to their offspring, at a corresponding age. The embryo is
thus left almost unaffected, and serves as a record of the past condition of the species. Hence it is that existing
species during the early stages of their development so often resemble ancient and extinct forms belonging to the
same class. On this view of the meaning of embryological resemblances, and indeed on any view, it is incredible
that an animal should have undergone such momentous and abrupt transformations as those above indicated, and
yet should not bear even a trace in its embryonic condition of any sudden modification, every detail in its
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The Origin of Species by means of Natural Selection
structure being developed by insensibly fine steps.
He who believes that some ancient form was transformed suddenly through an internal force or tendency into,
for instance, one furnished with wings, will be almost compelled to assume, in opposition to all analogy, that
many individuals varied simultaneously. It cannot be denied that such abrupt and great changes of structure are
widely different from those which most species apparently have undergone. He will further be compelled to
believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding
conditions, have been suddenly produced; and of such complex and wonderful co−adaptations, he will not be able
to assign a shadow of an explanation. He will be forced to admit that these great and sudden transformations have
left no trace of their action on the embryo. To admit all this is, as it seems to me, to enter into the realms of
miracle, and to leave those of science.
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The Origin of Species by means of Natural Selection
CHAPTER VIII. INSTINCT.
Instincts comparable with habits, but different in their origin −− Instincts graduated −− Aphides and ants −−
Instincts variable −− Domestic instincts, their origin −− Natural instincts of the cuckoo, molothrus, ostrich, and
parasitic bees −− Slave−making ants −− Hive−bee, its cell−making instinct −− Changes of instinct and structure
not necessarily simultaneous −− Difficulties of the theory of the Natural Selection of instincts −− Neuter or sterile
insects −− Summary.
Many instincts are so wonderful that their development will probably appear to the reader a difficulty
sufficient to overthrow my whole theory. I may here premise, that I have nothing to do with the origin of the
mental powers, any more than I have with that of life itself. We are concerned only with the diversities of instinct
and of the other mental faculties in animals of the same class.
I will not attempt any definition of instinct. It would be easy to show that several distinct mental actions are
commonly embraced by this term; but every one understands what is meant, when it is said that instinct impels
the cuckoo to migrate and to lay her eggs in other birds' nests. An action, which we ourselves require experience
to enable us to perform, when performed by an animal, more especially by a very young one, without experience,
and when performed by many individuals in the same way, without their knowing for what purpose it is
performed, is usually said to be instinctive. But I could show that none of these characters are universal. A little
dose of judgment or reason, as Pierre Huber expresses it, often comes into play, even with animals low in the
scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared instinct with habit. This comparison
gives, I think, an accurate notion of the frame of mind under which an instinctive action is performed, but not
necessarily of its origin. How unconsciously many habitual actions are performed, indeed not rarely in direct
opposition to our conscious will! yet they may be modified by the will or reason. Habits easily become associated
with other habits, with certain periods of time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between instincts and habits could be pointed out.
As in repeating a well−known song, so in instincts, one action follows another by a sort of rhythm; if a person be
interrupted in a song, or in repeating anything by rote, he is generally forced to go back to recover the habitual
train of thought: so P. Huber found it was with a caterpillar, which makes a very complicated hammock; for if he
took a caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a
hammock completed up only to the third stage, the caterpillar simply re−performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a hammock made up, for instance, to the third
stage, and were put into one finished up to the sixth stage, so that much of its work was already done for it, far
from deriving any benefit from this, it was much embarrassed, and, in order to complete its hammock, seemed
forced to start from the third stage, where it had left off, and thus tried to complete the already finished work.
If we suppose any habitual action to become inherited−−and it can be shown that this does sometimes
happen−−then the resemblance between what originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years old with wonderfully little practice, had
played a tune with no practice at all, be might truly be said to have done so instinctively. But it would be a serious
error to suppose that the greater number of instincts have been acquired by habit in one generation, and then
transmitted by inheritance to succeeding generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive−bee and of many ants, could not possibly have been
acquired by habit.
It will be universally admitted that instincts are as important as corporeal structures for the welfare of each
species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight
modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving and continually accumulating variations of
instinct to any extent that was profitable. It is thus, as I believe, that all the most complex and wonderful instincts
have originated. As modifications of corporeal structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with instincts. But I believe that the effects of habit are
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The Origin of Species by means of Natural Selection
in many cases of subordinate importance to the effects of the natural selection of what may be called spontaneous
variations of instincts;−−that is of variations produced by the same unknown causes which produce slight
deviations of bodily structure.
No complex instinct can possibly be produced through natural selection, except by the slow and gradual
accumulation of numerous, slight, yet profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which each complex instinct has been
acquired−−for these could be found only in the lineal ancestors of each species−−but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that
gradations of some kind are possible; and this we certainly can do. I have been surprised to find, making
allowance for the instincts of animals having been but little observed, except in Europe and North America, and
for no instinct being known among extinct species, how very generally gradations, leading to the most complex
instincts, can be discovered. Changes of instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different seasons of the year, or when placed under different
circumstances, etc.; in which case either the one or the other instinct might be preserved by natural selection. And
such instances of diversity of instinct in the same species can be shown to occur in nature.
Again, as in the case of corporeal structure, and conformably to my theory, the instinct of each species is good
for itself, but has never, as far as we can judge, been produced for the exclusive good of others. One of the
strongest instances of an animal apparently performing an action for the sole good of another, with which I am
acquainted, is that of aphides voluntarily yielding, as was first observed by Huber, their sweet excretion to ants:
that they do so voluntarily, the following facts show. I removed all the ants from a group of about a dozen aphides
on a dock−plant, and prevented their attendance during several hours. After this interval, I felt sure that the
aphides would want to excrete. I watched them for some time through a lens, but not one excreted; I then tickled
and stroked them with a hair in the same manner, as well as I could, as the ants do with their antennae; but not one
excreted. Afterwards, I allowed an ant to visit them, and it immediately seemed, by its eager way of running about
to be well aware what a rich flock it had discovered; it then began to play with its antennae on the abdomen first
of one aphis and then of another; and each, as soon as it felt the antennae, immediately lifted up its abdomen and
excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. Even the quite young aphides
behaved in this manner, showing that the action was instinctive, and not the result of experience. It is certain,
from the observations of Huber, that the aphides show no dislike to the ants: if the latter be not present they are at
last compelled to eject their excretion. But as the excretion is extremely viscid, it is no doubt a convenience to the
aphides to have it removed; therefore probably they do not excrete solely for the good of the ants. Although there
is no evidence that any animal performs an action for the exclusive good of another species, yet each tries to take
advantage of the instincts of others, as each takes advantage of the weaker bodily structure of other species. So
again certain instincts cannot be considered as absolutely perfect; but as details on this and other such points are
not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are
indispensable for the action of natural selection, as many instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts certainly do vary−−for instance, the migratory instinct, both in
extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in dependence on the
situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly
unknown to us. Audubon has given several remarkable cases of differences in the nests of the same species in the
northern and southern United States. Why, it has been asked, if instinct be variable, has it not granted to the bee
"the ability to use some other material when wax was deficient?" But what other natural material could bees use?
They will work, as I have seen, with wax hardened with vermilion or softened with lard. Andrew Knight observed
that his bees, instead of laboriously collecting propolis, used a cement of wax and turpentine, with which he had
covered decorticated trees. It has lately been shown that bees, instead of searching for pollen, will gladly use a
very different substance, namely, oatmeal. Fear of any particular enemy is certainly an instinctive quality, as may
be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the same enemy in
other animals. The fear of man is slowly acquired, as I have elsewhere shown, by the various animals which
inhabit desert islands; and we see an instance of this, even in England, in the greater wildness of all our large birds
in comparison with our small birds; for the large birds have been most persecuted by man. We may safely
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attribute the greater wildness of our large birds to this cause; for in uninhabited islands large birds are not more
fearful than small; and the magpie, so wary in England, is tame in Norway, as is the hooded crow in Egypt.
That the mental qualities of animals of the same kind, born in a state of nature, vary much, could be shown by
many facts. Several cases could also be adduced of occasional and strange habits in wild animals, which, if
advantageous to the species, might have given rise, through natural selection, to new instincts. But I am well
aware that these general statements, without the facts in detail, can produce but a feeble effect on the reader's
mind. I can only repeat my assurance, that I do not speak without good evidence.
INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.
The possibility, or even probability, of inherited variations of instinct in a state of nature will be strengthened
by briefly considering a few cases under domestication. We shall thus be enabled to see the part which habit and
the selection of so−called spontaneous variations have played in modifying the mental qualities of our domestic
animals. It is notorious how much domestic animals vary in their mental qualities. With cats, for instance, one
naturally takes to catching rats, and another mice, and these tendencies are known to be inherited. One cat,
according to Mr. St. John, always brought home game birds, another hares or rabbits, and another hunted on
marshy ground and almost nightly caught woodcocks or snipes. A number of curious and authentic instances
could be given of various shades of disposition and taste, and likewise of the oddest tricks, associated with certain
frames of mind or periods of time. But let us look to the familiar case of the breeds of dogs: it cannot be doubted
that young pointers (I have myself seen striking instances) will sometimes point and even back other dogs the
very first time that they are taken out; retrieving is certainly in some degree inherited by retrievers; and a tendency
to run round, instead of at, a flock of sheep, by shepherd−dogs. I cannot see that these actions, performed without
experience by the young, and in nearly the same manner by each individual, performed with eager delight by each
breed, and without the end being known−−for the young pointer can no more know that he points to aid his
master, than the white butterfly knows why she lays her eggs on the leaf of the cabbage−−I cannot see that these
actions differ essentially from true instincts. If we were to behold one kind of wolf, when young and without any
training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with a
peculiar gait; and another kind of wolf rushing round, instead of at, a herd of deer, and driving them to a distant
point, we should assuredly call these actions instinctive. Domestic instincts, as they may be called, are certainly
far less fixed than natural instincts; but they have been acted on by far less rigorous selection, and have been
transmitted for an incomparably shorter period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions are inherited, and how curiously they become
mingled, is well shown when different breeds of dogs are crossed. Thus it is known that a cross with a bull−dog
has affected for many generations the courage and obstinacy of greyhounds; and a cross with a greyhound has
given to a whole family of shepherd−dogs a tendency to hunt hares. These domestic instincts, when thus tested by
crossing, resemble natural instincts, which in a like manner become curiously blended together, and for a long
period exhibit traces of the instincts of either parent: for example, Le Roy describes a dog, whose
great−grandfather was a wolf, and this dog showed a trace of its wild parentage only in one way, by not coming in
a straight line to his master, when called.
Domestic instincts are sometimes spoken of as actions which have become inherited solely from
long−continued and compulsory habit, but this is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler−pigeon to tumble−−an action which, as I have witnessed, is performed
by young birds, that have never seen a pigeon tumble. We may believe that some one pigeon showed a slight
tendency to this strange habit, and that the long−continued selection of the best individuals in successive
generations made tumblers what they now are; and near Glasgow there are house−tumblers, as I hear from Mr.
Brent, which cannot fly eighteen inches high without going head over heels. It may be doubted whether any one
would have thought of training a dog to point, had not some one dog naturally shown a tendency in this line; and
this is known occasionally to happen, as I once saw, in a pure terrier: the act of pointing is probably, as many have
thought, only the exaggerated pause of an animal preparing to spring on its prey. When the first tendency to point
was once displayed, methodical selection and the inherited effects of compulsory training in each successive
generation would soon complete the work; and unconscious selection is still in progress, as each man tries to
procure, without intending to improve the breed, dogs which stand and hunt best. On the other hand, habit alone
in some cases has sufficed; hardly any animal is more difficult to tame than the young of the wild rabbit; scarcely
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The Origin of Species by means of Natural Selection
any animal is tamer than the young of the tame rabbit; but I can hardly suppose that domestic rabbits have often
been selected for tameness alone; so that we must attribute at least the greater part of the inherited change from
extreme wildness to extreme tameness, to habit and long−continued close confinement.
Natural instincts are lost under domestication: a remarkable instance of this is seen in those breeds of fowls
which very rarely or never become "broody," that is, never wish to sit on their eggs. Familiarity alone prevents
our seeing how largely and how permanently the minds of our domestic animals have been modified. It is scarcely
possible to doubt that the love of man has become instinctive in the dog. All wolves, foxes, jackals and species of
the cat genus, when kept tame, are most eager to attack poultry, sheep and pigs; and this tendency has been found
incurable in dogs which have been brought home as puppies from countries such as Tierra del Fuego and
Australia, where the savages do not keep these domestic animals. How rarely, on the other hand, do our civilised
dogs, even when quite young, require to be taught not to attack poultry, sheep, and pigs! No doubt they
occasionally do make an attack, and are then beaten; and if not cured, they are destroyed; so that habit and some
degree of selection have probably concurred in civilising by inheritance our dogs. On the other hand, young
chickens have lost wholly by habit, that fear of the dog and cat which no doubt was originally instinctive in them,
for I am informed by Captain Hutton that the young chickens of the parent stock, the Gallus bankiva, when reared
in India under a hen, are at first excessively wild. So it is with young pheasants reared in England under a hen. It
is not that chickens have lost all fear, but fear only of dogs and cats, for if the hen gives the danger chuckle they
will run (more especially young turkeys) from under her and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of allowing, as we see in wild ground−birds, their
mother to fly away. But this instinct retained by our chickens has become useless under domestication, for the
mother−hen has almost lost by disuse the power of flight.
Hence, we may conclude that under domestication instincts have been acquired and natural instincts have
been lost, partly by habit and partly by man selecting and accumulating, during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in our ignorance call an accident. In some
cases compulsory habit alone has sufficed to produce inherited mental changes; in other cases compulsory habit
has done nothing, and all has been the result of selection, pursued both methodically and unconsciously; but in
most cases habit and selection have probably concurred.
SPECIAL INSTINCTS.
We shall, perhaps, best understand how instincts in a state of nature have become modified by selection by
considering a few cases. I will select only three, namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave−making instinct of certain ants; and the cell−making power of the hive−bee: these two
latter instincts have generally and justly been ranked by naturalists as the most wonderful of all known instincts.
INSTINCTS OF THE CUCKOO.
It is supposed by some naturalists that the more immediate cause of the instinct of the cuckoo is that she lays
her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her
own eggs, those first laid would have to be left for some time unincubated or there would be eggs and young birds
of different ages in the same nest. If this were the case the process of laying and hatching might be inconveniently
long, more especially as she migrates at a very early period; and the first hatched young would probably have to
be fed by the male alone. But the American cuckoo is in this predicament, for she makes her own nest and has
eggs and young successively hatched, all at the same time. It has been both asserted and denied that the American
cuckoo occasionally lays her eggs in other birds' nests; but I have lately heard from Dr. Merrill, of Iowa, that he
once found in Illinois a young cuckoo, together with a young jay in the nest of a blue jay (Garrulus cristatus); and
as both were nearly full feathered, there could be no mistake in their identification. I could also give several
instances of various birds which have been known occasionally to lay their eggs in other birds' nests. Now let us
suppose that the ancient progenitor of our European cuckoo had the habits of the American cuckoo, and that she
occasionally laid an egg in another bird's nest. If the old bird profited by this occasional habit through being
enabled to emigrate earlier or through any other cause; or if the young were made more vigorous by advantage
being taken of the mistaken instinct of another species than when reared by their own mother, encumbered as she
could hardly fail to be by having eggs and young of different ages at the same time, then the old birds or the
fostered young would gain an advantage. And analogy would lead us to believe that the young thus reared would
be apt to follow by inheritance the occasional and aberrant habit of their mother, and in their turn would be apt to
CHAPTER VIII. INSTINCT. 116

The Origin of Species by means of Natural Selection
lay their eggs in other birds' nests, and thus be more successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo has been generated. It has, also recently been
ascertained on sufficient evidence, by Adolf Muller, that the cuckoo occasionally lays her eggs on the bare
ground, sits on them and feeds her young. This rare event is probably a case of reversion to the long−lost,
aboriginal instinct of nidification.
It has been objected that I have not noticed other related instincts and adaptations of structure in the cuckoo,
which are spoken of as necessarily co−ordinated. But in all cases, speculation on an instinct known to us only in a
single species, is useless, for we have hitherto had no facts to guide us. Until recently the instincts of the European
and of the non− parasitic American cuckoo alone were known; now, owing to Mr. Ramsay's observations, we
have learned something about three Australian species, which lay their eggs in other birds' nests. The chief points
to be referred to are three: first, that the common cuckoo, with rare exceptions, lays only one egg in a nest, so that
the large and voracious young bird receives ample food. Secondly, that the eggs are remarkably small, not
exceeding those of the skylark−−a bird about one−fourth as large as the cuckoo. That the small size of the egg is a
real case of adaptation we may infer from the fact of the mon−parasitic American cuckoo laying full−sized eggs.
Thirdly, that the young cuckoo, soon after birth, has the instinct, the strength and a properly shaped back for
ejecting its foster− brothers, which then perish from cold and hunger. This has been boldly called a beneficent
arrangement, in order that the young cuckoo may get sufficient food, and that its foster−brothers may perish
before they had acquired much feeling!
Turning now to the Australian species: though these birds generally lay only one egg in a nest, it is not rare to
find two and even three eggs in the same nest. In the bronze cuckoo the eggs vary greatly in size, from eight to ten
lines in length. Now, if it had been of an advantage to this species to have laid eggs even smaller than those now
laid, so as to have deceived certain foster−parents, or, as is more probable, to have been hatched within a shorter
period (for it is asserted that there is a relation between the size of eggs and the period of their incubation), then
there is no difficulty in believing that a race or species might have been formed which would have laid smaller
and smaller eggs; for these would have been more safely hatched and reared. Mr. Ramsay remarks that two of the
Australian cuckoos, when they lay their eggs in an open nest, manifest a decided preference for nests containing
eggs similar in colour to their own. The European species apparently manifests some tendency towards a similar
instinct, but not rarely departs from it, as is shown by her laying her dull and pale−coloured eggs in the nest of the
hedge−warbler with bright greenish−blue eggs. Had our cuckoo invariably displayed the above instinct, it would
assuredly have been added to those which it is assumed must all have been acquired together. The eggs of the
Australian bronze cuckoo vary, according to Mr. Ramsay, to an extraordinary degree in colour; so that in this
respect, as well as in size, natural selection might have secured and fixed any advantageous variation.
In the case of the European cuckoo, the offspring of the foster−parents are commonly ejected from the nest
within three days after the cuckoo is hatched; and as the latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of ejection was performed by the foster−parents themselves.
But he has now received a trustworthy account of a young cuckoo which was actually seen, while still blind and
not able even to hold up its own head, in the act of ejecting its foster−brothers. One of these was replaced in the
nest by the observer, and was again thrown out. With respect to the means by which this strange and odious
instinct was acquired, if it were of great importance for the young cuckoo, as is probably the case, to receive as
much food as possible soon after birth, I can see no special difficulty in its having gradually acquired, during
successive generations, the blind desire, the strength, and structure necessary for the work of ejection; for those
cuckoos which had such habits and structure best developed would be the most securely reared. The first step
towards the acquisition of the proper instinct might have been mere unintentional restlessness on the part of the
young bird, when somewhat advanced in age and strength; the habit having been afterwards improved, and
transmitted to an earlier age. I can see no more difficulty in this than in the unhatched young of other birds
acquiring the instinct to break through their own shells; or than in young snakes acquiring in their upper jaws, as
Owen has remarked, a transitory sharp tooth for cutting through the tough egg−shell. For if each part is liable to
individual variations at all ages, and the variations tend to be inherited at a corresponding or earlier
age−−propositions which cannot be disputed−−then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or fall together with the whole theory of
natural selection.
CHAPTER VIII. INSTINCT. 117

The Origin of Species by means of Natural Selection
Some species of Molothrus, a widely distinct genus of American birds, allied to our starlings, have parasitic
habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts.
The sexes of Molothrus badius are stated by an excellent observer, Mr. Hudson, sometimes to live promiscuously
together in flocks, and sometimes to pair. They either build a nest of their own or seize on one belonging to some
other bird, occasionally throwing out the nestlings of the stranger. They either lay their eggs in the nest thus
appropriated, or oddly enough build one for themselves on the top of it. They usually sit on their own eggs and
rear their own young; but Mr. Hudson says it is probable that they are occasionally parasitic, for he has seen the
young of this species following old birds of a distinct kind and clamouring to be fed by them. The parasitic habits
of another species of Molothrus, the M. bonariensis, are much more highly developed than those of the last, but
are still far from perfect. This bird, as far as it is known, invariably lays its eggs in the nests of strangers; but it is
remarkable that several together sometimes commence to build an irregular untidy nest of their own, placed in
singular ill−adapted situations, as on the leaves of a large thistle. They never, however, as far as Mr. Hudson has
ascertained, complete a nest for themselves. They often lay so many eggs−−from fifteen to twenty−−in the same
foster−nest, that few or none can possibly be hatched. They have, moreover, the extraordinary habit of pecking
holes in the eggs, whether of their own species or of their foster parents, which they find in the appropriated nests.
They drop also many eggs on the bare ground, which are thus wasted. A third species, the M. pecoris of North
America, has acquired instincts as perfect as those of the cuckoo, for it never lays more than one egg in a
foster−nest, so that the young bird is securely reared. Mr. Hudson is a strong disbeliever in evolution, but he
appears to have been so much struck by the imperfect instincts of the Molothrus bonariensis that he quotes my
words, and asks, "Must we consider these habits, not as especially endowed or created instincts, but as small
consequences of one general law, namely, transition?"
Various birds, as has already been remarked, occasionally lay their eggs in the nests of other birds. This habit
is not very uncommon with the Gallinaceae, and throws some light on the singular instinct of the ostrich. In this
family several hen birds unite and lay first a few eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of the hens laying a large number of eggs, but, as
with the cuckoo, at intervals of two or three days. The instinct, however, of the American ostrich, as in the case of
the Molothrus bonariensis, has not as yet been perfected; for a surprising number of eggs lie strewed over the
plains, so that in one day's hunting I picked up no less than twenty lost and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in the nests of other kinds of bees. This case is more
remarkable than that of the cuckoo; for these bees have not only had their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the pollen−collecting apparatus which would have
been indispensable if they had stored up food for their own young. Some species of Sphegidae (wasp−like insects)
are likewise parasitic; and M. Fabre has lately shown good reason for believing that, although the Tachytes nigra
generally makes its own burrow and stores it with paralysed prey for its own larvae, yet that, when this insect
finds a burrow already made and stored by another sphex, it takes advantage of the prize, and becomes for the
occasion parasitic. In this case, as with that of the Molothrus or cuckoo, I can see no difficulty in natural selection
making an occasional habit permanent, if of advantage to the species, and if the insect whose nest and stored food
are feloniously appropriated, be not thus exterminated.
SLAVE−MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica (Polyerges) rufescens by Pierre Huber, a better
observer even than his celebrated father. This ant is absolutely dependent on its slaves; without their aid, the
species would certainly become extinct in a single year. The males and fertile females do no work of any kind,
and the workers or sterile females, though most energetic and courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of feeding their own larvae. When the old nest is found
inconvenient, and they have to migrate, it is the slaves which determine the migration, and actually carry their
masters in their jaws. So utterly helpless are the masters, that when Huber shut up thirty of them without a slave,
but with plenty of the food which they like best, and with their larvae and pupae to stimulate them to work, they
did nothing; they could not even feed themselves, and many perished of hunger. Huber then introduced a single
slave (F. fusca), and she instantly set to work, fed and saved the survivors; made some cells and tended the larvae,
and put all to rights. What can be more extraordinary than these well−ascertained facts? If we had not known of
any other slave−making ant, it would have been hopeless to speculate how so wonderful an instinct could have
CHAPTER VIII. INSTINCT. 118

The Origin of Species by means of Natural Selection
been perfected.
Another species, Formica sanguinea, was likewise first discovered by P. Huber to be a slave−making ant. This
species is found in the southern parts of England, and its habits have been attended to by Mr. F. Smith, of the
British Museum, to whom I am much indebted for information on this and other subjects. Although fully trusting
to the statements of Huber and Mr. Smith, I tried to approach the subject in a sceptical frame of mind, as any one
may well be excused for doubting the existence of so extraordinary an instinct as that of making slaves. Hence, I
will give the observations which I made in some little detail. I opened fourteen nests of F. sanguinea, and found a
few slaves in all. Males and fertile females of the slave−species (F. fusca) are found only in their own proper
communities, and have never been observed in the nests of F. sanguinea. The slaves are black and not above half
the size of their red masters, so that the contrast in their appearance is great. When the nest is slightly disturbed,
the slaves occasionally come out, and like their masters are much agitated and defend the nest: when the nest is
much disturbed, and the larvae and pupae are exposed, the slaves work energetically together with their masters in
carrying them away to a place of safety. Hence, it is clear that the slaves feel quite at home. During the months of
June and July, on three successive years, I watched for many hours several nests in Surrey and Sussex, and never
saw a slave either leave or enter a nest. As, during these months, the slaves are very few in number, I thought that
they might behave differently when more numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June and August, both in Surrey and Hampshire, and has never seen the slaves, though
present in large numbers in August, either leave or enter the nest. Hence, he considers them as strictly household
slaves. The masters, on the other hand, may be constantly seen bringing in materials for the nest, and food of all
kinds. During the year 1860, however, in the month of July, I came across a community with an unusually large
stock of slaves, and I observed a few slaves mingled with their masters leaving the nest, and marching along the
same road to a tall Scotch−fir tree, twenty−five yards distant, which they ascended together, probably in search of
aphides or cocci. According to Huber, who had ample opportunities for observation, the slaves in Switzerland
habitually work with their masters in making the nest, and they alone open and close the doors in the morning and
evening; and, as Huber expressly states, their principal office is to search for aphides. This difference in the usual
habits of the masters and slaves in the two countries, probably depends merely on the slaves being captured in
greater numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one nest to another, and it was a most
interesting spectacle to behold the masters carefully carrying their slaves in their jaws instead of being carried by
them, as in the case of F. rufescens. Another day my attention was struck by about a score of the slave−makers
haunting the same spot, and evidently not in search of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as many as three of these ants clinging to the
legs of the slave−making F. sanguinea. The latter ruthlessly killed their small opponents and carried their dead
bodies as food to their nest, twenty−nine yards distant; but they were prevented from getting any pupae to rear as
slaves. I then dug up a small parcel of the pupae of F. fusca from another nest, and put them down on a bare spot
near the place of combat; they were eagerly seized and carried off by the tyrants, who perhaps fancied that, after
all, they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupae of another species, F. flava, with a few
of these little yellow ants still clinging to the fragments of their nest. This species is sometimes, though rarely,
made into slaves, as has been described by Mr. Smith. Although so small a species, it is very courageous, and I
have seen it ferociously attack other ants. In one instance I found to my surprise an independent community of F.
flava under a stone beneath a nest of the slave−making F. sanguinea; and when I had accidentally disturbed both
nests, the little ants attacked their big neighbours with surprising courage. Now I was curious to ascertain whether
F. sanguinea could distinguish the pupae of F. fusca, which they habitually make into slaves, from those of the
little and furious F. flava, which they rarely capture, and it was evident that they did at once distinguish them; for
we have seen that they eagerly and instantly seized the pupae of F. fusca, whereas they were much terrified when
they came across the pupae, or even the earth from the nest, of F. flava, and quickly ran away; but in about a
quarter of an hour, shortly after all the little yellow ants had crawled away, they took heart and carried off the
pupae.
One evening I visited another community of F. sanguinea, and found a number of these ants returning home
and entering their nests, carrying the dead bodies of F. fusca (showing that it was not a migration) and numerous
CHAPTER VIII. INSTINCT. 119

The Origin of Species by means of Natural Selection
pupae. I traced a long file of ants burdened with booty, for about forty yards back, to a very thick clump of heath,
whence I saw the last individual of F. sanguinea emerge, carrying a pupa; but I was not able to find the desolated
nest in the thick heath. The nest, however, must have been close at hand, for two or three individuals of F. fusca
were rushing about in the greatest agitation, and one was perched motionless with its own pupa in its mouth on
the top of a spray of heath, an image of despair over its ravaged home.
Such are the facts, though they did not need confirmation by me, in regard to the wonderful instinct of making
slaves. Let it be observed what a contrast the instinctive habits of F. sanguinea present with those of the
continental F. rufescens. The latter does not build its own nest, does not determine its own migrations, does not
collect food for itself or its young, and cannot even feed itself: it is absolutely dependent on its numerous slaves.
Formica sanguinea, on the other hand, possesses much fewer slaves, and in the early part of the summer
extremely few. The masters determine when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves seem to have the exclusive care of the
larvae, and the masters alone go on slave−making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly the slaves, tend and milk as it may be
called, their aphides; and thus both collect food for the community. In England the masters alone usually leave the
nest to collect building materials and food for themselves, their slaves and larvae. So that the masters in this
country receive much less service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not pretend to conjecture. But as ants which are not
slave−makers, will, as I have seen, carry off pupae of other species, if scattered near their nests, it is possible that
such pupae originally stored as food might become developed; and the foreign ants thus unintentionally reared
would then follow their proper instincts, and do what work they could. If their presence proved useful to the
species which had seized them−−if it were more advantageous to this species, to capture workers than to procreate
them−−the habit of collecting pupae, originally for food, might by natural selection be strengthened and rendered
permanent for the very different purpose of raising slaves. When the instinct was once acquired, if carried out to a
much less extent even than in our British F. sanguinea, which, as we have seen, is less aided by its slaves than the
same species in Switzerland, natural selection might increase and modify the instinct−−always supposing each
modification to be of use to the species−−until an ant was formed as abjectly dependent on its slaves as is the
Formica rufescens.
CELL−MAKING INSTINCT OF THE HIVE−BEE.
I will not here enter on minute details on this subject, but will merely give an outline of the conclusions at
which I have arrived. He must be a dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from mathematicians that bees have practically
solved a recondite problem, and have made their cells of the proper shape to hold the greatest possible amount of
honey, with the least possible consumption of precious wax in their construction. It has been remarked that a
skilful workman, with fitting tools and measures, would find it very difficult to make cells of wax of the true
form, though this is effected by a crowd of bees working in a dark hive. Granting whatever instincts you please, it
seems at first quite inconceivable how they can make all the necessary angles and planes, or even perceive when
they are correctly made. But the difficulty is not nearly so great as at first appears: all this beautiful work can be
shown, I think, to follow from a few simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown that the form of the cell stands in
close relation to the presence of adjoining cells; and the following view may, perhaps, be considered only as a
modification of his theory. Let us look to the great principle of gradation, and see whether Nature does not reveal
to us her method of work. At one end of a short series we have humble−bees, which use their old cocoons to hold
honey, sometimes adding to them short tubes of wax, and likewise making separate and very irregular rounded
cells of wax. At the other end of the series we have the cells of the hive−bee, placed in a double layer: each cell,
as is well known, is an hexagonal prism, with the basal edges of its six sides bevelled so as to join an inverted
pyramid, of three rhombs. These rhombs have certain angles, and the three which form the pyramidal base of a
single cell on one side of the comb, enter into the composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the hive−bee and the simplicity of those of the
humble−bee, we have the cells of the Mexican Melipona domestica, carefully described and figured by Pierre
Huber. The Melipona itself is intermediate in structure between the hive and humble bee, but more nearly related
CHAPTER VIII. INSTINCT. 120

The Origin of Species by means of Natural Selection
to the latter: it forms a nearly regular waxen comb of cylindrical cells, in which the young are hatched, and, in
addition, some large cells of wax for holding honey. These latter cells are nearly spherical and of nearly equal
sizes, and are aggregated into an irregular mass. But the important point to notice is, that these cells are always
made at that degree of nearness to each other that they would have intersected or broken into each other if the
spheres had been completed; but this is never permitted, the bees building perfectly flat walls of wax between the
spheres which thus tend to intersect. Hence, each cell consists of an outer spherical portion, and of two, three, or
more flat surfaces, according as the cell adjoins two, three or more other cells. When one cell rests on three other
cells, which, from the spheres being nearly of the same size, is very frequently and necessarily the case, the three
flat surfaces are united into a pyramid; and this pyramid, as Huber has remarked, is manifestly a gross imitation of
the three−sided pyramidal base of the cell of the hive−bee. As in the cells of the hive−bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction of three adjoining cells. It is obvious that the
Melipona saves wax, and what is more important, labour, by this manner of building; for the flat walls between
the adjoining cells are not double, but are of the same thickness as the outer spherical portions, and yet each flat
portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had made its spheres at some given distance
from each other, and had made them of equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would have been as perfect as the comb of the hive−bee. Accordingly I wrote to Professor
Miller, of Cambridge, and this geometer has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:−
If a number of equal spheres be described with their centres placed in two parallel layers; with the centre of
each sphere at the distance of radius x sqrt(2) or radius x 1.41421 (or at some lesser distance), from the centres of
the six surrounding spheres in the same layer; and at the same distance from the centres of the adjoining spheres
in the other and parallel layer; then, if planes of intersection between the several spheres in both layers be formed,
there will result a double layer of hexagonal prisms united together by pyramidal bases formed of three rhombs;
and the rhombs and the sides of the hexagonal prisms will have every angle identically the same with the best
measurements which have been made of the cells of the hive−bee. But I hear from Professor Wyman, who has
made numerous careful measurements, that the accuracy of the workmanship of the bee has been greatly
exaggerated; so much so, that whatever the typical form of the cell may be, it is rarely, if ever, realised.
Hence we may safely conclude that, if we could slightly modify the instincts already possessed by the
Melipona, and in themselves not very wonderful, this bee would make a structure as wonderfully perfect as that of
the hive−bee. We must suppose the Melipona to have the power of forming her cells truly spherical, and of equal
sizes; and this would not be very surprising, seeing that she already does so to a certain extent, and seeing what
perfectly cylindrical burrows many insects make in wood, apparently by turning round on a fixed point. We must
suppose the Melipona to arrange her cells in level layers, as she already does her cylindrical cells; and we must
further suppose, and this is the greatest difficulty, that she can somehow judge accurately at what distance to stand
from her fellow−labourers when several are making their spheres; but she is already so far enabled to judge of
distance, that she always describes her spheres so as to intersect to a certain extent; and then she unites the points
of intersection by perfectly flat surfaces. By such modifications of instincts which in themselves are not very
wonderful−−hardly more wonderful than those which guide a bird to make its nest−−I believe that the hive−bee
has acquired, through natural selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following the example of Mr. Tegetmeier, I separated two
combs, and put between them a long, thick, rectangular strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made them wider and wider until they were converted
into shallow basins, appearing to the eye perfectly true or parts of a sphere, and of about the diameter of a cell. It
was most interesting to observe that, wherever several bees had begun to excavate these basins near together, they
had begun their work at such a distance from each other that by the time the basins had acquired the above stated
width (i.e. about the width of an ordinary cell), and were in depth about one sixth of the diameter of the sphere of
which they formed a part, the rims of the basins intersected or broke into each other. As soon as this occurred, the
bees ceased to excavate, and began to build up flat walls of wax on the lines of intersection between the basins, so
that each hexagonal prism was built upon the scalloped edge of a smooth basin, instead of on the straight edges of
a three−sided pyramid as in the case of ordinary cells.
CHAPTER VIII. INSTINCT. 121

The Origin of Species by means of Natural Selection
I then put into the hive, instead of a thick, rectangular piece of wax, a thin and narrow, knife−edged ridge,
coloured with vermilion. The bees instantly began on both sides to excavate little basins near to each other, in the
same way as before; but the ridge of wax was so thin, that the bottoms of the basins, if they had been excavated to
the same depth as in the former experiment, would have broken into each other from the opposite sides. The bees,
however, did not suffer this to happen, and they stopped their excavations in due time; so that the basins, as soon
as they had been a little deepened, came to have flat bases; and these flat bases, formed by thin little plates of the
vermilion wax left ungnawed, were situated, as far as the eye could judge, exactly along the planes of imaginary
intersection between the basins on the opposite side of the ridge of wax. In some parts, only small portions, in
other parts, large portions of a rhombic plate were thus left between the opposed basins, but the work, from the
unnatural state of things, had not been neatly performed. The bees must have worked at very nearly the same rate
in circularly gnawing away and deepening the basins on both sides of the ridge of vermilion wax, in order to have
thus succeeded in leaving flat plates between the basins, by stopping work at the planes of intersection.
Considering how flexible thin wax is, I do not see that there is any difficulty in the bees, whilst at work on the
two sides of a strip of wax, perceiving when they have gnawed the wax away to the proper thinness, and then
stopping their work. In ordinary combs it has appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed half−completed rhombs at the base of a
just−commenced cell, which were slightly concave on one side, where I suppose that the bees had excavated too
quickly, and convex on the opposed side where the bees had worked less quickly. In one well−marked instance, I
put the comb back into the hive, and allowed the bees to go on working for a short time, and again examined the
cell, and I found that the rhombic plate had been completed, and had become PERFECTLY FLAT: it was
absolutely impossible, from the extreme thinness of the little plate, that they could have effected this by gnawing
away the convex side; and I suspect that the bees in such cases stand in the opposed cells and push and bend the
ductile and warm wax (which as I have tried is easily done) into its proper intermediate plane, and thus flatten it.
>From the experiment of the ridge of vermilion wax we can see that, if the bees were to build for themselves a
thin wall of wax, they could make their cells of the proper shape, by standing at the proper distance from each
other, by excavating at the same rate, and by endeavouring to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may be clearly seen by examining the edge of a growing comb,
do make a rough, circumferential wall or rim all round the comb; and they gnaw this away from the opposite
sides, always working circularly as they deepen each cell. They do not make the whole three−sided pyramidal
base of any one cell at the same time, but only that one rhombic plate which stands on the extreme growing
margin, or the two plates, as the case may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements differ from those made by the justly
celebrated elder Huber, but I am convinced of their accuracy; and if I had space, I could show that they are
conformable with my theory.
Huber's statement, that the very first cell is excavated out of a little parallel−sided wall of wax, is not, as far as
I have seen, strictly correct; the first commencement having always been a little hood of wax; but I will not here
enter on details. We see how important a part excavation plays in the construction of the cells; but it would be a
great error to suppose that the bees cannot build up a rough wall of wax in the proper position−−that is, along the
plane of intersection between two adjoining spheres. I have several specimens showing clearly that they can do
this. Even in the rude circumferential rim or wall of wax round a growing comb, flexures may sometimes be
observed, corresponding in position to the planes of the rhombic basal plates of future cells. But the rough wall of
wax has in every case to be finished off, by being largely gnawed away on both sides. The manner in which the
bees build is curious; they always make the first rough wall from ten to twenty times thicker than the excessively
thin finished wall of the cell, which will ultimately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin cutting it away equally on both sides near the
ground, till a smooth, very thin wall is left in the middle; the masons always piling up the cut−away cement, and
adding fresh cement on the summit of the ridge. We shall thus have a thin wall steadily growing upward but
always crowned by a gigantic coping. From all the cells, both those just commenced and those completed, being
thus crowned by a strong coping of wax, the bees can cluster and crawl over the comb without injuring the
delicate hexagonal walls. These walls, as Professor Miller has kindly ascertained for me, vary greatly in thickness;
being, on an average of twelve measurements made near the border of the comb, 1/352 of an inch in thickness;
CHAPTER VIII. INSTINCT. 122

The Origin of Species by means of Natural Selection
whereas the basal rhomboidal plates are thicker, nearly in the proportion of three to two, having a mean thickness,
from twenty−one measurements, of 1/229 of an inch. By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the cells are made, that a multitude of bees all
work together; one bee after working a short time at one cell going to another, so that, as Huber has stated, a score
of individuals work even at the commencement of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls of a single cell, or the extreme margin of the circumferential rim of a
growing comb, with an extremely thin layer of melted vermilion wax; and I invariably found that the colour was
most delicately diffused by the bees−−as delicately as a painter could have done it with his brush−−by atoms of
the coloured wax having been taken from the spot on which it had been placed, and worked into the growing
edges of the cells all round. The work of construction seems to be a sort of balance struck between many bees, all
instinctively standing at the same relative distance from each other, all trying to sweep equal spheres, and then
building up, or leaving ungnawed, the planes of intersection between these spheres. It was really curious to note in
cases of difficulty, as when two pieces of comb met at an angle, how often the bees would pull down and rebuild
in different ways the same cell, sometimes recurring to a shape which they had at first rejected.
When bees have a place on which they can stand in their proper positions for working−−for instance, on a slip
of wood, placed directly under the middle of a comb growing downwards, so that the comb has to be built over
one face of the slip−−in this case the bees can lay the foundations of one wall of a new hexagon, in its strictly
proper place, projecting beyond the other completed cells. It suffices that the bees should be enabled to stand at
their proper relative distances from each other and from the walls of the last completed cells, and then, by striking
imaginary spheres, they can build up a wall intermediate between two adjoining spheres; but, as far as I have
seen, they never gnaw away and finish off the angles of a cell till a large part both of that cell and of the adjoining
cells has been built. This capacity in bees of laying down under certain circumstances a rough wall in its proper
place between two just−commenced cells, is important, as it bears on a fact, which seems at first subversive of the
foregoing theory; namely, that the cells on the extreme margin of wasp−combs are sometimes strictly hexagonal;
but I have not space here to enter on this subject. Nor does there seem to me any great difficulty in a single insect
(as in the case of a queen−wasp) making hexagonal cells, if she were to work alternately on the inside and outside
of two or three cells commenced at the same time, always standing at the proper relative distance from the parts of
the cells just begun, sweeping spheres or cylinders, and building up intermediate planes.
As natural selection acts only by the accumulation of slight modifications of structure or instinct, each
profitable to the individual under its conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the present perfect plan of construction, could
have profited the progenitors of the hive−bee? I think the answer is not difficult: cells constructed like those of the
bee or the wasp gain in strength, and save much in labour and space, and in the materials of which they are
constructed. With respect to the formation of wax, it is known that bees are often hard pressed to get sufficient
nectar; and I am informed by Mr. Tegetmeier that it has been experimentally proved that from twelve to fifteen
pounds of dry sugar are consumed by a hive of bees for the secretion of a pound of wax; so that a prodigious
quantity of fluid nectar must be collected and consumed by the bees in a hive for the secretion of the wax
necessary for the construction of their combs. Moreover, many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable to support a large stock of bees during the winter; and
the security of the hive is known mainly to depend on a large number of bees being supported. Hence the saving
of wax by largely saving honey, and the time consumed in collecting the honey, must be an important element of
success any family of bees. Of course the success of the species may be dependent on the number of its enemies,
or parasites, or on quite distinct causes, and so be altogether independent of the quantity of honey which the bees
can collect. But let us suppose that this latter circumstance determined, as it probably often has determined,
whether a bee allied to our humble−bees could exist in large numbers in any country; and let us further suppose
that the community lived through the winter, and consequently required a store of honey: there can in this case be
no doubt that it would be an advantage to our imaginary humble−bee if a slight modification of her instincts led
her to make her waxen cells near together, so as to intersect a little; for a wall in common even to two adjoining
cells would save some little labour and wax. Hence, it would continually be more and more advantageous to our
humble−bees, if they were to make their cells more and more regular, nearer together, and aggregated into a mass,
CHAPTER VIII. INSTINCT. 123

The Origin of Species by means of Natural Selection
like the cells of the Melipona; for in this case a large part of the bounding surface of each cell would serve to
bound the adjoining cells, and much labour and wax would be saved. Again, from the same cause, it would be
advantageous to the Melipona, if she were to make her cells closer together, and more regular in every way than
at present; for then, as we have seen, the spherical surfaces would wholly disappear and be replaced by plane
surfaces; and the Melipona would make a comb as perfect as that of the hive−bee. Beyond this stage of perfection
in architecture, natural selection could not lead; for the comb of the hive−bee, as far as we can see, is absolutely
perfect in economising labour and wax.
Thus, as I believe, the most wonderful of all known instincts, that of the hive−bee, can be explained by natural
selection having taken advantage of numerous, successive, slight modifications of simpler instincts; natural
selection having, by slow degrees, more and more perfectly led the bees to sweep equal spheres at a given
distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection.
The bees, of course, no more knowing that they swept their spheres at one particular distance from each other,
than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates; the motive
power of the process of natural selection having been the construction of cells of due strength and of the proper
size and shape for the larvae, this being effected with the greatest possible economy of labour and wax; that
individual swarm which thus made the best cells with least labour, and least waste of honey in the secretion of
wax, having succeeded best, and having transmitted their newly−acquired economical instincts to new swarms,
which in their turn will have had the best chance of succeeding in the struggle for existence.
OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS: NEUTER
AND STERILE INSECTS.
It has been objected to the foregoing view of the origin of instincts that "the variations of structure and of
instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without
an immediate corresponding change in the other would have been fatal." The force of this objection rests entirely
on the assumption that the changes in the instincts and structure are abrupt. To take as an illustration the case of
the larger titmouse, (Parus major) alluded to in a previous chapter; this bird often holds the seeds of the yew
between its feet on a branch, and hammers with its beak till it gets at the kernel. Now what special difficulty
would there be in natural selection preserving all the slight individual variations in the shape of the beak, which
were better and better adapted to break open the seeds, until a beak was formed, as well constructed for this
purpose as that of the nuthatch, at the same time that habit, or compulsion, or spontaneous variations of taste, led
the bird to become more and more of a seed−eater? In this case the beak is supposed to be slowly modified by
natural selection, subsequently to, but in accordance with, slowly changing habits or taste; but let the feet of the
titmouse vary and grow larger from correlation with the beak, or from any other unknown cause, and it is not
improbable that such larger feet would lead the bird to climb more and more until it acquired the remarkable
climbing instinct and power of the nuthatch. In this case a gradual change of structure is supposed to lead to
changed instinctive habits. To take one more case: few instincts are more remarkable than that which leads the
swift of the Eastern Islands to make its nest wholly of inspissated saliva. Some birds build their nests of mud,
believed to be moistened with saliva; and one of the swifts of North America makes its nest (as I have seen) of
sticks agglutinated with saliva, and even with flakes of this substance. Is it then very improbable that the natural
selection of individual swifts, which secreted more and more saliva, should at last produce a species with instincts
leading it to neglect other materials and to make its nest exclusively of inspissated saliva? And so in other cases. It
must, however, be admitted that in many instances we cannot conjecture whether it was instinct or structure which
first varied.
No doubt many instincts of very difficult explanation could be opposed to the theory of natural
selection−−cases, in which we cannot see how an instinct could have originated; cases, in which no intermediate
gradations are known to exist; cases of instincts of such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically the same in animals so remote in the scale of
nature that we cannot account for their similarity by inheritance from a common progenitor, and consequently
must believe that they were independently acquired through natural selection. I will not here enter on these
several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and
actually fatal to the whole theory. I allude to the neuters or sterile females in insect communities: for these neuters
often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile,
CHAPTER VIII. INSTINCT. 124

The Origin of Species by means of Natural Selection
they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I will here take only a single case, that of
working or sterile ants. How the workers have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that some insects and other articulate animals
in a state of nature occasionally become sterile; and if such insects had been social, and it had been profitable to
the community that a number should have been annually born capable of work, but incapable of procreation, I can
see no especial difficulty in this having been effected through natural selection. But I must pass over this
preliminary difficulty. The great difficulty lies in the working ants differing widely from both the males and the
fertile females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes,
and in instinct. As far as instinct alone is concerned, the wonderful difference in this respect between the workers
and the perfect females would have been better exemplified by the hive−bee. If a working ant or other neuter
insect had been an ordinary animal, I should have unhesitatingly assumed that all its characters had been slowly
acquired through natural selection; namely, by individuals having been born with slight profitable modifications,
which were inherited by the offspring, and that these again varied and again were selected, and so onwards. But
with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could
never have transmitted successively acquired modifications of structure or instinct to its progeny. It may well be
asked how it is possible to reconcile this case with the theory of natural selection?
First, let it be remembered that we have innumerable instances, both in our domestic productions and in those
in a state of nature, of all sorts of differences of inherited structure which are correlated with certain ages and with
either sex. We have differences correlated not only with one sex, but with that short period when the reproductive
system is active, as in the nuptial plumage of many birds, and in the hooked jaws of the male salmon. We have
even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the
male sex; for oxen of certain breeds have longer horns than the oxen of other breeds, relatively to the length of the
horns in both the bulls and cows of these same breeds. Hence, I can see no great difficulty in any character
becoming correlated with the sterile condition of certain members of insect communities; the difficulty lies in
understanding how such correlated modifications of structure could have been slowly accumulated by natural
selection.
This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered
that selection may be applied to the family, as well as to the individual, and may thus gain the desired end.
Breeders of cattle wish the flesh and fat to be well marbled together. An animal thus characterized has been
slaughtered, but the breeder has gone with confidence to the same stock and has succeeded. Such faith may be
placed in the power of selection that a breed of cattle, always yielding oxen with extraordinarily long horns,
could, it is probable, be formed by carefully watching which individual bulls and cows, when matched, produced
oxen with the longest horns; and yet no one ox would ever have propagated its kind. Here is a better and real
illustration: According to M. Verlot, some varieties of the double annual stock, from having been long and
carefully selected to the right degree, always produce a large proportion of seedlings bearing double and quite
sterile flowers, but they likewise yield some single and fertile plants. These latter, by which alone the variety can
be propagated, may be compared with the fertile male and female ants, and the double sterile plants with the
neuters of the same community. As with the varieties of the stock, so with social insects, selection has been
applied to the family, and not to the individual, for the sake of gaining a serviceable end. Hence, we may conclude
that slight modifications of structure or of instinct, correlated with the sterile condition of certain members of the
community, have proved advantageous; consequently the fertile males and females have flourished, and
transmitted to their fertile offspring a tendency to produce sterile members with the same modifications. This
process must have been repeated many times, until that prodigious amount of difference between the fertile and
sterile females of the same species has been produced which we see in many social insects.
But we have not as yet touched on the acme of the difficulty; namely, the fact that the neuters of several ants
differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree,
and are thus divided into two or even three castes. The castes, moreover, do not generally graduate into each
other, but are perfectly well defined; being as distinct from each other as are any two species of the same genus,
or rather as any two genera of the same family. Thus, in Eciton, there are working and soldier neuters, with jaws
and instincts extraordinarily different: in Cryptocerus, the workers of one caste alone carry a wonderful sort of
CHAPTER VIII. INSTINCT. 125

The Origin of Species by means of Natural Selection
shield on their heads, the use of which is quite unknown: in the Mexican Myrmecocystus, the workers of one
caste never leave the nest; they are fed by the workers of another caste, and they have an enormously developed
abdomen which secretes a sort of honey, supplying the place of that excreted by the aphides, or the domestic
cattle as they may be called, which our European ants guard and imprison.
It will indeed be thought that I have an overweening confidence in the principle of natural selection, when I do
not admit that such wonderful and well−established facts at once annihilate the theory. In the simpler case of
neuter insects all of one caste, which, as I believe, have been rendered different from the fertile males and females
through natural selection, we may conclude from the analogy of ordinary variations, that the successive, slight,
profitable modifications did not first arise in all the neuters in the same nest, but in some few alone; and that by
the survival of the communities with females which produced most neuters having the advantageous modification,
all the neuters ultimately came to be thus characterized. According to this view we ought occasionally to find in
the same nest neuter−insects, presenting gradations of structure; and this we do find, even not rarely, considering
how few neuter−insects out of Europe have been carefully examined. Mr. F. Smith has shown that the neuters of
several British ants differ surprisingly from each other in size and sometimes in colour; and that the extreme
forms can be linked together by individuals taken out of the same nest: I have myself compared perfect gradations
of this kind. It sometimes happens that the larger or the smaller sized workers are the most numerous; or that both
large and small are numerous, while those of an intermediate size are scanty in numbers. Formica flava has larger
and smaller workers, with some few of intermediate size; and, in this species, as Mr. F. Smith has observed, the
larger workers have simple eyes (ocelli), which, though small, can be plainly distinguished, whereas the smaller
workers have their ocelli rudimentary. Having carefully dissected several specimens of these workers, I can affirm
that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their
proportionately lesser size; and I fully believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate condition. So that here we have two bodies of sterile
workers in the same nest, differing not only in size, but in their organs of vision, yet connected by some few
members in an intermediate condition. I may digress by adding, that if the smaller workers had been the most
useful to the community, and those males and females had been continually selected, which produced more and
more of the smaller workers, until all the workers were in this condition; we should then have had a species of ant
with neuters in nearly the same condition as those of Myrmica. For the workers of Myrmica have not even
rudiments of ocelli, though the male and female ants of this genus have well−developed ocelli.
I may give one other case: so confidently did I expect occasionally to find gradations of important structures
between the different castes of neuters in the same species, that I gladly availed myself of Mr. F. Smith's offer of
numerous specimens from the same nest of the driver ant (Anomma) of West Africa. The reader will perhaps best
appreciate the amount of difference in these workers by my giving, not the actual measurements, but a strictly
accurate illustration: the difference was the same as if we were to see a set of workmen building a house, of whom
many were five feet four inches high, and many sixteen feet high; but we must in addition suppose that the larger
workmen had heads four instead of three times as big as those of the smaller men, and jaws nearly five times as
big. The jaws, moreover, of the working ants of the several sizes differed wonderfully in shape, and in the form
and number of the teeth. But the important fact for us is that, though the workers can be grouped into castes of
different sizes, yet they graduate insensibly into each other, as does the widely−different structure of their jaws. I
speak confidently on this latter point, as Sir J. Lubbock made drawings for me, with the camera lucida, of the jaws
which I dissected from the workers of the several sizes. Mr. Bates, in his interesting "Naturalist on the Amazons,"
has described analogous cases.
With these facts before me, I believe that natural selection, by acting on the fertile ants or parents, could form
a species which should regularly produce neuters, all of large size with one form of jaw, or all of small size with
widely different jaws; or lastly, and this is the greatest difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure; a graduated series having first been
formed, as in the case of the driver ant, and then the extreme forms having been produced in greater and greater
numbers, through the survival of the parents which generated them, until none with an intermediate structure were
produced.
An analogous explanation has been given by Mr. Wallace, of the equally complex case, of certain Malayan
butterflies regularly appearing under two or even three distinct female forms; and by Fritz Muller, of certain
CHAPTER VIII. INSTINCT. 126

The Origin of Species by means of Natural Selection
Brazilian crustaceans likewise appearing under two widely distinct male forms. But this subject need not here be
discussed.
I have now explained how, I believe, the wonderful fact of two distinctly defined castes of sterile workers
existing in the same nest, both widely different from each other and from their parents, has originated. We can see
how useful their production may have been to a social community of ants, on the same principle that the division
of labour is useful to civilised man. Ants, however, work by inherited instincts and by inherited organs or tools,
while man works by acquired knowledge and manufactured instruments. But I must confess, that, with all my
faith in natural selection, I should never have anticipated that this principle could have been efficient in so high a
degree, had not the case of these neuter insects led me to this conclusion. I have, therefore, discussed this case, at
some little but wholly insufficient length, in order to show the power of natural selection, and likewise because
this is by far the most serious special difficulty which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount of modification may be effected by the
accumulation of numerous, slight, spontaneous variations, which are in any way profitable, without exercise or
habit having been brought into play. For peculiar habits, confined to the workers of sterile females, however long
they might be followed, could not possibly affect the males and fertile females, which alone leave descendants. I
am surprised that no one has advanced this demonstrative case of neuter insects, against the well−known doctrine
of inherited habit, as advanced by Lamarck.
SUMMARY.
I have endeavoured in this chapter briefly to show that the mental qualities of our domestic animals vary, and
that the variations are inherited. Still more briefly I have attempted to show that instincts vary slightly in a state of
nature. No one will dispute that instincts are of the highest importance to each animal. Therefore, there is no real
difficulty, under changing conditions of life, in natural selection accumulating to any extent slight modifications
of instinct which are in any way useful. In many cases habit or use and disuse have probably come into play. I do
not pretend that the facts given in this chapter strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand, the fact that instincts are not always
absolutely perfect and are liable to mistakes; that no instinct can be shown to have been produced for the good of
other animals, though animals take advantage of the instincts of others; that the canon in natural history, of
"Natura non facit saltum," is applicable to instincts as well as to corporeal structure, and is plainly explicable on
the foregoing views, but is otherwise inexplicable−−all tend to corroborate the theory of natural selection.
This theory is also strengthened by some few other facts in regard to instincts; as by that common case of
closely allied, but distinct, species, when inhabiting distant parts of the world and living under considerably
different conditions of life, yet often retaining nearly the same instincts. For instance, we can understand, on the
principle of inheritance, how it is that the thrush of tropical South America lines its nest with mud, in the same
peculiar manner as does our British thrush; how it is that the Hornbills of Africa and India have the same
extraordinary instinct of plastering up and imprisoning the females in a hole in a tree, with only a small hole left
in the plaster through which the males feed them and their young when hatched; how it is that the male wrens
(Troglodytes) of North America, build "cock−nests," to roost in, like the males of our Kitty−wrens,−−a habit
wholly unlike that of any other known bird. Finally, it may not be a logical deduction, but to my imagination it is
far more satisfactory to look at such instincts as the young cuckoo ejecting its foster−brothers, ants making slaves,
the larvae of ichneumonidae feeding within the live bodies of caterpillars, not as specially endowed or created
instincts, but as small consequences of one general law leading to the advancement of all organic
beings−−namely, multiply, vary, let the strongest live and the weakest die.
CHAPTER VIII. INSTINCT. 127

The Origin of Species by means of Natural Selection
CHAPTER IX. HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids −− Sterility various in degree, not universal,
affected by close interbreeding, removed by domestication −− Laws governing the sterility of hybrids −− Sterility
not a special endowment, but incidental on other differences, not accumulated by natural selection −− Causes of
the sterility of first crosses and of hybrids −− Parallelism between the effects of changed conditions of life and of
crossing −− Dimorphism and trimorphism −− Fertility of varieties when crossed and of their mongrel offspring
not universal −− Hybrids and mongrels compared independently of their fertility −− Summary.
The view commonly entertained by naturalists is that species, when intercrossed, have been specially
endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly probable, for
species living together could hardly have been kept distinct had they been capable of freely crossing. The subject
is in many ways important for us, more especially as the sterility of species when first crossed, and that of their
hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable degrees
of sterility. It is an incidental result of differences in the reproductive systems of the parent−species.
In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been
confounded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced from
them.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they
produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both plants and animals; though the formative
organs themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual
elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction probably has been slurred over, owing to the sterility in both cases
being looked on as a special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to be descended from common parents, when
crossed, and likewise the fertility of their mongrel offspring, is, with reference to my theory, of equal importance
with the sterility of species; for it seems to make a broad and clear distinction between varieties and species.
DEGREES OF STERILITY.
First, for the sterility of species when crossed and of their hybrid offspring. It is impossible to study the
several memoirs and works of those two conscientious and admirable observers, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed with the high generality of some degree
of sterility. Kolreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he found two
forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks them as
varieties. Gartner, also, makes the rule equally universal; and he disputes the entire fertility of Kolreuter's ten
cases. But in these and in many other cases, Gartner is obliged carefully to count the seeds, in order to show that
there is any degree of sterility. He always compares the maximum number of seeds produced by two species when
first crossed, and the maximum produced by their hybrid offspring, with the average number produced by both
pure parent−species in a state of nature. But causes of serious error here intervene: a plant, to be hybridised, must
be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimented on by Gartner were potted, and were kept in a
chamber in his house. That these processes are often injurious to the fertility of a plant cannot be doubted; for
Gartner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some degree impaired. Moreover, as Gartner
repeatedly crossed some forms, such as the common red and blue pimpernels (Anagallis arvensis and coerulea),
which the best botanists rank as varieties, and found them absolutely sterile, we may doubt whether many species
are really so sterile, when intercrossed, as he believed.
It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and
CHAPTER IX. HYBRIDISM. 128

The Origin of Species by means of Natural Selection
graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required than that the two most experienced observers
who have ever lived, namely Kolreuter and Gartner, arrived at diametrically opposite conclusions in regard to
some of the very same forms. It is also most instructive to compare−−but I have not space here to enter on
details−−the evidence advanced by our best botanists on the question whether certain doubtful forms should be
ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same
observer from experiments made during different years. It can thus be shown that neither sterility nor fertility
affords any certain distinction between species and varieties. The evidence from this source graduates away, and
is doubtful in the same degree as is the evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though Gartner was enabled to rear some
hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never increases, but generally decreases greatly and
suddenly. With respect to this decrease, it may first be noticed that when any deviation in structure or constitution
is common to both parents, this is often transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I believe that their fertility has been diminished
in nearly all these cases by an independent cause, namely, by too close interbreeding. I have made so many
experiments and collected so many facts, showing on the one hand that an occasional cross with a distinct
individual or variety increases the vigour and fertility of the offspring, and on the other hand that very close
interbreeding lessens their vigour and fertility, that I cannot doubt the correctness of this conclusion. Hybrids are
seldom raised by experimentalists in great numbers; and as the parent−species, or other allied hybrids, generally
grow in the same garden, the visits of insects must be carefully prevented during the flowering season: hence
hybrids, if left to themselves, will generally be fertilised during each generation by pollen from the same flower;
and this would probably be injurious to their fertility, already lessened by their hybrid origin. I am strengthened in
this conviction by a remarkable statement repeatedly made by Gartner, namely, that if even the less fertile hybrids
be artificially fertilised with hybrid pollen of the same kind, their fertility, notwithstanding the frequent ill effects
from manipulation, sometimes decidedly increases, and goes on increasing. Now, in the process of artificial
fertilisation, pollen is as often taken by chance (as I know from my own experience) from the anthers of another
flower, as from the anthers of the flower itself which is to be fertilised; so that a cross between two flowers,
though probably often on the same plant, would be thus effected. Moreover, whenever complicated experiments
are in progress, so careful an observer as Gartner would have castrated his hybrids, and this would have insured in
each generation a cross with pollen from a distinct flower, either from the same plant or from another plant of the
same hybrid nature. And thus, the strange fact of an increase of fertility in the successive generations of
ARTIFICIALLY FERTILISED hybrids, in contrast with those spontaneously self− fertilised, may, as I believe,
be accounted for by too close interbreeding having been avoided.
Now let us turn to the results arrived at by a third most experienced hybridiser, namely, the Hon. and Rev. W.
Herbert. He is as emphatic in his conclusion that some hybrids are perfectly fertile−−as fertile as the pure
parent−species−−as are Kolreuter and Gartner that some degree of sterility between distinct species is a universal
law of nature. He experimented on some of the very same species as did Gartner. The difference in their results
may, I think, be in part accounted for by Herbert's great horticultural skill, and by his having hot−houses at his
command. Of his many important statements I will here give only a single one as an example, namely, that "every
ovule in a pod of Crinum capense fertilised by C. revolutum produced a plant, which I never saw to occur in a
case of its natural fecundation." So that here we have perfect, or even more than commonly perfect fertility, in a
first cross between two distinct species.
This case of the Crinum leads me to refer to a singular fact, namely, that individual plants of certain species of
Lobelia, Verbascum and Passiflora, can easily be fertilised by the pollen from a distinct species, but not by pollen
from the same plant, though this pollen can be proved to be perfectly sound by fertilising other plants or species.
In the genus Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various orchids as shown by Mr.
Scott and Fritz Muller, all the individuals are in this peculiar condition. So that with some species, certain
abnormal individuals, and in other species all the individuals, can actually be hybridised much more readily than
they can be fertilised by pollen from the same individual plant! To give one instance, a bulb of Hippeastrum
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aulicum produced four flowers; three were fertilised by Herbert with their own pollen, and the fourth was
subsequently fertilised by the pollen of a compound hybrid descended from three distinct species: the result was
that "the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely, whereas the
pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good
seed, which vegetated freely." Mr. Herbert tried similar experiments during many years, and always with the same
result. These cases serve to show on what slight and mysterious causes the lesser or greater fertility of a species
sometimes depends.
The practical experiments of horticulturists, though not made with scientific precision, deserve some notice. It
is notorious in how complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia,
Rhododendron, etc., have been crossed, yet many of these hybrids seed freely. For instance, Herbert asserts that a
hybrid from Calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit, "reproduces
itself as perfectly as if it had been a natural species from the mountains of Chile." I have taken some pains to
ascertain the degree of fertility of some of the complex crosses of Rhododendrons, and I am assured that many of
them are perfectly fertile. Mr. C. Noble, for instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. ponticum and catawbiense, and that this hybrid "seeds as freely as it is possible to imagine." Had
hybrids, when fairly treated, always gone on decreasing in fertility in each successive generation, as Gartner
believed to be the case, the fact would have been notorious to nurserymen. Horticulturists raise large beds of the
same hybrid, and such alone are fairly treated, for by insect agency the several individuals are allowed to cross
freely with each other, and the injurious influence of close interbreeding is thus prevented. Any one may readily
convince himself of the efficiency of insect agency by examining the flowers of the more sterile kinds of hybrid
Rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen brought from other
flowers.
In regard to animals, much fewer experiments have been carefully tried than with plants. If our systematic
arrangements can be trusted, that is, if the genera of animals are as distinct from each other as are the genera of
plants, then we may infer that animals more widely distinct in the scale of nature can be crossed more easily than
in the case of plants; but the hybrids themselves are, I think, more sterile. It should, however, be borne in mind
that, owing to few animals breeding freely under confinement, few experiments have been fairly tried: for
instance, the canary−bird has been crossed with nine distinct species of finches, but, as not one of these breeds
freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their
hybrids, should be perfectly fertile. Again, with respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the
same time from different parents, so as to avoid the ill effects of close interbreeding. On the contrary, brothers and
sisters have usually been crossed in each successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids
should have gone on increasing.
Although I know of hardly any thoroughly well−authenticated cases of perfectly fertile hybrid animals, I have
reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P.
torquatus, are perfectly fertile. M. Quatrefages states that the hybrids from two moths (Bombyx cynthia and
arrindia) were proved in Paris to be fertile inter se for eight generations. It has lately been asserted that two such
distinct species as the hare and rabbit, when they can be got to breed together, produce offspring, which are highly
fertile when crossed with one of the parent−species. The hybrids from the common and Chinese geese (A.
cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this
country with either pure parent, and in one single instance they have bred inter se. This was effected by Mr.
Eyton, who raised two hybrids from the same parents, but from different hatches; and from these two birds he
raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these
cross−bred geese must be far more fertile; for I am assured by two eminently capable judges, namely Mr. Blyth
and Captain Hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they
are kept for profit, where neither pure parent−species exists, they must certainly be highly or perfectly fertile.
With our domesticated animals, the various races when crossed together are quite fertile; yet in many cases
they are descended from two or more wild species. From this fact we must conclude either that the aboriginal
parent−species at first produced perfectly fertile hybrids, or that the hybrids subsequently reared under
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domestication became quite fertile. This latter alternative, which was first propounded by Pallas, seems by far the
most probable, and can, indeed, hardly be doubted. It is, for instance, almost certain that our dogs are descended
from several wild stocks; yet, with perhaps the exception of certain indigenous domestic dogs of South America,
all are quite fertile together; but analogy makes me greatly doubt, whether the several aboriginal species would at
first have freely bred together and have produced quite fertile hybrids. So again I have lately acquired decisive
evidence that the crossed offspring from the Indian humped and common cattle are inter se perfectly fertile; and
from the observations by Rutimeyer on their important osteological differences, as well as from those by Mr.
Blyth on their differences in habits, voice, constitution, etc., these two forms must be regarded as good and
distinct species. The same remarks may be extended to the two chief races of the pig. We must, therefore, either
give up the belief of the universal sterility of species when crossed; or we must look at this sterility in animals, not
as an indelible characteristic, but as one capable of being removed by domestication.
Finally, considering all the ascertained facts on the intercrossing of plants and animals, it may be concluded
that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot,
under our present state of knowledge, be considered as absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the laws governing the sterility of first crosses and of hybrids. Our
chief object will be to see whether or not these laws indicate that species have been specially endowed with this
quality, in order to prevent their crossing and blending together in utter confusion. The following conclusions are
drawn up chiefly from Gartner's admirable work on the hybridisation of plants. I have taken much pains to
ascertain how far they apply to animals, and, considering how scanty our knowledge is in regard to hybrid
animals, I have been surprised to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from
zero to perfect fertility. It is surprising in how many curious ways this gradation can be shown; but only the barest
outline of the facts can here be given. When pollen from a plant of one family is placed on the stigma of a plant of
a distinct family, it exerts no more influence than so much inorganic dust. From this absolute zero of fertility, the
pollen of different species applied to the stigma of some one species of the same genus, yields a perfect gradation
in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in
certain abnormal cases, even to an excess of fertility, beyond that which the plant's own pollen produces. So in
hybrids themselves, there are some which never have produced, and probably never would produce, even with the
pollen of the pure parents, a single fertile seed: but in some of these cases a first trace of fertility may be detected,
by the pollen of one of the pure parent−species causing the flower of the hybrid to wither earlier than it otherwise
would have done; and the early withering of the flower is well known to be a sign of incipient fertilisation. From
this extreme degree of sterility we have self−fertilised hybrids producing a greater and greater number of seeds up
to perfect fertility.
The hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring,
are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the
hybrids thus produced−−two classes of facts which are generally confounded together−−is by no means strict.
There are many cases, in which two pure species, as in the genus Verbascum, can be united with unusual facility,
and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. On the other hand, there are
species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are
very fertile. Even within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is
that of pure species. But the fertility of first crosses is likewise innately variable; for it is not always the same in
degree when the same two species are crossed under the same circumstances; it depends in part upon the
constitution of the individuals which happen to have been chosen for the experiment. So it is with hybrids, for
their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same
capsule and exposed to the same conditions.
By the term systematic affinity is meant, the general resemblance between species in structure and
constitution. Now the fertility of first crosses, and of the hybrids produced from them, is largely governed by their
systematic affinity. This is clearly shown by hybrids never having been raised between species ranked by
systematists in distinct families; and on the other hand, by very closely allied species generally uniting with
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facility. But the correspondence between systematic affinity and the facility of crossing is by no means strict. A
multitude of cases could be given of very closely allied species which will not unite, or only with extreme
difficulty; and on the other hand of very distinct species which unite with the utmost facility. In the same family
there may be a genus, as Dianthus, in which very many species can most readily be crossed; and another genus, as
Silene, in which the most persevering efforts have failed to produce between extremely close species a single
hybrid. Even within the limits of the same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of almost any other genus; but Gartner
found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be
fertilised, by no less than eight other species of Nicotiana. Many analogous facts could be given.
No one has been able to point out what kind or what amount of difference, in any recognisable character, is
sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and general
appearance, and having strongly marked differences in every part of the flower, even in the pollen, in the fruit,
and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and evergreen trees, plants
inhabiting different stations and fitted for extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance, of a female−ass being first crossed
by a stallion, and then a mare by a male−ass: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of making reciprocal crosses. Such cases are
highly important, for they prove that the capacity in any two species to cross is often completely independent of
their systematic affinity, that is of any difference in their structure or constitution, excepting in their reproductive
systems. The diversity of the result in reciprocal crosses between the same two species was long ago observed by
Kolreuter. To give an instance: Mirabilis jalapa can easily be fertilised by the pollen of M. longiflora, and the
hybrids thus produced are sufficiently fertile; but Kolreuter tried more than two hundred times, during eight
following years, to fertilise reciprocally M. longiflora with the pollen of M. jalapa, and utterly failed. Several
other equally striking cases could be given. Thuret has observed the same fact with certain sea−weeds or Fuci.
Gartner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a
lesser degree. He has observed it even between closely related forms (as Matthiola annua and glabra) which many
botanists rank only as varieties. It is also a remarkable fact that hybrids raised from reciprocal crosses, though of
course compounded of the very same two species, the one species having first been used as the father and then as
the mother, though they rarely differ in external characters, yet generally differ in fertility in a small, and
occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance, some species have a remarkable power
of crossing with other species; other species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all necessarily go together. There are certain
hybrids which, instead of having, as is usual, an intermediate character between their two parents, always closely
resemble one of them; and such hybrids, though externally so like one of their pure parent−species, are with rare
exceptions extremely sterile. So again among hybrids which are usually intermediate in structure between their
parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure
parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the
same capsule have a considerable degree of fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that
when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero
to perfect fertility, or even to fertility under certain conditions in excess; that their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is innately variable; that it is by no means
always the same in degree in the first cross and in the hybrids produced from this cross; that the fertility of
hybrids is not related to the degree in which they resemble in external appearance either parent; and lastly, that the
facility of making a first cross between any two species is not always governed by their systematic affinity or
degree of resemblance to each other. This latter statement is clearly proved by the difference in the result of
reciprocal crosses between the same two species, for, according as the one species or the other is used as the
father or the mother, there is generally some difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from reciprocal crosses often differ in fertility.
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Now do these complex and singular rules indicate that species have been endowed with sterility simply to
prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely different in
degree, when various species are crossed, all of which we must suppose it would be equally important to keep
from blending together? Why should the degree of sterility be innately variable in the individuals of the same
species? Why should some species cross with facility and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why should there often be so great a difference in
the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of
hybrids been permitted? To grant to species the special power of producing hybrids, and then to stop their further
propagation by different degrees of sterility, not strictly related to the facility of the first union between their
parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility, both of first
crosses and of hybrids, is simply incidental or dependent on unknown differences in their reproductive systems;
the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the same two species,
the male sexual element of the one will often freely act on the female sexual element of the other, but not in a
reversed direction. It will be advisable to explain a little more fully, by an example, what I mean by sterility being
incidental on other differences, and not a specially endowed quality. As the capacity of one plant to be grafted or
budded on another is unimportant for their welfare in a state of nature, I presume that no one will suppose that this
capacity is a SPECIALLY endowed quality, but will admit that it is incidental on differences in the laws of
growth of the two plants. We can sometimes see the reason why one tree will not take on another from differences
in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, etc.; but in a
multitude of cases we can assign no reason whatever. Great diversity in the size of two plants, one being woody
and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different
climates, does not always prevent the two grafting together. As in hybridisation, so with grafting, the capacity is
limited by systematic affinity, for no one has been able to graft together trees belonging to quite distinct families;
and, on the other hand, closely allied species and varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed by systematic
affinity. Although many distinct genera within the same family have been grafted together, in other cases species
of the same genus will not take on each other. The pear can be grafted far more readily on the quince, which is
ranked as a distinct genus, than on the apple, which is a member of the same genus. Even different varieties of the
pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on
certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in different INDIVIDUALS of the same two
species in crossing; so Sagaret believes this to be the case with different individuals of the same two species in
being grafted together. As in reciprocal crosses, the facility of effecting an union is often very far from equal, so it
sometimes is in grafting. The common gooseberry, for instance, cannot be grafted on the currant, whereas the
currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids which have their reproductive organs in an imperfect condition, is a
different case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet
these two distinct classes of cases run to a large extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted
with no great difficulty on a fourth species, when thus grafted were rendered barren. On the other hand, certain
species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots. We are
reminded by this latter fact of the extraordinary cases of Hippeastrum, Passiflora, etc., which seed much more
freely when fertilised with the pollen of a distinct species than when fertilised with pollen from the same plant.
We thus see that, although there is a clear and great difference between the mere adhesion of grafted stocks
and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of
parallelism in the results of grafting and of crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted on each other as incidental on unknown
differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first
crosses are incidental on unknown differences in their reproductive systems. These differences in both cases
follow, to a certain extent, as might have been expected, systematic affinity, by which term every kind of
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resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem
to indicate that the greater or lesser difficulty of either grafting or crossing various species has been a special
endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of
specific forms as in the case of grafting it is unimportant for their welfare.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids
might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like
any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of
another variety. For it would clearly be advantageous to two varieties or incipient species if they could be kept
from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary
that he should keep them separate. In the first place, it may be remarked that species inhabiting distinct regions
are often sterile when crossed; now it could clearly have been of no advantage to such separated species to have
been rendered mutually sterile, and consequently this could not have been effected through natural selection; but
it may perhaps be argued, that, if a species was rendered sterile with some one compatriot, sterility with other
species would follow as a necessary contingency. In the second place, it is almost as much opposed to the theory
of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should
have been rendered utterly impotent on a second form, while at the same time the male element of this second
form is enabled freely to fertilise the first form; for this peculiar state of the reproductive system could hardly
have been advantageous to either species.
In considering the probability of natural selection having come into action, in rendering species mutually
sterile, the greatest difficulty will be found to lie in the existence of many graduated steps, from slightly lessened
fertility to absolute sterility. It may be admitted that it would profit an incipient species, if it were rendered in
some slight degree sterile when crossed with its parent form or with some other variety; for thus fewer bastardised
and deteriorated offspring would be produced to commingle their blood with the new species in process of
formation. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be
increased through natural selection to that high degree which is common with so many species, and which is
universal with species which have been differentiated to a generic or family rank, will find the subject
extraordinarily complex. After mature reflection, it seems to me that this could not have been effected through
natural selection. Take the case of any two species which, when crossed, produced few and sterile offspring; now,
what is there which could favour the survival of those individuals which happened to be endowed in a slightly
higher degree with mutual infertility, and which thus approached by one small step towards absolute sterility? Yet
an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with
many species, for a multitude are mutually quite barren. With sterile neuter insects we have reason to believe that
modifications in their structure and fertility have been slowly accumulated by natural selection, from an
advantage having been thus indirectly given to the community to which they belonged over other communities of
the same species; but an individual animal not belonging to a social community, if rendered slightly sterile when
crossed with some other variety, would not thus itself gain any advantage or indirectly give any advantage to the
other individuals of the same variety, thus leading to their preservation.
But it would be superfluous to discuss this question in detail: for with plants we have conclusive evidence that
the sterility of crossed species must be due to some principle, quite independent of natural selection. Both Gartner
and Kolreuter have proved that in genera including numerous species, a series can be formed from species which
when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by
the pollen of certain other species, for the germen swells. It is here manifestly impossible to select the more sterile
individuals, which have already ceased to yield seeds; so that this acme of sterility, when the germen alone is
effected, cannot have been gained through selection; and from the laws governing the various grades of sterility
being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be,
is the same or nearly the same in all cases.
We will now look a little closer at the probable nature of the differences between species which induce
sterility in first crosses and in hybrids. In the case of first crosses, the greater or less difficulty in effecting a union
and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for
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The Origin of Species by means of Natural Selection
the pollen−tubes to reach the ovarium. It has also been observed that when the pollen of one species is placed on
the stigma of a distantly allied species, though the pollen−tubes protrude, they do not penetrate the stigmatic
surface. Again, the male element may reach the female element, but be incapable of causing an embryo to be
developed, as seems to have been the case with some of Thuret's experiments on Fuci. No explanation can be
given of these facts, any more than why certain trees cannot be grafted on others. Lastly, an embryo may be
developed, and then perish at an early period. This latter alternative has not been sufficiently attended to; but I
believe, from observations communicated to me by Mr. Hewitt, who has had great experience in hybridising
pheasants and fowls, that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr.
Salter has recently given the results of an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had been fertilised; and in the majority of the
fertilised eggs, the embryos had either been partially developed and had then perished, or had become nearly
mature, but the young chickens had been unable to break through the shell. Of the chickens which were born,
more than four−fifths died within the first few days, or at latest weeks, "without any obvious cause, apparently
from mere inability to live;" so that from the 500 eggs only twelve chickens were reared. With plants, hybridized
embryos probably often perish in a like manner; at least it is known that hybrids raised from very distinct species
are sometimes weak and dwarfed, and perish at an early age; of which fact Max Wichura has recently given some
striking cases with hybrid willows. It may be here worth noticing that in some cases of parthenogenesis, the
embryos within the eggs of silk moths which had not been fertilised, pass through their early stages of
development and then perish like the embryos produced by a cross between distinct species. Until becoming
acquainted with these facts, I was unwilling to believe in the frequent early death of hybrid embryos; for hybrids,
when once born, are generally healthy and long−lived, as we see in the case of the common mule. Hybrids,
however, are differently circumstanced before and after birth: when born and living in a country where their two
parents live, they are generally placed under suitable conditions of life. But a hybrid partakes of only half of the
nature and constitution of its mother; it may therefore, before birth, as long as it is nourished within its mother's
womb, or within the egg or seed produced by the mother, be exposed to conditions in some degree unsuitable, and
consequently be liable to perish at an early period; more especially as all very young beings are eminently
sensitive to injurious or unnatural conditions of life. But after all, the cause more probably lies in some
imperfection in the original act of impregnation, causing the embryo to be imperfectly developed, rather than in
the conditions to which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is
somewhat different. I have more than once alluded to a large body of facts showing that, when animals and plants
are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases the sterility is independent of general health, and
is often accompanied by excess of size or great luxuriance. In both cases the sterility occurs in various degrees; in
both, the male element is the most liable to be affected; but sometimes the female more than the male. In both, the
tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered
impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids. On the
other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility; and
certain species in a group will produce unusually fertile hybrids. No one can tell till he tries, whether any
particular animal will breed under confinement, or any exotic plant seed freely under culture; nor can he tell till he
tries, whether any two species of a genus will produce more or less sterile hybrids. Lastly, when organic beings
are placed during several generations under conditions not natural to them, they are extremely liable to vary,
which seems to be partly due to their reproductive systems having been specially affected, though in a lesser
degree than when sterility ensues. So it is with hybrids, for their offspring in successive generations are eminently
liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are
produced by the unnatural crossing of two species, the reproductive system, independently of the general state of
health, is affected in a very similar manner. In the one case, the conditions of life have been disturbed, though
often in so slight a degree as to be inappreciable by us; in the other case, or that of hybrids, the external conditions
have remained the same, but the organisation has been disturbed by two distinct structures and constitutions,
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including of course the reproductive systems, having been blended into one. For it is scarcely possible that two
organisations should be compounded into one, without some disturbance occurring in the development, or
periodical action, or mutual relations of the different parts and organs one to another or to the conditions of life.
When hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same
compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable,
does not diminish; it is even apt to increase, this being generally the result, as before explained, of too close
interbreeding. The above view of the sterility of hybrids being caused by two constitutions being compounded
into one has been strongly maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the above or any other view, several facts with
respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses;
or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure
parent. Nor do I pretend that the foregoing remarks go to the root of the matter: no explanation is offered why an
organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show is, that in
two cases, in some respects allied, sterility is the common result−−in the one case from the conditions of life
having been disturbed, in the other case from the organisation having been disturbed by two organisations being
compounded into one.
A similar parallelism holds good with an allied yet very different class of facts. It is an old and almost
universal belief, founded on a considerable body of evidence, which I have elsewhere given, that slight changes in
the conditions of life are beneficial to all living things. We see this acted on by farmers and gardeners in their
frequent exchanges of seed, tubers, etc., from one soil or climate to another, and back again. During the
convalescence of animals, great benefit is derived from almost any change in their habits of life. Again, both with
plants and animals, there is the clearest evidence that a cross between individuals of the same species, which
differ to a certain extent, gives vigour and fertility to the offspring; and that close interbreeding continued during
several generations between the nearest relations, if these be kept under the same conditions of life, almost always
leads to decreased size, weakness, or sterility.
Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on
the other hand, that slight crosses, that is, crosses between the males and females of the same species, which have
been subjected to slightly different conditions, or which have slightly varied, give vigour and fertility to the
offspring. But, as we have seen, organic beings long habituated to certain uniform conditions under a state of
nature, when subjected, as under confinement, to a considerable change in their conditions, very frequently are
rendered more or less sterile; and we know that a cross between two forms that have become widely or
specifically different, produce hybrids which are almost always in some degree sterile. I am fully persuaded that
this double parallelism is by no means an accident or an illusion. He who is able to explain why the elephant, and
a multitude of other animals, are incapable of breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so generally sterile. He will at the same time
be able to explain how it is that the races of some of our domesticated animals, which have often been subjected
to new and not uniform conditions, are quite fertile together, although they are descended from distinct species,
which would probably have been sterile if aboriginally crossed. The above two parallel series of facts seem to be
connected together by some common but unknown bond, which is essentially related to the principle of life; this
principle, according to Mr. Herbert Spencer, being that life depends on, or consists in, the incessant action and
reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this
tendency is slightly disturbed by any change, the vital forces gain in power.
RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be found to throw some light on hybridism. Several plants
belonging to distinct orders present two forms, which exist in about equal numbers and which differ in no respect
except in their reproductive organs; one form having a long pistil with short stamens, the other a short pistil with
long stamens; the two having differently sized pollen−grains. With trimorphic plants there are three forms
likewise differing in the lengths of their pistils and stamens, in the size and colour of the pollen−grains, and in
some other respects; and as in each of the three forms there are two sets of stamens, the three forms possess
altogether six sets of stamens and three kinds of pistils. These organs are so proportioned in length to each other
that half the stamens in two of the forms stand on a level with the stigma of the third form. Now I have shown,
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The Origin of Species by means of Natural Selection
and the result has been confirmed by other observers, that in order to obtain full fertility with these plants, it is
necessary that the stigma of the one form should be fertilised by pollen taken from the stamens of corresponding
height in another form. So that with dimorphic species two unions, which may be called legitimate, are fully
fertile; and two, which may be called illegitimate, are more or less infertile. With trimorphic species six unions
are legitimate, or fully fertile, and twelve are illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic and trimorphic plants, when they are
illegitimately fertilised, that is by pollen taken from stamens not corresponding in height with the pistil, differs
much in degree, up to absolute and utter sterility; just in the same manner as occurs in crossing distinct species.
As the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or
less favourable, so I have found it with illegitimate unions. It is well known that if pollen of a distinct species be
placed on the stigma of a flower, and its own pollen be afterwards, even after a considerable interval of time,
placed on the same stigma, its action is so strongly prepotent that it generally annihilates the effect of the foreign
pollen; so it is with the pollen of the several forms of the same species, for legitimate pollen is strongly prepotent
over illegitimate pollen, when both are placed on the same stigma. I ascertained this by fertilising several flowers,
first illegitimately, and twenty−four hours afterwards legitimately, with pollen taken from a peculiarly coloured
variety, and all the seedlings were similarly coloured; this shows that the legitimate pollen, though applied
twenty−four hours subsequently, had wholly destroyed or prevented the action of the previously applied
illegitimate pollen. Again, as in making reciprocal crosses between the same two species, there is occasionally a
great difference in the result, so the same thing occurs with trimorphic plants; for instance, the mid−styled form of
Lythrum salicaria was illegitimately fertilised with the greatest ease by pollen from the longer stamens of the
short−styled form, and yielded many seeds; but the latter form did not yield a single seed when fertilised by the
longer stamens of the mid−styled form.
In all these respects, and in others which might be added, the forms of the same undoubted species, when
illegitimately united, behave in exactly the same manner as do two distinct species when crossed. This led me
carefully to observe during four years many seedlings, raised from several illegitimate unions. The chief result is
that these illegitimate plants, as they may be called, are not fully fertile. It is possible to raise from dimorphic
species, both long−styled and short−styled illegitimate plants, and from trimorphic plants all three illegitimate
forms. These can then be properly united in a legitimate manner. When this is done, there is no apparent reason
why they should not yield as many seeds as did their parents when legitimately fertilised. But such is not the case.
They are all infertile, in various degrees; some being so utterly and incurably sterile that they did not yield during
four seasons a single seed or even seed−capsule. The sterility of these illegitimate plants, when united with each
other in a legitimate manner, may be strictly compared with that of hybrids when crossed inter se. If, on the other
hand, a hybrid is crossed with either pure parent−species, the sterility is usually much lessened: and so it is when
an illegitimate plant is fertilised by a legitimate plant. In the same manner as the sterility of hybrids does not
always run parallel with the difficulty of making the first cross between the two parent−species, so that sterility of
certain illegitimate plants was unusually great, while the sterility of the union from which they were derived was
by no means great. With hybrids raised from the same seed− capsule the degree of sterility is innately variable, so
it is in a marked manner with illegitimate plants. Lastly, many hybrids are profuse and persistent flowerers, while
other and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; exactly similar cases occur
with the illegitimate offspring of various dimorphic and trimorphic plants.
Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. It is
hardly an exaggeration to maintain that illegitimate plants are hybrids, produced within the limits of the same
species by the improper union of certain forms, while ordinary hybrids are produced from an improper union
between so−called distinct species. We have also already seen that there is the closest similarity in all respects
between first illegitimate unions and first crosses between distinct species. This will perhaps be made more fully
apparent by an illustration; we may suppose that a botanist found two well−marked varieties (and such occur) of
the long−styled form of the trimorphic Lythrum salicaria, and that he determined to try by crossing whether they
were specifically distinct. He would find that they yielded only about one−fifth of the proper number of seed, and
that they behaved in all the other above specified respects as if they had been two distinct species. But to make the
case sure, he would raise plants from his supposed hybridised seed, and he would find that the seedlings were
miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might
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then maintain that he had actually proved, in accordance with the common view, that his two varieties were as
good and as distinct species as any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are important, because they show us, first, that the
physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific
distinction; secondly, because we may conclude that there is some unknown bond which connects the infertility of
illegitimate unions with that of their illegitimate offspring, and we are led to extend the same view to first crosses
and hybrids; thirdly, because we find, and this seems to me of especial importance, that two or three forms of the
same species may exist and may differ in no respect whatever, either in structure or in constitution, relatively to
external conditions, and yet be sterile when united in certain ways. For we must remember that it is the union of
the sexual elements of individuals of the same form, for instance, of two long− styled forms, which results in
sterility; while it is the union of the sexual elements proper to two distinct forms which is fertile. Hence the case
appears at first sight exactly the reverse of what occurs, in the ordinary unions of the individuals of the same
species and with crosses between distinct species. It is, however, doubtful whether this is really so; but I will not
enlarge on this obscure subject.
We may, however, infer as probable from the consideration of dimorphic and trimorphic plants, that the
sterility of distinct species when crossed and of their hybrid progeny, depends exclusively on the nature of their
sexual elements, and not on any difference in their structure or general constitution. We are also led to this same
conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united
with great difficulty, with the female of a second species, while the converse cross can be effected with perfect
facility. That excellent observer, Gartner, likewise concluded that species when crossed are sterile owing to
differences confined to their reproductive systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.
It may be urged as an overwhelming argument that there must be some essential distinction between species
and varieties inasmuch as the latter, however much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some exceptions, presently to be given, I fully
admit that this is the rule. But the subject is surrounded by difficulties, for, looking to varieties produced under
nature, if two forms hitherto reputed to be varieties be found in any degree sterile together, they are at once
ranked by most naturalists as species. For instance, the blue and red pimpernel, which are considered by most
botanists as varieties, are said by Gartner to be quite sterile when crossed, and he consequently ranks them as
undoubted species. If we thus argue in a circle, the fertility of all varieties produced under nature will assuredly
have to be granted.
If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still
involved in some doubt. For when it is stated, for instance, that certain South American indigenous domestic dogs
do not readily unite with European dogs, the explanation which will occur to everyone, and probably the true one,
is that they are descended from aboriginally distinct species. Nevertheless the perfect fertility of so many
domestic races, differing widely from each other in appearance, for instance, those of the pigeon, or of the
cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when intercrossed. Several considerations, however, render
the fertility of domestic varieties less remarkable. In the first place, it may be observed that the amount of external
difference between two species is no sure guide to their degree of mutual sterility, so that similar differences in
the case of varieties would be no sure guide. It is certain that with species the cause lies exclusively in differences
in their sexual constitution. Now the varying conditions to which domesticated animals and cultivated plants have
been subjected, have had so little tendency towards modifying the reproductive system in a manner leading to
mutual sterility, that we have good grounds for admitting the directly opposite doctrine of Pallas, namely, that
such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their
natural state probably would have been in some degree sterile when crossed, become perfectly fertile together.
With plants, so far is cultivation from giving a tendency towards sterility between distinct species, that in several
well− authenticated cases already alluded to, certain plants have been affected in an opposite manner, for they
have become self−impotent, while still retaining the capacity of fertilising, and being fertilised by, other species.
If the Pallasian doctrine of the elimination of sterility through long−continued domestication be admitted, and it
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The Origin of Species by means of Natural Selection
can hardly be rejected, it becomes in the highest degree improbable that similar conditions long− continued
should likewise induce this tendency; though in certain cases, with species having a peculiar constitution, sterility
might occasionally be thus caused. Thus, as I believe, we can understand why, with domesticated animals,
varieties have not been produced which are mutually sterile; and why with plants only a few such cases,
immediately to be given, have been observed.
The real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become
mutually infertile when crossed, but why this has so generally occurred with natural varieties, as soon as they
have been permanently modified in a sufficient degree to take rank as species. We are far from precisely knowing
the cause; nor is this surprising, seeing how profoundly ignorant we are in regard to the normal and abnormal
action of the reproductive system. But we can see that species, owing to their struggle for existence with
numerous competitors, will have been exposed during long periods of time to more uniform conditions, than have
domestic varieties; and this may well make a wide difference in the result. For we know how commonly wild
animals and plants, when taken from their natural conditions and subjected to captivity, are rendered sterile; and
the reproductive functions of organic beings which have always lived under natural conditions would probably in
like manner be eminently sensitive to the influence of an unnatural cross. Domesticated productions, on the other
hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in
their conditions of life, and which can now generally resist with undiminished fertility repeated changes of
conditions, might be expected to produce varieties, which would be little liable to have their reproductive powers
injuriously affected by the act of crossing with other varieties which had originated in a like manner.
I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But it is
impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which
I will briefly abstract. The evidence is at least as good as that from which we believe in the sterility of a multitude
of species. The evidence is also derived from hostile witnesses, who in all other cases consider fertility and
sterility as safe criterions of specific distinction. Gartner kept, during several years, a dwarf kind of maize with
yellow seeds, and a tall variety with red seeds growing near each other in his garden; and although these plants
have separated sexes, they never naturally crossed. He then fertilised thirteen flowers of the one kind with pollen
of the other; but only a single head produced any seed, and this one head produced only five grains. Manipulation
in this case could not have been injurious, as the plants have separated sexes. No one, I believe, has suspected that
these varieties of maize are distinct species; and it is important to notice that the hybrid plants thus raised were
themselves PERFECTLY fertile; so that even Gartner did not venture to consider the two varieties as specifically
distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he
asserts that their mutual fertilisation is by so much the less easy as their differences are greater. How far these
experiments may be trusted, I know not; but the forms experimented on are ranked by Sagaret, who mainly
founds his classification by the test of infertility, as varieties, and Naudin has come to the same conclusion.
The following case is far more remarkable, and seems at first incredible; but it is the result of an astonishing
number of experiments made during many years on nine species of Verbascum, by so good an observer and so
hostile a witness as Gartner: namely, that the yellow and white varieties when crossed produce less seed than the
similarly coloured varieties of the same species. Moreover, he asserts that, when yellow and white varieties of one
species are crossed with yellow and white varieties of a DISTINCT species, more seed is produced by the crosses
between the similarly coloured flowers, than between those which are differently coloured. Mr. Scott also has
experimented on the species and varieties of Verbascum; and although unable to confirm Gartner's results on the
crossing of the distinct species, he finds that the dissimilarly coloured varieties of the same species yield fewer
seeds, in the proportion of eighty−six to 100, than the similarly coloured varieties. Yet these varieties differ in no
respect, except in the colour of their flowers; and one variety can sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact
that one particular variety of the common tobacco was more fertile than the other varieties, when crossed with a
widely distinct species. He experimented on five forms which are commonly reputed to be varieties, and which he
tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile.
But one of these five varieties, when used either as the father or mother, and crossed with the Nicotiana glutinosa,
always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed
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The Origin of Species by means of Natural Selection
with N. glutinosa. Hence the reproductive system of this one variety must have been in some manner and in some
degree modified.
>From these facts it can no longer be maintained that varieties when crossed are invariably quite fertile. From
the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety, if proved
to be infertile in any degree, would almost universally be ranked as a species; from man attending only to external
characters in his domestic varieties, and from such varieties not having been exposed for very long periods to
uniform conditions of life; from these several considerations we may conclude that fertility does not constitute a
fundamental distinction between varieties and species when crossed. The general sterility of crossed species may
safely be looked at, not as a special acquirement or endowment, but as incidental on changes of an unknown
nature in their sexual elements.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of species and of varieties when crossed may be
compared in several other respects. Gartner, whose strong wish it was to draw a distinct line between species and
varieties, could find very few, and, as it seems to me, quite unimportant differences between the so−called hybrid
offspring of species, and the so−called mongrel offspring of varieties. And, on the other hand, they agree most
closely in many important respects.
I shall here discuss this subject with extreme brevity. The most important distinction is, that in the first
generation mongrels are more variable than hybrids; but Gartner admits that hybrids from species which have
long been cultivated are often variable in the first generation; and I have myself seen striking instances of this
fact. Gartner further admits that hybrids between very closely allied species are more variable than those from
very distinct species; and this shows that the difference in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated for several generations, an extreme amount of variability in
the offspring in both cases is notorious; but some few instances of both hybrids and mongrels long retaining a
uniform character could be given. The variability, however, in the successive generations of mongrels is, perhaps,
greater than in hybrids.
This greater variability in mongrels than in hybrids does not seem at all surprising. For the parents of
mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural
varieties), and this implies that there has been recent variability; which would often continue and would augment
that arising from the act of crossing. The slight variability of hybrids in the first generation, in contrast with that in
the succeeding generations, is a curious fact and deserves attention. For it bears on the view which I have taken of
one of the causes of ordinary variability; namely, that the reproductive system, from being eminently sensitive to
changed conditions of life, fails under these circumstances to perform its proper function of producing offspring
closely similar in all respects to the parent−form. Now, hybrids in the first generation are descended from species
(excluding those long cultivated) which have not had their reproductive systems in any way affected, and they are
not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are
highly variable.
But to return to our comparison of mongrels and hybrids: Gartner states that mongrels are more liable than
hybrids to revert to either parent form; but this, if it be true, is certainly only a difference in degree. Moreover,
Gartner expressly states that the hybrids from long cultivated plants are more subject to reversion than hybrids
from species in their natural state; and this probably explains the singular difference in the results arrived at by
different observers. Thus Max Wichura doubts whether hybrids ever revert to their parent forms, and he
experimented on uncultivated species of willows, while Naudin, on the other hand, insists in the strongest terms
on the almost universal tendency to reversion in hybrids, and he experimented chiefly on cultivated plants.
Gartner further states that when any two species, although most closely allied to each other, are crossed with a
third species, the hybrids are widely different from each other; whereas if two very distinct varieties of one
species are crossed with another species, the hybrids do not differ much. But this conclusion, as far as I can make
out, is founded on a single experiment; and seems directly opposed to the results of several experiments made by
Kolreuter.
Such alone are the unimportant differences which Gartner is able to point out between hybrid and mongrel
plants. On the other hand, the degrees and kinds of resemblance in mongrels and in hybrids to their respective
parents, more especially in hybrids produced from nearly related species, follow, according to Gartner the same
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The Origin of Species by means of Natural Selection
laws. When two species are crossed, one has sometimes a prepotent power of impressing its likeness on the
hybrid. So I believe it to be with varieties of plants; and with animals, one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a reciprocal cross generally resemble each
other closely, and so it is with mongrel plants from a reciprocal cross. Both hybrids and mongrels can be reduced
to either pure parent form, by repeated crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but the subject is here much complicated, partly
owing to the existence of secondary sexual characters; but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the other, both when one species is crossed with another and
when one variety is crossed with another variety. For instance, I think those authors are right who maintain that
the ass has a prepotent power over the horse, so that both the mule and the hinny resemble more closely the ass
than the horse; but that the prepotency runs more strongly in the male than in the female ass, so that the mule,
which is an offspring of the male ass and mare, is more like an ass than is the hinny, which is the offspring of the
female−ass and stallion.
Much stress has been laid by some authors on the supposed fact, that it is only with mongrels that the
offspring are not intermediate in character, but closely resemble one of their parents; but this does sometimes
occur with hybrids, yet I grant much less frequently than with mongrels. Looking to the cases which I have
collected of cross−bred animals closely resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly appeared−−such as albinism, melanism,
deficiency of tail or horns, or additional fingers and toes; and do not relate to characters which have been slowly
acquired through selection. A tendency to sudden reversions to the perfect character of either parent would, also,
be much more likely to occur with mongrels, which are descended from varieties often suddenly produced and
semi−monstrous in character, than with hybrids, which are descended from species slowly and naturally
produced. On the whole, I entirely agree with Dr. Prosper Lucas, who, after arranging an enormous body of facts
with respect to animals, comes to the conclusion that the laws of resemblance of the child to its parents are the
same, whether the two parents differ little or much from each other, namely, in the union of individuals of the
same variety, or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, in all other respects there seems to be a general and
close similarity in the offspring of crossed species, and of crossed varieties. If we look at species as having been
specially created, and at varieties as having been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species
and varieties.
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The Origin of Species by means of Natural Selection
SUMMARY OF CHAPTER.
First crosses between forms, sufficiently distinct to be ranked as species, and their hybrids, are very generally,
but not universally, sterile. The sterility is of all degrees, and is often so slight that the most careful
experimentalists have arrived at diametrically opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently susceptible to action of favourable and
unfavourable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by
several curious and complex laws. It is generally different, and sometimes widely different in reciprocal crosses
between the same two species. It is not always equal in degree in a first cross and in the hybrids produced from
this cross.
In the same manner as in grafting trees, the capacity in one species or variety to take on another, is incidental
on differences, generally of an unknown nature, in their vegetative systems, so in crossing, the greater or less
facility of one species to unite with another is incidental on unknown differences in their reproductive systems.
There is no more reason to think that species have been specially endowed with various degrees of sterility to
prevent their crossing and blending in nature, than to think that trees have been specially endowed with various
and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our
forests.
The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection. In the
case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death
of the embryo. In the case of hybrids, it apparently depends on their whole organisation having been disturbed by
being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects
pure species, when exposed to new and unnatural conditions of life. He who will explain these latter cases will be
able to explain the sterility of hybrids. This view is strongly supported by a parallelism of another kind: namely,
that, firstly, slight changes in the conditions of life add to the vigour and fertility of all organic beings; and
secondly, that the crossing of forms, which have been exposed to slightly different conditions of life, or which
have varied, favours the size, vigour and fertility of their offspring. The facts given on the sterility of the
illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, perhaps render it
probable that some unknown bond in all cases connects the degree of fertility of first unions with that of their
offspring. The consideration of these facts on dimorphism, as well as of the results of reciprocal crosses, clearly
leads to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their
sexual elements. But why, in the case of distinct species, the sexual elements should so generally have become
more or less modified, leading to their mutual infertility, we do not know; but it seems to stand in some close
relation to species having been exposed for long periods of time to nearly uniform conditions of life.
It is not surprising that the difficulty in crossing any two species, and the sterility of their hybrid offspring,
should in most cases correspond, even if due to distinct causes: for both depend on the amount of difference
between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, and the capacity of being grafted together−−though this latter capacity
evidently depends on widely different circumstances−−should all run, to a certain extent, parallel with the
systematic affinity of the forms subjected to experiment; for systematic affinity includes resemblances of all
kinds.
First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their
mongrel offspring, are very generally, but not, as is so often stated, invariably fertile. Nor is this almost universal
and perfect fertility surprising, when it is remembered how liable we are to argue in a circle with respect to
varieties in a state of nature; and when we remember that the greater number of varieties have been produced
under domestication by the selection of mere external differences, and that they have not been long exposed to
uniform conditions of life. It should also be especially kept in mind, that long−continued domestication tends to
eliminate sterility, and is therefore little likely to induce this same quality. Independently of the question of
fertility, in all other respects there is the closest general resemblance between hybrids and mongrels, in their
variability, in their power of absorbing each other by repeated crosses, and in their inheritance of characters from
SUMMARY OF CHAPTER. 142

The Origin of Species by means of Natural Selection
both parent−forms. Finally, then, although we are as ignorant of the precise cause of the sterility of first crosses
and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the
facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties.
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The Origin of Species by means of Natural Selection
CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day −− On the nature of extinct intermediate varieties;
on their number −− On the lapse of time, as inferred from the rate of denudation and of deposition number −− On
the lapse of time as estimated by years −− On the poorness of our palaeontological collections −− On the
intermittence of geological formations −− On the denudation of granitic areas −− On the absence of intermediate
varieties in any one formation −− On the sudden appearance of groups of species −− On their sudden appearance
in the lowest known fossiliferous strata −− Antiquity of the habitable earth.
In the sixth chapter I enumerated the chief objections which might be justly urged against the views
maintained in this volume. Most of them have now been discussed. One, namely, the distinctness of specific
forms and their not being blended together by innumerable transitional links, is a very obvious difficulty. I
assigned reasons why such links do not commonly occur at the present day under the circumstances apparently
most favourable for their presence, namely, on an extensive and continuous area with graduated physical
conditions. I endeavoured to show, that the life of each species depends in a more important manner on the
presence of other already defined organic forms, than on climate, and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be
beaten out and exterminated during the course of further modification and improvement. The main cause,
however, of innumerable intermediate links not now occurring everywhere throughout nature depends, on the
very process of natural selection, through which new varieties continually take the places of and supplant their
parent−forms. But just in proportion as this process of extermination has acted on an enormous scale, so must the
number of intermediate varieties, which have formerly existed, be truly enormous. Why then is not every
geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any
such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be
urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.
In the first place, it should always be borne in mind what sort of intermediate forms must, on the theory, have
formerly existed. I have found it difficult, when looking at any two species, to avoid picturing to myself forms
DIRECTLY intermediate between them. But this is a wholly false view; we should always look for forms
intermediate between each species and a common but unknown progenitor; and the progenitor will generally have
differed in some respects from all its modified descendants. To give a simple illustration: the fantail and pouter
pigeons are both descended from the rock−pigeon; if we possessed all the intermediate varieties which have ever
existed, we should have an extremely close series between both and the rock−pigeon; but we should have no
varieties directly intermediate between the fantail and pouter; none, for instance, combining a tail somewhat
expanded with a crop somewhat enlarged, the characteristic features of these two breeds. These two breeds,
moreover, have become so much modified, that, if we had no historical or indirect evidence regarding their origin,
it would not have been possible to have determined from a mere comparison of their structure with that of the
rock−pigeon, C. livia, whether they had descended from this species or from some other allied species, such as C.
oenas.
So with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no
reason to suppose that links directly intermediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organisation much general resemblance to the
tapir and to the horse; but in some points of structure may have differed considerably from both, even perhaps
more than they differ from each other. Hence, in all such cases, we should be unable to recognise the parent−form
of any two or more species, even if we closely compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain of the intermediate links.
It is just possible, by the theory, that one of two living forms might have descended from the other; for
instance, a horse from a tapir; and in this case DIRECT intermediate links will have existed between them. But
such a case would imply that one form had remained for a very long period unaltered, whilst its descendants had
undergone a vast amount of change; and the principle of competition between organism and organism, between
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child and parent, will render this a very rare event; for in all cases the new and improved forms of life tend to
supplant the old and unimproved forms.
By the theory of natural selection all living species have been connected with the parent−species of each
genus, by differences not greater than we see between the natural and domestic varieties of the same species at the
present day; and these parent−species, now generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to the common ancestor of each great class. So that
the number of intermediate and transitional links, between all living and extinct species, must have been
inconceivably great. But assuredly, if this theory be true, such have lived upon the earth.
ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT OF
DENUDATION.
Independently of our not finding fossil remains of such infinitely numerous connecting links, it may be
objected that time cannot have sufficed for so great an amount of organic change, all changes having been
effected slowly. It is hardly possible for me to recall to the reader who is not a practical geologist, the facts
leading the mind feebly to comprehend the lapse of time. He who can read Sir Charles Lyell's grand work on the
Principles of Geology, which the future historian will recognise as having produced a revolution in natural
science, and yet does not admit how vast have been the past periods of time, may at once close this volume. Not
that it suffices to study the Principles of Geology, or to read special treatises by different observers on separate
formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation, or
even of each stratum. We can best gain some idea of past time by knowing the agencies at work; and learning
how deeply the surface of the land has been denuded, and how much sediment has been deposited. As Lyell has
well remarked, the extent and thickness of our sedimentary formations are the result and the measure of the
denudation which the earth's crust has elsewhere undergone. Therefore a man should examine for himself the
great piles of superimposed strata, and watch the rivulets bringing down mud, and the waves wearing away the
sea−cliffs, in order to comprehend something about the duration of past time, the monuments of which we see all
around us.
It is good to wander along the coast, when formed of moderately hard rocks, and mark the process of
degradation. The tides in most cases reach the cliffs only for a short time twice a day, and the waves eat into them
only when they are charged with sand or pebbles; for there is good evidence that pure water effects nothing in
wearing away rock. At last the base of the cliff is undermined, huge fragments fall down, and these remaining
fixed, have to be worn away atom by atom, until after being reduced in size they can be rolled about by the waves,
and then they are more quickly ground into pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded
and how seldom they are rolled about! Moreover, if we follow for a few miles any line of rocky cliff, which is
undergoing degradation, we find that it is only here and there, along a short length or round a promontory, that the
cliffs are at the present time suffering. The appearance of the surface and the vegetation show that elsewhere years
have elapsed since the waters washed their base.
We have, however, recently learned from the observations of Ramsay, in the van of many excellent
observers−−of Jukes, Geikie, Croll and others, that subaerial degradation is a much more important agency than
coast−action, or the power of the waves. The whole surface of the land is exposed to the chemical action of the air
and of the rainwater, with its dissolved carbonic acid, and in colder countries to frost; the disintegrated matter is
carried down even gentle slopes during heavy rain, and to a greater extent than might be supposed, especially in
arid districts, by the wind; it is then transported by the streams and rivers, which, when rapid deepen their
channels, and triturate the fragments. On a rainy day, even in a gently undulating country, we see the effects of
subaerial degradation in the muddy rills which flow down every slope. Messrs. Ramsay and Whitaker have
shown, and the observation is a most striking one, that the great lines of escarpment in the Wealden district and
those ranging across England, which formerly were looked at as ancient sea−coasts, cannot have been thus
formed, for each line is composed of one and the same formation, while our sea−cliffs are everywhere formed by
the intersection of various formations. This being the case, we are compelled to admit that the escarpments owe
their origin in chief part to the rocks of which they are composed, having resisted subaerial denudation better than
the surrounding surface; this surface consequently has been gradually lowered, with the lines of harder rock left
projecting. Nothing impresses the mind with the vast duration of time, according to our ideas of time, more
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The Origin of Species by means of Natural Selection
forcibly than the conviction thus gained that subaerial agencies, which apparently have so little power, and which
seem to work so slowly, have produced great results.
When thus impressed with the slow rate at which the land is worn away through subaerial and littoral action, it
is good, in order to appreciate the past duration of time, to consider, on the one hand, the masses of rock which
have been removed over many extensive areas, and on the other hand the thickness of our sedimentary
formations. I remember having been much struck when viewing volcanic islands, which have been worn by the
waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of
the lava−streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once
extended into the open ocean. The same story is told still more plainly by faults−−those great cracks along which
the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of
feet; for since the crust cracked, and it makes no great difference whether the upheaval was sudden, or, as most
geologists now believe, was slow and effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally visible. The Craven fault, for instance, extends
for upward of thirty miles, and along this line the vertical displacement of the strata varies from 600 to 3,000 feet.
Professor Ramsay has published an account of a downthrow in Anglesea of 2,300 feet; and he informs me that he
fully believes that there is one in Merionethshire of 12,000 feet; yet in these cases there is nothing on the surface
of the land to show such prodigious movements; the pile of rocks on either side of the crack having been
smoothly swept away.
On the other hand, in all parts of the world the piles of sedimentary strata are of wonderful thickness. In the
Cordillera, I estimated one mass of conglomerate at ten thousand feet; and although conglomerates have probably
been accumulated at a quicker rate than finer sediments, yet from being formed of worn and rounded pebbles,
each of which bears the stamp of time, they are good to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum thickness, from actual measurement in most cases, of the
successive formations in DIFFERENT parts of Great Britain; and this is the result: Feet Palaeozoic strata (not
including igneous beds)..57,154 Secondary strata................................13,190 Tertiary
strata..................................2,240
−−making altogether 72,584 feet; that is, very nearly thirteen and three−quarters British miles. Some of these
formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent.
Moreover, between each successive formation we have, in the opinion of most geologists, blank periods of
enormous length. So that the lofty pile of sedimentary rocks in Britain gives but an inadequate idea of the time
which has elapsed during their accumulation. The consideration of these various facts impresses the mind almost
in the same manner as does the vain endeavour to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an interesting paper, remarks that we do not err "in
forming too great a conception of the length of geological periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and then at the figures representing several million years,
the two produce a totally different effect on the mind, and the figures are at once pronounced too small. In regard
to subaerial denudation, Mr. Croll shows, by calculating the known amount of sediment annually brought down
by certain rivers, relatively to their areas of drainage, that 1,000 feet of solid rock, as it became gradually
disintegrated, would thus be removed from the mean level of the whole area in the course of six million years.
This seems an astonishing result, and some considerations lead to the suspicion that it may be too large, but if
halved or quartered it is still very surprising. Few of us, however, know what a million really means: Mr. Croll
gives the following illustration: Take a narrow strip of paper, eighty−three feet four inches in length, and stretch it
along the wall of a large hall; then mark off at one end the tenth of an inch. This tenth of an inch will represent
one hundred years, and the entire strip a million years. But let it be borne in mind, in relation to the subject of this
work, what a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above
dimensions. Several eminent breeders, during a single lifetime, have so largely modified some of the higher
animals, which propagate their kind much more slowly than most of the lower animals, that they have formed
what well deserves to be called a new sub−breed. Few men have attended with due care to any one strain for more
than half a century, so that a hundred years represents the work of two breeders in succession. It is not to be
supposed that species in a state of nature ever change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer with the effects which follow from
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The Origin of Species by means of Natural Selection
unconscious selection, that is, the preservation of the most useful or beautiful animals, with no intention of
modifying the breed; but by this process of unconscious selection, various breeds have been sensibly changed in
the course of two or three centuries.
Species, however, probably change much more slowly, and within the same country only a few change at the
same time. This slowness follows from all the inhabitants of the same country being already so well adapted to
each other, that new places in the polity of nature do not occur until after long intervals, due to the occurrence of
physical changes of some kind, or through the immigration of new forms. Moreover, variations or individual
differences of the right nature, by which some of the inhabitants might be better fitted to their new places under
the altered circumstance, would not always occur at once. Unfortunately we have no means of determining,
according to the standard of years, how long a period it takes to modify a species; but to the subject of time we
must return.
ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS.
Now let us turn to our richest museums, and what a paltry display we behold! That our collections are
imperfect is admitted by every one. The remark of that admirable palaeontologist, Edward Forbes, should never
be forgotten, namely, that very many fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot. Only a small portion of the surface of the earth
has been geologically explored, and no part with sufficient care, as the important discoveries made every year in
Europe prove. No organism wholly soft can be preserved. Shells and bones decay and disappear when left on the
bottom of the sea, where sediment is not accumulating. We probably take a quite erroneous view, when we
assume that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed
and preserve fossil remains. Throughout an enormously large proportion of the ocean, the bright blue tint of the
water bespeaks its purity. The many cases on record of a formation conformably covered, after an immense
interval of time, by another and later formation, without the underlying bed having suffered in the interval any
wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered
condition. The remains which do become embedded, if in sand or gravel, will, when the beds are upraised,
generally be dissolved by the percolation of rain water charged with carbonic acid. Some of the many kinds of
animals which live on the beach between high and low water mark seem to be rarely preserved. For instance, the
several species of the Chthamalinae (a sub−family of sessile cirripedes) coat the rocks all over the world in
infinite numbers: they are all strictly littoral, with the exception of a single Mediterranean species, which inhabits
deep water and this has been found fossil in Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is known that the genus Chthamalus existed during the Chalk period. Lastly, many great
deposits, requiring a vast length of time for their accumulation, are entirely destitute of organic remains, without
our being able to assign any reason: one of the most striking instances is that of the Flysch formation, which
consists of shale and sandstone, several thousand, occasionally even six thousand feet in thickness, and extending
for at least 300 miles from Vienna to Switzerland; and although this great mass has been most carefully searched,
no fossils, except a few vegetable remains, have been found.
With respect to the terrestrial productions which lived during the Secondary and Palaeozoic periods, it is
superfluous to state that our evidence is fragmentary in an extreme degree. For instance, until recently not a
land−shell was known belonging to either of these vast periods, with the exception of one species discovered by
Sir C. Lyell and Dr. Dawson in the carboniferous strata of North America; but now land−shells have been found
in the lias. In regard to mammiferous remains, a glance at the historical table published in Lyell's Manual, will
bring home the truth, how accidental and rare is their preservation, far better than pages of detail. Nor is their
rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been
discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging
to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely results from another and more important cause than any
of the foregoing; namely, from the several formations being separated from each other by wide intervals of time.
This doctrine has been emphatically admitted by many geologists and palaeontologists, who, like E. Forbes,
entirely disbelieve in the change of species. When we see the formations tabulated in written works, or when we
follow them in nature, it is difficult to avoid believing that they are closely consecutive. But we know, for
instance, from Sir R. Murchison's great work on Russia, what wide gaps there are in that country between the
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The Origin of Species by means of Natural Selection
superimposed formations; so it is in North America, and in many other parts of the world. The most skilful
geologist, if his attention had been confined exclusively to these large territories, would never have suspected that
during the periods which were blank and barren in his own country, great piles of sediment, charged with new and
peculiar forms of life, had elsewhere been accumulated. And if, in every separate territory, hardly any idea can be
formed of the length of time which has elapsed between the consecutive formations, we may infer that this could
nowhere be ascertained. The frequent and great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography of the surrounding lands, whence the sediment was
derived, accord with the belief of vast intervals of time having elapsed between each formation.
We can, I think, see why the geological formations of each region are almost invariably intermittent; that is,
have not followed each other in close sequence. Scarcely any fact struck me more when examining many hundred
miles of the South American coasts, which have been upraised several hundred feet within the recent period, than
the absence of any recent deposits sufficiently extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna, tertiary beds are so poorly developed that no
record of several successive and peculiar marine faunas will probably be preserved to a distant age. A little
reflection will explain why, along the rising coast of the western side of South America, no extensive formations
with recent or tertiary remains can anywhere be found, though the supply of sediment must for ages have been
great, from the enormous degradation of the coast rocks and from the muddy streams entering the sea. The
explanation, no doubt, is that the littoral and sub−littoral deposits are continually worn away, as soon as they are
brought up by the slow and gradual rising of the land within the grinding action of the coast−waves.
We may, I think, conclude that sediment must be accumulated in extremely thick, solid, or extensive masses,
in order to withstand the incessant action of the waves, when first upraised and during subsequent oscillations of
level, as well as the subsequent subaerial degradation. Such thick and extensive accumulations of sediment may
be formed in two ways; either in profound depths of the sea, in which case the bottom will not be inhabited by so
many and such varied forms of life as the more shallow seas; and the mass when upraised will give an imperfect
record of the organisms which existed in the neighbourhood during the period of its accumulation. Or sediment
may be deposited to any thickness and extent over a shallow bottom, if it continue slowly to subside. In this latter
case, as long as the rate of subsidence and supply of sediment nearly balance each other, the sea will remain
shallow and favourable for many and varied forms, and thus a rich fossiliferous formation, thick enough, when
upraised, to resist a large amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are throughout the greater part of their thickness
RICH IN FOSSILS, have thus been formed during subsidence. Since publishing my views on this subject in 1845,
I have watched the progress of geology, and have been surprised to note how author after author, in treating of
this or that great formation, has come to the conclusion that it was accumulated during subsidence. I may add, that
the only ancient tertiary formation on the west coast of South America, which has been bulky enough to resist
such degradation as it has as yet suffered, but which will hardly last to a distant geological age, was deposited
during a downward oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area has undergone numerous slow oscillations of level, and
apparently these oscillations have affected wide spaces. Consequently, formations rich in fossils and sufficiently
thick and extensive to resist subsequent degradation, will have been formed over wide spaces during periods of
subsidence, but only where the supply of sediment was sufficient to keep the sea shallow and to embed and
preserve the remains before they had time to decay. On the other hand, as long as the bed of the sea remained
stationary, THICK deposits cannot have been accumulated in the shallow parts, which are the most favourable to
life. Still less can this have happened during the alternate periods of elevation; or, to speak more accurately, the
beds which were then accumulated will generally have been destroyed by being upraised and brought within the
limits of the coast−action.
These remarks apply chiefly to littoral and sublittoral deposits. In the case of an extensive and shallow sea,
such as that within a large part of the Malay Archipelago, where the depth varies from thirty or forty to sixty
fathoms, a widely extended formation might be formed during a period of elevation, and yet not suffer
excessively from denudation during its slow upheaval; but the thickness of the formation could not be great, for
owing to the elevatory movement it would be less than the depth in which it was formed; nor would the deposit be
much consolidated, nor be capped by overlying formations, so that it would run a good chance of being worn
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away by atmospheric degradation and by the action of the sea during subsequent oscillations of level. It has,
however, been suggested by Mr. Hopkins, that if one part of the area, after rising and before being denuded,
subsided, the deposit formed during the rising movement, though not thick, might afterwards become protected by
fresh accumulations, and thus be preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds of considerable horizontal extent have rarely been
completely destroyed. But all geologists, excepting the few who believe that our present metamorphic schists and
plutonic rocks once formed the primordial nucleus of the globe, will admit that these latter rocks have been
stripped of their covering to an enormous extent. For it is scarcely possible that such rocks could have been
solidified and crystallised while uncovered; but if the metamorphic action occurred at profound depths of the
ocean, the former protecting mantle of rock may not have been very thick. Admitting then that gneiss,
mica−schist, granite, diorite, etc., were once necessarily covered up, how can we account for the naked and
extensive areas of such rocks in many parts of the world, except on the belief that they have subsequently been
completely denuded of all overlying strata? That such extensive areas do exist cannot be doubted: the granitic
region of Parime is described by Humboldt as being at least nineteen times as large as Switzerland. South of the
Amazon, Boue colours an area composed of rocks of this nature as equal to that of Spain, France, Italy, part of
Germany, and the British Islands, all conjoined. This region has not been carefully explored, but from the
concurrent testimony of travellers, the granitic area is very large: thus Von Eschwege gives a detailed section of
these rocks, stretching from Rio de Janeiro for 260 geographical miles inland in a straight line; and I travelled for
150 miles in another direction, and saw nothing but granitic rocks. Numerous specimens, collected along the
whole coast, from near Rio de Janeiro to the mouth of the Plata, a distance of 1,100 geographical miles, were
examined by me, and they all belonged to this class. Inland, along the whole northern bank of the Plata, I saw,
besides modern tertiary beds, only one small patch of slightly metamorphosed rock, which alone could have
formed a part of the original capping of the granitic series. Turning to a well−known region, namely, to the United
States and Canada, as shown in Professor H.D. Rogers' beautiful map, I have estimated the areas by cutting out
and weighing the paper, and I find that the metamorphic (excluding the "semi−metamorphic") and granite rocks
exceed, in the proportion of 19 to 12.5, the whole of the newer Palaeozoic formations. In many regions the
metamorphic and granite rocks would be found much more widely extended than they appear to be, if all the
sedimentary beds were removed which rest unconformably on them, and which could not have formed part of the
original mantle under which they were crystallised. Hence, it is probable that in some parts of the world whole
formations have been completely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of elevation the area of the land and of the
adjoining shoal parts of the sea will be increased and new stations will often be formed−−all circumstances
favourable, as previously explained, for the formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand, during subsidence, the inhabited area and
number of inhabitants will decrease (excepting on the shores of a continent when first broken up into an
archipelago), and consequently during subsidence, though there will be much extinction, few new varieties or
species will be formed; and it is during these very periods of subsidence that the deposits which are richest in
fossils have been accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.
>From these several considerations it cannot be doubted that the geological record, viewed as a whole, is
extremely imperfect; but if we confine our attention to any one formation, it becomes much more difficult to
understand why we do not therein find closely graduated varieties between the allied species which lived at its
commencement and at its close. Several cases are on record of the same species presenting varieties in the upper
and lower parts of the same formation. Thus Trautschold gives a number of instances with Ammonites, and
Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the successive
beds of a fresh−water formation in Switzerland. Although each formation has indisputably required a vast number
of years for its deposition, several reasons can be given why each should not commonly include a graduated series
of links between the species which lived at its commencement and close, but I cannot assign due proportional
weight to the following considerations.
Although each formation may mark a very long lapse of years, each probably is short compared with the
period requisite to change one species into another. I am aware that two palaeontologists, whose opinions are
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The Origin of Species by means of Natural Selection
worthy of much deference, namely Bronn and Woodward, have concluded that the average duration of each
formation is twice or thrice as long as the average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us from coming to any just conclusion on this head. When we see a species first appearing
in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously
existed. So again, when we find a species disappearing before the last layers have been deposited, it would be
equally rash to suppose that it then became extinct. We forget how small the area of Europe is compared with the
rest of the world; nor have the several stages of the same formation throughout Europe been correlated with
perfect accuracy.
We may safely infer that with marine animals of all kinds there has been a large amount of migration due to
climatal and other changes; and when we see a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for instance, that several species appear somewhat
earlier in the palaeozoic beds of North America than in those of Europe; time having apparently been required for
their migration from the American to the European seas. In examining the latest deposits, in various quarters of
the world, it has everywhere been noted, that some few still existing species are common in the deposit, but have
become extinct in the immediately surrounding sea; or, conversely, that some are now abundant in the
neighbouring sea, but are rare or absent in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the glacial epoch, which forms only a part of
one whole geological period; and likewise to reflect on the changes of level, on the extreme change of climate,
and on the great lapse of time, all included within this same glacial period. Yet it may be doubted whether, in any
quarter of the world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on accumulating within
the same area during the whole of this period. It is not, for instance, probable that sediment was deposited during
the whole of the glacial period near the mouth of the Mississippi, within that limit of depth at which marine
animals can best flourish: for we know that great geographical changes occurred in other parts of America during
this space of time. When such beds as were deposited in shallow water near the mouth of the Mississippi during
some part of the glacial period shall have been upraised, organic remains will probably first appear and disappear
at different levels, owing to the migrations of species and to geographical changes. And in the distant future, a
geologist, examining these beds, would be tempted to conclude that the average duration of life of the embedded
fossils had been less than that of the glacial period, instead of having been really far greater, that is, extending
from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the
deposit must have gone on continuously accumulating during a long period, sufficient for the slow process of
modification; hence, the deposit must be a very thick one; and the species undergoing change must have lived in
the same district throughout the whole time. But we have seen that a thick formation, fossiliferous throughout its
entire thickness, can accumulate only during a period of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species may live on the same space, the supply of sediment must
nearly counterbalance the amount of subsidence. But this same movement of subsidence will tend to submerge the
area whence the sediment is derived, and thus diminish the supply, whilst the downward movement continues. In
fact, this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one palaeontologist that very thick deposits are usually barren
of organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of formations in any country, has generally
been intermittent in its accumulation. When we see, as is so often the case, a formation composed of beds of
widely different mineralogical composition, we may reasonably suspect that the process of deposition has been
more or less interrupted. Nor will the closest inspection of a formation give us any idea of the length of time
which its deposition may have consumed. Many instances could be given of beds, only a few feet in thickness,
representing formations which are elsewhere thousands of feet in thickness, and which must have required an
enormous period for their accumulation; yet no one ignorant of this fact would have even suspected the vast lapse
of time represented by the thinner formation. Many cases could be given of the lower beds of a formation having
been upraised, denuded, submerged, and then re−covered by the upper beds of the same formation−−facts,
showing what wide, yet easily overlooked, intervals have occurred in its accumulation. In other cases we have the
plainest evidence in great fossilised trees, still standing upright as they grew, of many long intervals of time and
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changes of level during the process of deposition, which would not have been suspected, had not the trees been
preserved: thus Sir C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick in Nova Scotia, with
ancient root−bearing strata, one above the other, at no less than sixty−eight different levels. Hence, when the same
species occurs at the bottom, middle, and top of a formation, the probability is that it has not lived on the same
spot during the whole period of deposition, but has disappeared and reappeared, perhaps many times, during the
same geological period. Consequently if it were to undergo a considerable amount of modification during the
deposition of any one geological formation, a section would not include all the fine intermediate gradations which
must on our theory have existed, but abrupt, though perhaps slight, changes of form.
It is all−important to remember that naturalists have no golden rule by which to distinguish species and
varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of
difference between any two forms, they rank both as species, unless they are enabled to connect them together by
the closest intermediate gradations; and this, from the reasons just assigned, we can seldom hope to effect in any
one geological section. Supposing B and C to be two species, and a third, A, to be found in an older and
underlying bed; even if A were strictly intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be closely connected by intermediate varieties with either one or
both forms. Nor should it be forgotten, as before explained, that A might be the actual progenitor of B and C, and
yet would not necessarily be strictly intermediate between them in all respects. So that we might obtain the
parent−species and its several modified descendants from the lower and upper beds of the same formation, and
unless we obtained numerous transitional gradations, we should not recognise their blood−relationship, and
should consequently rank them as distinct species.
It is notorious on what excessively slight differences many palaeontologists have founded their species; and
they do this the more readily if the specimens come from different sub−stages of the same formation. Some
experienced conchologists are now sinking many of the very fine species of D'Orbigny and others into the rank of
varieties; and on this view we do find the kind of evidence of change which on the theory we ought to find. Look
again at the later tertiary deposits, which include many shells believed by the majority of naturalists to be identical
with existing species; but some excellent naturalists, as Agassiz and Pictet, maintain that all these tertiary species
are specifically distinct, though the distinction is admitted to be very slight; so that here, unless we believe that
these eminent naturalists have been misled by their imaginations, and that these late tertiary species really present
no difference whatever from their living representatives, or unless we admit, in opposition to the judgment of
most naturalists, that these tertiary species are all truly distinct from the recent, we have evidence of the frequent
occurrence of slight modifications of the kind required. If we look to rather wider intervals of time, namely, to
distinct but consecutive stages of the same great formation, we find that the embedded fossils, though universally
ranked as specifically different, yet are far more closely related to each other than are the species found in more
widely separated formations; so that here again we have undoubted evidence of change in the direction required
by the theory; but to this latter subject I shall return in the following chapter.
With animals and plants that propagate rapidly and do not wander much, there is reason to suspect, as we have
formerly seen, that their varieties are generally at first local; and that such local varieties do not spread widely and
supplant their parent−form until they have been modified and perfected in some considerable degree. According
to this view, the chance of discovering in a formation in any one country all the early stages of transition between
any two forms, is small, for the successive changes are supposed to have been local or confined to some one spot.
Most marine animals have a wide range; and we have seen that with plants it is those which have the widest
range, that oftenest present varieties, so that, with shells and other marine animals, it is probable that those which
had the widest range, far exceeding the limits of the known geological formations in Europe, have oftenest given
rise, first to local varieties and ultimately to new species; and this again would greatly lessen the chance of our
being able to trace the stages of transition in any one geological formation.
It is a more important consideration, leading to the same result, as lately insisted on by Dr. Falconer, namely,
that the period during which each species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without undergoing any change.
It should not be forgotten, that at the present day, with perfect specimens for examination, two forms can
seldom be connected by intermediate varieties, and thus proved to be the same species, until many specimens are
collected from many places; and with fossil species this can rarely be done. We shall, perhaps, best perceive the
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improbability of our being enabled to connect species by numerous, fine, intermediate, fossil links, by asking
ourselves whether, for instance, geologists at some future period will be able to prove that our different breeds of
cattle, sheep, horses, and dogs are descended from a single stock or from several aboriginal stocks; or, again,
whether certain sea−shells inhabiting the shores of North America, which are ranked by some conchologists as
distinct species from their European representatives, and by other conchologists as only varieties, are really
varieties, or are, as it is called, specifically distinct. This could be effected by the future geologist only by his
discovering in a fossil state numerous intermediate gradations; and such success is improbable in the highest
degree.
It has been asserted over and over again, by writers who believe in the immutability of species, that geology
yields no linking forms. This assertion, as we shall see in the next chapter, is certainly erroneous. As Sir J.
Lubbock has remarked, "Every species is a link between other allied forms." If we take a genus having a score of
species, recent and extinct, and destroy four−fifths of them, no one doubts that the remainder will stand much
more distinct from each other. If the extreme forms in the genus happen to have been thus destroyed, the genus
itself will stand more distinct from other allied genera. What geological research has not revealed, is the former
existence of infinitely numerous gradations, as fine as existing varieties, connecting together nearly all existing
and extinct species. But this ought not to be expected; yet this has been repeatedly advanced as a most serious
objection against my views.
It may be worth while to sum up the foregoing remarks on the causes of the imperfection of the geological
record under an imaginary illustration. The Malay Archipelago is about the size of Europe from the North Cape to
the Mediterranean, and from Britain to Russia; and therefore equals all the geological formations which have been
examined with any accuracy, excepting those of the United States of America. I fully agree with Mr. Godwin−
Austen, that the present condition of the Malay Archipelago, with its numerous large islands separated by wide
and shallow seas, probably represents the former state of Europe, while most of our formations were
accumulating. The Malay Archipelago is one of the richest regions in organic beings; yet if all the species were to
be collected which have ever lived there, how imperfectly would they represent the natural history of the world!
But we have every reason to believe that the terrestrial productions of the archipelago would be preserved in
an extremely imperfect manner in the formations which we suppose to be there accumulating. Not many of the
strictly littoral animals, or of those which lived on naked submarine rocks, would be embedded; and those
embedded in gravel or sand would not endure to a distant epoch. Wherever sediment did not accumulate on the
bed of the sea, or where it did not accumulate at a sufficient rate to protect organic bodies from decay, no remains
could be preserved.
Formations rich in fossils of many kinds, and of thickness sufficient to last to an age as distant in futurity as
the secondary formations lie in the past, would generally be formed in the archipelago only during periods of
subsidence. These periods of subsidence would be separated from each other by immense intervals of time, during
which the area would be either stationary or rising; whilst rising, the fossiliferous formations on the steeper shores
would be destroyed, almost as soon as accumulated, by the incessant coast−action, as we now see on the shores of
South America. Even throughout the extensive and shallow seas within the archipelago, sedimentary beds could
hardly be accumulated of great thickness during the periods of elevation, or become capped and protected by
subsequent deposits, so as to have a good chance of enduring to a very distant future. During the periods of
subsidence, there would probably be much extinction of life; during the periods of elevation, there would be much
variation, but the geological record would then be less perfect.
It may be doubted whether the duration of any one great period of subsidence over the whole or part of the
archipelago, together with a contemporaneous accumulation of sediment, would EXCEED the average duration of
the same specific forms; and these contingencies are indispensable for the preservation of all the transitional
gradations between any two or more species. If such gradations were not all fully preserved, transitional varieties
would merely appear as so many new, though closely allied species. It is also probable that each great period of
subsidence would be interrupted by oscillations of level, and that slight climatical changes would intervene during
such lengthy periods; and in these cases the inhabitants of the archipelago would migrate, and no closely
consecutive record of their modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range thousands of miles beyond its confines;
and analogy plainly leads to the belief that it would be chiefly these far−ranging species, though only some of
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them, which would oftenest produce new varieties; and the varieties would at first be local or confined to one
place, but if possessed of any decided advantage, or when further modified and improved, they would slowly
spread and supplant their parent−forms. When such varieties returned to their ancient homes, as they would differ
from their former state in a nearly uniform, though perhaps extremely slight degree, and as they would be found
embedded in slightly different sub−stages of the same formation, they would, according to the principles followed
by many palaeontologists, be ranked as new and distinct species.
If then there be some degree of truth in these remarks, we have no right to expect to find, in our geological
formations, an infinite number of those fine transitional forms, which, on our theory, have connected all the past
and present species of the same group into one long and branching chain of life. We ought only to look for a few
links, and such assuredly we do find−−some more distantly, some more closely, related to each other; and these
links, let them be ever so close, if found in different stages of the same formation, would, by many
palaeontologists, be ranked as distinct species. But I do not pretend that I should ever have suspected how poor
was the record in the best preserved geological sections, had not the absence of innumerable transitional links
between the species which lived at the commencement and close of each formation, pressed so hardly on my
theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.
The abrupt manner in which whole groups of species suddenly appear in certain formations, has been urged
by several palaeontologists−−for instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the belief in
the transmutation of species. If numerous species, belonging to the same genera or families, have really started
into life at once, the fact would be fatal to the theory of evolution through natural selection. For the development
by this means of a group of forms, all of which are descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long before their modified descendants. But we
continually overrate the perfection of the geological record, and falsely infer, because certain genera or families
have not been found beneath a certain stage, that they did not exist before that stage. In all cases positive
palaeontological evidence may be implicitly trusted; negative evidence is worthless, as experience has so often
shown. We continually forget how large the world is, compared with the area over which our geological
formations have been carefully examined; we forget that groups of species may elsewhere have long existed, and
have slowly multiplied, before they invaded the ancient archipelagoes of Europe and the United States. We do not
make due allowance for the enormous intervals of time which have elapsed between our consecutive formations,
longer perhaps in many cases than the time required for the accumulation of each formation. These intervals will
have given time for the multiplication of species from some one parent−form: and in the succeeding formation,
such groups or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it might require a long succession of ages to adapt an
organism to some new and peculiar line of life, for instance, to fly through the air; and consequently that the
transitional forms would often long remain confined to some one region; but that, when this adaptation had once
been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short
time would be necessary to produce many divergent forms, which would spread rapidly and widely throughout
the world. Professor Pictet, in his excellent Review of this work, in commenting on early transitional forms, and
taking birds as an illustration, cannot see how the successive modifications of the anterior limbs of a supposed
prototype could possibly have been of any advantage. But look at the penguins of the Southern Ocean; have not
these birds their front limbs in this precise intermediate state of "neither true arms nor true wings?" Yet these
birds hold their place victoriously in the battle for life; for they exist in infinite numbers and of many kinds. I do
not suppose that we here see the real transitional grades through which the wings of birds have passed; but what
special difficulty is there in believing that it might profit the modified descendants of the penguin, first to become
enabled to flap along the surface of the sea like the logger−headed duck, and ultimately to rise from its surface
and glide through the air?
I will now give a few examples to illustrate the foregoing remarks, and to show how liable we are to error in
supposing that whole groups of species have suddenly been produced. Even in so short an interval as that between
the first and second editions of Pictet's great work on Palaeontology, published in 1844−46 and in 1853−57, the
conclusions on the first appearance and disappearance of several groups of animals have been considerably
modified; and a third edition would require still further changes. I may recall the well−known fact that in
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geological treatises, published not many years ago, mammals were always spoken of as having abruptly come in
at the commencement of the tertiary series. And now one of the richest known accumulations of fossil mammals
belongs to the middle of the secondary series; and true mammals have been discovered in the new red sandstone
at nearly the commencement of this great series. Cuvier used to urge that no monkey occurred in any tertiary
stratum; but now extinct species have been discovered in India, South America and in Europe, as far back as the
miocene stage. Had it not been for the rare accident of the preservation of footsteps in the new red sandstone of
the United States, who would have ventured to suppose that no less than at least thirty different bird−like animals,
some of gigantic size, existed during that period? Not a fragment of bone has been discovered in these beds. Not
long ago, palaeontologists maintained that the whole class of birds came suddenly into existence during the
eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the
deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long
lizard−like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been
discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this how
little we as yet know of the former inhabitants of the world.
I may give another instance, which, from having passed under my own eyes has much struck me. In a memoir
on Fossil Sessile Cirripedes, I stated that, from the large number of existing and extinct tertiary species; from the
extraordinary abundance of the individuals of many species all over the world, from the Arctic regions to the
equator, inhabiting various zones of depths, from the upper tidal limits to fifty fathoms; from the perfect manner
in which specimens are preserved in the oldest tertiary beds; from the ease with which even a fragment of a valve
can be recognised; from all these circumstances, I inferred that, had sessile cirripedes existed during the
secondary periods, they would certainly have been preserved and discovered; and as not one species had then
been discovered in beds of this age, I concluded that this great group had been suddenly developed at the
commencement of the tertiary series. This was a sore trouble to me, adding, as I then thought, one more instance
of the abrupt appearance of a great group of species. But my work had hardly been published, when a skilful
palaeontologist, M. Bosquet, sent me a drawing of a perfect specimen of an unmistakable sessile cirripede, which
he had himself extracted from the chalk of Belgium. And, as if to make the case as striking as possible, this
cirripede was a Chthamalus, a very common, large, and ubiquitous genus, of which not one species has as yet
been found even in any tertiary stratum. Still more recently, a Pyrgoma, a member of a distinct subfamily of
sessile cirripedes, has been discovered by Mr. Woodward in the upper chalk; so that we now have abundant
evidence of the existence of this group of animals during the secondary period.
The case most frequently insisted on by palaeontologists of the apparently sudden appearance of a whole
group of species, is that of the teleostean fishes, low down, according to Agassiz, in the Chalk period. This group
includes the large majority of existing species. But certain Jurassic and Triassic forms are now commonly
admitted to be teleostean; and even some palaeozoic forms have thus been classed by one high authority. If the
teleosteans had really appeared suddenly in the northern hemisphere at the commencement of the chalk formation,
the fact would have been highly remarkable; but it would not have formed an insuperable difficulty, unless it
could likewise have been shown that at the same period the species were suddenly and simultaneously developed
in other quarters of the world. It is almost superfluous to remark that hardly any fossil−fish are known from south
of the equator; and by running through Pictet's Palaeontology it will be seen that very few species are known from
several formations in Europe. Some few families of fish now have a confined range; the teleostean fishes might
formerly have had a similarly confined range, and after having been largely developed in some one sea, have
spread widely. Nor have we any right to suppose that the seas of the world have always been so freely open from
south to north as they are at present. Even at this day, if the Malay Archipelago were converted into land, the
tropical parts of the Indian Ocean would form a large and perfectly enclosed basin, in which any great group of
marine animals might be multiplied; and here they would remain confined, until some of the species became
adapted to a cooler climate, and were enabled to double the southern capes of Africa or Australia, and thus reach
other and distant seas.
>From these considerations, from our ignorance of the geology of other countries beyond the confines of
Europe and the United States, and from the revolution in our palaeontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash to dogmatize on the succession of organic
forms throughout the world, as it would be for a naturalist to land for five minutes on a barren point in Australia,
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and then to discuss the number and range of its productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much more serious. I allude to the manner in which species
belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same
group are descended from a single progenitor, apply with equal force to the earliest known species. For instance,
it cannot be doubted that all the Cambrian and Silurian trilobites are descended from some one crustacean, which
must have lived long before the Cambrian age, and which probably differed greatly from any known animal.
Some of the most ancient animals, as the Nautilus, Lingula, etc., do not differ much from living species; and it
cannot on our theory be supposed, that these old species were the progenitors of all the species belonging to the
same groups which have subsequently appeared, for they are not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited
long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with living creatures. Here we encounter a
formidable objection; for it seems doubtful whether the earth, in a fit state for the habitation of living creatures,
has lasted long enough. Sir W. Thompson concludes that the consolidation of the crust can hardly have occurred
less than twenty or more than four hundred million years ago, but probably not less than ninety−eight or more
than two hundred million years. These very wide limits show how doubtful the data are; and other elements may
have hereafter to be introduced into the problem. Mr. Croll estimates that about sixty million years have elapsed
since the Cambrian period, but this, judging from the small amount of organic change since the commencement of
the Glacial epoch, appears a very short time for the many and great mutations of life, which have certainly
occurred since the Cambrian formation; and the previous one hundred and forty million years can hardly be
considered as sufficient for the development of the varied forms of life which already existed during the Cambrian
period. It is, however, probable, as Sir William Thompson insists, that the world at a very early period was
subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes
would have tended to induce changes at a corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods
prior to the Cambrian system, I can give no satisfactory answer. Several eminent geologists, with Sir R.
Murchison at their head, were until recently convinced that we beheld in the organic remains of the lowest
Silurian stratum the first dawn of life. Other highly competent judges, as Lyell and E. Forbes, have disputed this
conclusion. We should not forget that only a small portion of the world is known with accuracy. Not very long
ago M. Barrande added another and lower stage, abounding with new and peculiar species, beneath the then
known Silurian system; and now, still lower down in the Lower Cambrian formation, Mr Hicks has found South
Wales beds rich in trilobites, and containing various molluscs and annelids. The presence of phosphatic nodules
and bituminous matter, even in some of the lowest azotic rocks, probably indicates life at these periods; and the
existence of the Eozoon in the Laurentian formation of Canada is generally admitted. There are three great series
of strata beneath the Silurian system in Canada, in the lowest of which the Eozoon is found. Sir W. Logan states
that their "united thickness may possibly far surpass that of all the succeeding rocks, from the base of the
palaeozoic series to the present time. We are thus carried back to a period so remote, that the appearance of the
so−called primordial fauna (of Barrande) may by some be considered as a comparatively modern event." The
Eozoon belongs to the most lowly organised of all classes of animals, but is highly organised for its class; it
existed in countless numbers, and, as Dr. Dawson has remarked, certainly preyed on other minute organic beings,
which must have lived in great numbers. Thus the words, which I wrote in 1859, about the existence of living
beings long before the Cambrian period, and which are almost the same with those since used by Sir W. Logan,
have proved true. Nevertheless, the difficulty of assigning any good reason for the absence of vast piles of strata
rich in fossils beneath the Cambrian system is very great. It does not seem probable that the most ancient beds
have been quite worn away by denudation, or that their fossils have been wholly obliterated by metamorphic
action, for if this had been the case we should have found only small remnants of the formations next succeeding
them in age, and these would always have existed in a partially metamorphosed condition. But the descriptions
which we possess of the Silurian deposits over immense territories in Russia and in North America, do not
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support the view that the older a formation is the more invariably it has suffered extreme denudation and
metamorphism.
The case at present must remain inexplicable; and may be truly urged as a valid argument against the views
here entertained. To show that it may hereafter receive some explanation, I will give the following hypothesis.
>From the nature of the organic remains which do not appear to have inhabited profound depths, in the several
formations of Europe and of the United States; and from the amount of sediment, miles in thickness, of which the
formations are composed, we may infer that from first to last large islands or tracts of land, whence the sediment
was derived, occurred in the neighbourhood of the now existing continents of Europe and North America. This
same view has since been maintained by Agassiz and others. But we do not know what was the state of things in
the intervals between the several successive formations; whether Europe and the United States during these
intervals existed as dry land, or as a submarine surface near land, on which sediment was not deposited, or as the
bed of an open and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the land, we see them studded with many
islands; but hardly one truly oceanic island (with the exception of New Zealand, if this can be called a truly
oceanic island) is as yet known to afford even a remnant of any palaeozoic or secondary formation. Hence, we
may perhaps infer, that during the palaeozoic and secondary periods, neither continents nor continental islands
existed where our oceans now extend; for had they existed, palaeozoic and secondary formations would in all
probability have been accumulated from sediment derived from their wear and tear; and would have been at least
partially upheaved by the oscillations of level, which must have intervened during these enormously long periods.
If, then, we may infer anything from these facts, we may infer that, where our oceans now extend, oceans have
extended from the remotest period of which we have any record; and on the other hand, that where continents
now exist, large tracts of land have existed, subjected, no doubt, to great oscillations of level, since the Cambrian
period. The coloured map appended to my volume on Coral Reefs, led me to conclude that the great oceans are
still mainly areas of subsidence, the great archipelagoes still areas of oscillations of level, and the continents areas
of elevation. But we have no reason to assume that things have thus remained from the beginning of the world.
Our continents seem to have been formed by a preponderance, during many oscillations of level, of the force of
elevation. But may not the areas of preponderant movement have changed in the lapse of ages? At a period long
antecedent to the Cambrian epoch, continents may have existed where oceans are now spread out, and clear and
open oceans may have existed where our continents now stand. Nor should we be justified in assuming that if, for
instance, the bed of the Pacific Ocean were now converted into a continent we should there find sedimentary
formations, in recognisable condition, older than the Cambrian strata, supposing such to have been formerly
deposited; for it might well happen that strata which had subsided some miles nearer to the centre of the earth, and
which had been pressed on by an enormous weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the surface. The immense areas in some
parts of the world, for instance in South America, of naked metamorphic rocks, which must have been heated
under great pressure, have always seemed to me to require some special explanation; and we may perhaps believe
that we see in these large areas the many formations long anterior to the Cambrian epoch in a completely
metamorphosed and denuded condition.
The several difficulties here discussed, namely, that, though we find in our geological formations many links
between the species which now exist and which formerly existed, we do not find infinitely numerous fine
transitional forms closely joining them all together. The sudden manner in which several groups of species first
appear in our European formations, the almost entire absence, as at present known, of formations rich in fossils
beneath the Cambrian strata, are all undoubtedly of the most serious nature. We see this in the fact that the most
eminent palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. Forbes, etc., and all our greatest
geologists, as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But Sir Charles Lyell now gives the support of his high authority to the opposite side, and
most geologists and palaeontologists are much shaken in their former belief. Those who believe that the
geological record is in any degree perfect, will undoubtedly at once reject my theory. For my part, following out
Lyell's metaphor, I look at the geological record as a history of the world imperfectly kept and written in a
changing dialect. Of this history we possess the last volume alone, relating only to two or three countries. Of this
volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines.
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Each word of the slowly−changing language, more or less different in the successive chapters, may represent the
forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly
introduced. On this view the difficulties above discussed are greatly diminished or even disappear.
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The Origin of Species by means of Natural Selection
CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species −− On their different rates of change −− Species once
lost do not reappear −− Groups of species follow the same general rules in their appearance and disappearance as
do single species −− On extinction −− On simultaneous changes in the forms of life throughout the world −− On
the affinities of extinct species to each other and to living species −− On the state of development of ancient forms
−− On the succession of the same types within the same areas −− Summary of preceding and present chapters.
Let us now see whether the several facts and laws relating to the geological succession of organic beings
accord best with the common view of the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.
New species have appeared very slowly, one after another, both on the land and in the waters. Lyell has
shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between the stages, and to make the proportion between the lost and existing
forms more gradual. In some of the most recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are extinct, and only one or two are new, having appeared there for the first time, either
locally, or, as far as we know, on the face of the earth. The secondary formations are more broken; but, as Bronn
has remarked, neither the appearance nor disappearance of the many species embedded in each formation has
been simultaneous.
Species belonging to different genera and classes have not changed at the same rate, or in the same degree. In
the older tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. Falconer
has given a striking instance of a similar fact, for an existing crocodile is associated with many lost mammals and
reptiles in the sub−Himalayan deposits. The Silurian Lingula differs but little from the living species of this
genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly. The productions
of the land seem to have changed at a quicker rate than those of the sea, of which a striking instance has been
observed in Switzerland. There is some reason to believe that organisms high in the scale, change more quickly
than those that are low: though there are exceptions to this rule. The amount of organic change, as Pictet has
remarked, is not the same in each successive so−called formation. Yet if we compare any but the most closely
related formations, all the species will be found to have undergone some change. When a species has once
disappeared from the face of the earth, we have no reason to believe that the same identical form ever reappears.
The strongest apparent exception to this latter rule is that of the so−called "colonies" of M. Barrande, which
intrude for a period in the midst of an older formation, and then allow the pre−existing fauna to reappear; but
Lyell's explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems
satisfactory.
These several facts accord well with our theory, which includes no fixed law of development, causing all the
inhabitants of an area to change abruptly, or simultaneously, or to an equal degree. The process of modification
must be slow, and will generally affect only a few species at the same time; for the variability of each species is
independent of that of all others. Whether such variations or individual differences as may arise will be
accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of
permanent modification, will depend on many complex contingencies−−on the variations being of a beneficial
nature, on the freedom of intercrossing, on the slowly changing physical conditions of the country, on the
immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into
competition. Hence it is by no means surprising that one species should retain the same identical form much
longer than others; or, if changing, should change in a less degree. We find similar relations between the existing
inhabitants of distinct countries; for instance, the land−shells and coleopterous insects of Madeira have come to
differ considerably from their nearest allies on the continent of Europe, whereas the marine shells and birds have
remained unaltered. We can perhaps understand the apparently quicker rate of change in terrestrial and in more
highly organised productions compared with marine and lower productions, by the more complex relations of the
higher beings to their organic and inorganic conditions of life, as explained in a former chapter. When many of
the inhabitants of any area have become modified and improved, we can understand, on the principle of
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competition, and from the all−important relations of organism to organism in the struggle for life, that any form
which did not become in some degree modified and improved, would be liable to extermination. Hence, we see
why all the species in the same region do at last, if we look to long enough intervals of time, become modified;
for otherwise they would become extinct.
In members of the same class the average amount of change, during long and equal periods of time, may,
perhaps, be nearly the same; but as the accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have been almost necessarily accumulated
at wide and irregularly intermittent intervals of time; consequently the amount of organic change exhibited by the
fossils embedded in consecutive formations is not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at hazard, in an ever slowly changing drama.
We can clearly understand why a species when once lost should never reappear, even if the very same
conditions of life, organic and inorganic, should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the place of another species in the economy of
nature, and thus supplant it; yet the two forms−−the old and the new−−would not be identically the same; for both
would almost certainly inherit different characters from their distinct progenitors; and organisms already differing
would vary in a different manner. For instance, it is possible, if all our fantail−pigeons were destroyed, that
fanciers might make a new breed hardly distinguishable from the present breed; but if the parent rock−pigeon
were likewise destroyed, and under nature we have every reason to believe that parent forms are generally
supplanted and exterminated by their improved offspring, it is incredible that a fantail, identical with the existing
breed, could be raised from any other species of pigeon, or even from any other well established race of the
domestic pigeon, for the successive variations would almost certainly be in some degree different, and the
newly−formed variety would probably inherit from its progenitor some characteristic differences.
Groups of species, that is, genera and families, follow the same general rules in their appearance and
disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. A group,
when it has once disappeared, never reappears; that is, its existence, as long as it lasts, is continuous. I am aware
that there are some apparent exceptions to this rule, but the exceptions are surprisingly few, so few that E. Forbes,
Pictet, and Woodward (though all strongly opposed to such views as I maintain) admit its truth; and the rule
strictly accords with the theory. For all the species of the same group, however long it may have lasted, are the
modified descendants one from the other, and all from a common progenitor. In the genus Lingula, for instance,
the species which have successively appeared at all ages must have been connected by an unbroken series of
generations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that whole groups of species sometimes falsely appear to have been abruptly
developed; and I have attempted to give an explanation of this fact, which if true would be fatal to my views. But
such cases are certainly exceptional; the general rule being a gradual increase in number, until the group reaches
its maximum, and then, sooner or later, a gradual decrease. If the number of the species included within a genus,
or the number of the genera within a family, be represented by a vertical line of varying thickness, ascending
through the successive geological formations, in which the species are found, the line will sometimes falsely
appear to begin at its lower end, not in a sharp point, but abruptly; it then gradually thickens upwards, often
keeping of equal thickness for a space, and ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the species of a group is strictly conformable with
the theory; for the species of the same genus, and the genera of the same family, can increase only slowly and
progressively; the process of modification and the production of a number of allied forms necessarily being a slow
and gradual process, one species first giving rise to two or three varieties, these being slowly converted into
species, which in their turn produce by equally slow steps other varieties and species, and so on, like the
branching of a great tree from a single stem, till the group becomes large.
ON EXTINCTION.
We have as yet only spokesn incidentally of the disappearance of species and of groups of species. On the
theory of natural selection, the extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of the earth having been swept away by
catastrophes at successive periods is very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to this conclusion. On the contrary, we
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have every reason to believe, from the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from another, and finally from the world. In some
few cases, however, as by the breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an island, the process of extinction may have been
rapid. Both single species and whole groups of species last for very unequal periods; some groups, as we have
seen, have endured from the earliest known dawn of life to the present day; some have disappeared before the
close of the palaeozoic period. No fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the extinction of a whole group of species is
generally a slower process than their production: if their appearance and disappearance be represented, as before,
by a vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks the
progress of extermination, than at its lower end, which marks the first appearance and the early increase in
number of the species. In some cases, however, the extermination of whole groups, as of ammonites, towards the
close of the secondary period, has been wonderfully sudden.
The extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed
that, as the individual has a definite length of life, so have species a definite duration. No one can have marvelled
more than I have done at the extinction of species. When I found in La Plata the tooth of a horse embedded with
the remains of Mastodon, Megatherium, Toxodon and other extinct monsters, which all co−existed with still
living shells at a very late geological period, I was filled with astonishment; for, seeing that the horse, since its
introduction by the Spaniards into South America, has run wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse under
conditions of life apparently so favourable. But my astonishment was groundless. Professor Owen soon perceived
that the tooth, though so like that of the existing horse, belonged to an extinct species. Had this horse been still
living, but in some degree rare, no naturalist would have felt the least surprise at its rarity; for rarity is the attribute
of a vast number of species of all classes, in all countries. If we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditions of life; but what that something is, we can hardly ever tell.
On the supposition of the fossil horse still existing as a rare species, we might have felt certain, from the analogy
of all other mammals, even of the slow−breeding elephant, and from the history of the naturalisation of the
domestic horse in South America, that under more favourable conditions it would in a very few years have
stocked the whole continent. But we could not have told what the unfavourable conditions were which checked its
increase, whether some one or several contingencies, and at what period of the horse's life, and in what degree
they severally acted. If the conditions had gone on, however slowly, becoming less and less favourable, we
assuredly should not have perceived the fact, yet the fossil horse would certainly have become rarer and rarer, and
finally extinct−−its place being seized on by some more successful competitor.
It is most difficult always to remember that the increase of every living creature is constantly being checked
by unperceived hostile agencies; and that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have heard surprise repeatedly expressed at such
great monsters as the Mastodon and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary, would in some cases determine, as has been
remarked by Owen, quicker extermination, from the greater amount of requisite food. Before man inhabited India
or Africa, some cause must have checked the continued increase of the existing elephant. A highly capable judge,
Dr. Falconer, believes that it is chiefly insects which, from incessantly harassing and weakening the elephant in
India, check its increase; and this was Bruce's conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood−sucking bats determine the existence of the larger naturalised quadrupeds in several
parts of South America.
We see in many cases in the more recent tertiary formations that rarity precedes extinction; and we know that
this has been the progress of events with those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely, that to admit that species generally become
rare before they become extinct−−to feel no surprise at the rarity of a species, and yet to marvel greatly when the
species ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death−−to
feel no surprise at sickness, but, when the sick man dies, to wonder and to suspect that he died by some deed of
violence.
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The theory of natural selection is grounded on the belief that each new variety and ultimately each new
species, is produced and maintained by having some advantage over those with which it comes into competition;
and the consequent extinction of less−favoured forms almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported far and near, like our short−horn
cattle, and takes the place of other breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both those naturally and artificially produced, are bound together. In flourishing
groups, the number of new specific forms which have been produced within a given time has at some periods
probably been greater than the number of the old specific forms which have been exterminated; but we know that
species have not gone on indefinitely increasing, at least during the later geological epochs, so that, looking to
later times, we may believe that the production of new forms has caused the extinction of about the same number
of old forms.
The competition will generally be most severe, as formerly explained and illustrated by examples, between the
forms which are most like each other in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent−species; and if many new forms have been developed from
any one species, the nearest allies of that species, i.e. the species of the same genus, will be the most liable to
extermination. Thus, as I believe, a number of new species descended from one species, that is a new genus,
comes to supplant an old genus, belonging to the same family. But it must often have happened that a new species
belonging to some one group has seized on the place occupied by a species belonging to a distinct group, and thus
have caused its extermination. If many allied forms be developed from the successful intruder, many will have to
yield their places; and it will generally be the allied forms, which will suffer from some inherited inferiority in
common. But whether it be species belonging to the same or to a distinct class, which have yielded their places to
other modified and improved species, a few of the sufferers may often be preserved for a long time, from being
fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they will have
escaped severe competition. For instance, some species of Trigonia, a great genus of shells in the secondary
formations, survive in the Australian seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. Therefore, the utter extinction of a group is generally, as we have seen, a
slower process than its production.
With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the close
of the palaeozoic period, and of Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our consecutive formations; and in these
intervals there may have been much slow extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of an area, many of the older species will
have been exterminated in a correspondingly rapid manner; and the forms which thus yield their places will
commonly be allied, for they will partake of the same inferiority in common.
Thus, as it seems to me, the manner in which single species and whole groups of species become extinct
accords well with the theory of natural selection. We need not marvel at extinction; if we must marvel, let it be at
our presumption in imagining for a moment that we understand the many complex contingencies on which the
existence of each species depends. If we forget for an instant that each species tends to increase inordinately, and
that some check is always in action, yet seldom perceived by us, the whole economy of nature will be utterly
obscured. Whenever we can precisely say why this species is more abundant in individuals than that; why this
species and not another can be naturalised in a given country; then, and not until then, we may justly feel surprise
why we cannot account for the extinction of any particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.
Scarcely any palaeontological discovery is more striking than the fact that the forms of life change almost
simultaneously throughout the world. Thus our European Chalk formation can be recognised in many distant
regions, under the most different climates, where not a fragment of the mineral chalk itself can be found; namely,
in North America, in equatorial South America, in Tierra del Fuego, at the Cape of Good Hope, and in the
peninsula of India. For at these distant points, the organic remains in certain beds present an unmistakable
resemblance to those of the Chalk. It is not that the same species are met with; for in some cases not one species is
identically the same, but they belong to the same families, genera, and sections of genera, and sometimes are
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similarly characterised in such trifling points as mere superficial sculpture. Moreover, other forms, which are not
found in the Chalk of Europe, but which occur in the formations either above or below, occur in the same order at
these distant points of the world. In the several successive palaeozoic formations of Russia, Western Europe and
North America, a similar parallelism in the forms of life has been observed by several authors; so it is, according
to Lyell, with the European and North American tertiary deposits. Even if the few fossil species which are
common to the Old and New Worlds were kept wholly out of view, the general parallelism in the successive
forms of life, in the palaeozoic and tertiary stages, would still be manifest, and the several formations could be
easily correlated.
These observations, however, relate to the marine inhabitants of the world: we have not sufficient data to
judge whether the productions of the land and of fresh water at distant points change in the same parallel manner.
We may doubt whether they have thus changed: if the Megatherium, Mylodon, Macrauchenia, and Toxodon had
been brought to Europe from La Plata, without any information in regard to their geological position, no one
would have suspected that they had co−existed with sea−shells all still living; but as these anomalous monsters
co−existed with the Mastodon and Horse, it might at least have been inferred that they had lived during one of the
later tertiary stages.
When the marine forms of life are spoken of as having changed simultaneously throughout the world, it must
not be supposed that this expression relates to the same year, or even to the same century, or even that it has a
very strict geological sense; for if all the marine animals now living in Europe, and all those that lived in Europe
during the pleistocene period (a very remote period as measured by years, including the whole glacial epoch)
were compared with those now existing in South America or in Australia, the most skilful naturalist would hardly
be able to say whether the present or the pleistocene inhabitants of Europe resembled most closely those of the
southern hemisphere. So, again, several highly competent observers maintain that the existing productions of the
United States are more closely related to those which lived in Europe during certain late tertiary stages, than to the
present inhabitants of Europe; and if this be so, it is evident that fossiliferous beds now deposited on the shores of
North America would hereafter be liable to be classed with somewhat older European beds. Nevertheless, looking
to a remotely future epoch, there can be little doubt that all the more modern MARINE formations, namely, the
upper pliocene, the pleistocene and strictly modern beds of Europe, North and South America, and Australia,
from containing fossil remains in some degree allied, and from not including those forms which are found only in
the older underlying deposits, would be correctly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously in the above large sense, at distant parts of the world,
has greatly struck those admirable observers, MM. de Verneuil and d'Archiac. After referring to the parallelism of
the palaeozoic forms of life in various parts of Europe, they add, "If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous phenomena, it will appear certain that all
these modifications of species, their extinction, and the introduction of new ones, cannot be owing to mere
changes in marine currents or other causes more or less local and temporary, but depend on general laws which
govern the whole animal kingdom." M. Barrande has made forcible remarks to precisely the same effect. It is,
indeed, quite futile to look to changes of currents, climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we treat of the present distribution of
organic beings, and find how slight is the relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout the world, is explicable on the theory
of natural selection. New species are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their own country, give birth to the greatest
number of new varieties or incipient species. We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest and most widely diffused, producing the greatest number of new
varieties. It is also natural that the dominant, varying and far−spreading species, which have already invaded, to a
certain extent, the territories of other species, should be those which would have the best chance of spreading still
further, and of giving rise in new countries to other new varieties and species. The process of diffusion would
often be very slow, depending on climatal and geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they might have to pass, but in the course of
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time the dominant forms would generally succeed in spreading and would ultimately prevail. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of distinct continents than with the marine
inhabitants of the continuous sea. We might therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous, succession of the same forms
of life throughout the world, accords well with the principle of new species having been formed by dominant
species spreading widely and varying; the new species thus produced being themselves dominant, owing to their
having had some advantage over their already dominant parents, as well as over other species; and again
spreading, varying, and producing new forms. The old forms which are beaten and which yield their places to the
new and victorious forms, will generally be allied in groups, from inheriting some inferiority in common; and,
therefore, as new and improved groups spread throughout the world, old groups disappear from the world; and the
succession of forms everywhere tends to correspond both in their first appearance and final disappearance.
There is one other remark connected with this subject worth making. I have given my reasons for believing
that most of our great formations, rich in fossils, were deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned, occurred during the periods when the bed of the sea was
either stationary or rising, and likewise when sediment was not thrown down quickly enough to embed and
preserve organic remains. During these long and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and extinction, and that there was much migration from other
parts of the world. As we have reason to believe that large areas are affected by the same movement, it is probable
that strictly contemporaneous formations have often been accumulated over very wide spaces in the same quarter
of the world; but we are very far from having any right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When two formations have been deposited in
two regions during nearly, but not exactly, the same period, we should find in both, from the causes explained in
the foregoing paragraphs, the same general succession in the forms of life; but the species would not exactly
correspond; for there will have been a little more time in the one region than in the other for modification,
extinction, and immigration.
I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his admirable Memoirs on the eocene
deposits of England and France, is able to draw a close general parallelism between the successive stages in the
two countries; but when he compares certain stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same genera, yet the species themselves differ
in a manner very difficult to account for considering the proximity of the two areas, unless, indeed, it be assumed
that an isthmus separated two seas inhabited by distinct, but contemporaneous faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also, shows that there is a striking general
parallelism in the successive Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a surprising
amount of difference in the species. If the several formations in these regions have not been deposited during the
same exact periods−−a formation in one region often corresponding with a blank interval in the other−−and if in
both regions the species have gone on slowly changing during the accumulation of the several formations and
during the long intervals of time between them; in this case the several formations in the two regions could be
arranged in the same order, in accordance with the general succession of the forms of life, and the order would
falsely appear to be strictly parallel; nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.
Let us now look to the mutual affinities of extinct and living species. All fall into a few grand classes; and this
fact is at once explained on the principle of descent. The more ancient any form is, the more, as a general rule, it
differs from living forms. But, as Buckland long ago remarked, extinct species can all be classed either in still
existing groups, or between them. That the extinct forms of life help to fill up the intervals between existing
genera, families, and orders, is certainly true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances. If we confine our attention either to the
living or to the extinct species of the same class, the series is far less perfect than if we combine both into one
general system. In the writings of Professor Owen we continually meet with the expression of generalised forms,
as applied to extinct animals; and in the writings of Agassiz, of prophetic or synthetic types; and these terms
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imply that such forms are, in fact, intermediate or connecting links. Another distinguished palaeontologist, M.
Gaudry, has shown in the most striking manner that many of the fossil mammals discovered by him in Attica
serve to break down the intervals between existing genera. Cuvier ranked the Ruminants and Pachyderms as two
of the most distinct orders of mammals; but so many fossil links have been disentombed that Owen has had to
alter the whole classification, and has placed certain Pachyderms in the same sub−order with ruminants; for
example, he dissolves by gradations the apparently wide interval between the pig and the camel. The Ungulata or
hoofed quadrupeds are now divided into the even−toed or odd−toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one will deny that the Hipparion is
intermediate between the existing horse and certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given to it by Professor Gervais
expresses, and which cannot be placed in any existing order. The Sirenia form a very distinct group of the
mammals, and one of the most remarkable peculiarities in existing dugong and lamentin is the entire absence of
hind limbs, without even a rudiment being left; but the extinct Halitherium had, according to Professor Flower, an
ossified thigh−bone "articulated to a well−defined acetabulum in the pelvis," and it thus makes some approach to
ordinary hoofed quadrupeds, to which the Sirenia are in other respects allied. The cetaceans or whales are widely
different from all other mammals, but the tertiary Zeuglodon and Squalodon, which have been placed by some
naturalists in an order by themselves, are considered by Professor Huxley to be undoubtedly cetaceans, "and to
constitute connecting links with the aquatic carnivora."
Even the wide interval between birds and reptiles has been shown by the naturalist just quoted to be partially
bridged over in the most unexpected manner, on the one hand, by the ostrich and extinct Archeopteryx, and on the
other hand by the Compsognathus, one of the Dinosaurians−−that group which includes the most gigantic of all
terrestrial reptiles. Turning to the Invertebrata, Barrande asserts, a higher authority could not be named, that he is
every day taught that, although palaeozoic animals can certainly be classed under existing groups, yet that at this
ancient period the groups were not so distinctly separated from each other as they now are.
Some writers have objected to any extinct species, or group of species, being considered as intermediate
between any two living species, or groups of species. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms or groups, the objection is probably valid. But in a
natural classification many fossil species certainly stand between living species, and some extinct genera between
living genera, even between genera belonging to distinct families. The most common case, especially with respect
to very distinct groups, such as fish and reptiles, seems to be that, supposing them to be distinguished at the
present day by a score of characters, the ancient members are separated by a somewhat lesser number of
characters, so that the two groups formerly made a somewhat nearer approach to each other than they now do.
It is a common belief that the more ancient a form is, by so much the more it tends to connect by some of its
characters groups now widely separated from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it would be difficult to prove the truth
of the proposition, for every now and then even a living animal, as the Lepidosiren, is discovered having affinities
directed towards very distinct groups. Yet if we compare the older Reptiles and Batrachians, the older Fish, the
older Cephalopods, and the eocene Mammals, with the recent members of the same classes, we must admit that
there is truth in the remark.
Let us see how far these several facts and inferences accord with the theory of descent with modification. As
the subject is somewhat complex, I must request the reader to turn to the diagram in the fourth chapter. We may
suppose that the numbered letters in italics represent genera, and the dotted lines diverging from them the species
in each genus. The diagram is much too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive geological formations, and all the forms
beneath the uppermost line may be considered as extinct. The three existing genera, a14, q14, p14, will form a
small family; b14 and f14, a closely allied family or subfamily; and o14, i14, m14, a third family. These three
families, together with the many extinct genera on the several lines of descent diverging from the parent form (A)
will form an order; for all will have inherited something in common from their ancient progenitor. On the
principle of the continued tendency to divergence of character, which was formerly illustrated by this diagram, the
more recent any form is the more it will generally differ from its ancient progenitor. Hence, we can understand the
rule that the most ancient fossils differ most from existing forms. We must not, however, assume that divergence
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of character is a necessary contingency; it depends solely on the descendants from a species being thus enabled to
seize on many and different places in the economy of nature. Therefore it is quite possible, as we have seen in the
case of some Silurian forms, that a species might go on being slightly modified in relation to its slightly altered
conditions of life, and yet retain throughout a vast period the same general characteristics. This is represented in
the diagram by the letter F14.
All the many forms, extinct and recent, descended from (A), make, as before remarked, one order; and this
order, from the continued effects of extinction and divergence of character, has become divided into several
sub−families and families, some of which are supposed to have perished at different periods, and some to have
endured to the present day.
By looking at the diagram we can see that if many of the extinct forms supposed to be embedded in the
successive formations, were discovered at several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for instance, the genera a1, a5, a10, f8, m3,
m6, m9, were disinterred, these three families would be so closely linked together that they probably would have
to be united into one great family, in nearly the same manner as has occurred with ruminants and certain
pachyderms. Yet he who objected to consider as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for they are intermediate, not directly, but only by a long
and circuitous course through many widely different forms. If many extinct forms were to be discovered above
one of the middle horizontal lines or geological formations−−for instance, above No. VI.−−but none from beneath
this line, then only two of the families (those on the left hand a14, etc., and b14, etc.) would have to be united into
one; and there would remain two families which would be less distinct from each other than they were before the
discovery of the fossils. So again, if the three families formed of eight genera (a14 to m14), on the uppermost line,
be supposed to differ from each other by half−a−dozen important characters, then the families which existed at a
period marked VI would certainly have differed from each other by a less number of characters; for they would at
this early stage of descent have diverged in a less degree from their common progenitor. Thus it comes that
ancient and extinct genera are often in a greater or less degree intermediate in character between their modified
descendants, or between their collateral relations.
Under nature the process will be far more complicated than is represented in the diagram; for the groups will
have been more numerous; they will have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the geological record, and that in a very
broken condition, we have no right to expect, except in rare cases, to fill up the wide intervals in the natural
system, and thus to unite distinct families or orders. All that we have a right to expect is, that those groups which
have, within known geological periods, undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ less from each other in some of their
characters than do the existing members of the same groups; and this by the concurrent evidence of our best
palaeontologists is frequently the case.
Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the
extinct forms of life to each other and to living forms, are explained in a satisfactory manner. And they are wholly
inexplicable on any other view.
On this same theory, it is evident that the fauna during any one great period in the earth's history will be
intermediate in general character between that which preceded and that which succeeded it. Thus the species
which lived at the sixth great stage of descent in the diagram are the modified offspring of those which lived at the
fifth stage, and are the parents of those which became still more modified at the seventh stage; hence they could
hardly fail to be nearly intermediate in character between the forms of life above and below. We must, however,
allow for the entire extinction of some preceding forms, and in any one region for the immigration of new forms
from other regions, and for a large amount of modification during the long and blank intervals between the
successive formations. Subject to these allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas. I need give only one instance, namely,
the manner in which the fossils of the Devonian system, when this system was first discovered, were at once
recognised by palaeontologists as intermediate in character between those of the overlying carboniferous and
underlying Silurian systems. But each fauna is not necessarily exactly intermediate, as unequal intervals of time
have elapsed between consecutive formations.
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It is no real objection to the truth of the statement that the fauna of each period as a whole is nearly
intermediate in character between the preceding and succeeding faunas, that certain genera offer exceptions to the
rule. For instance, the species of mastodons and elephants, when arranged by Dr. Falconer in two series−−in the
first place according to their mutual affinities, and in the second place according to their periods of existence−−do
not accord in arrangement. The species extreme in character are not the oldest or the most recent; nor are those
which are intermediate in character, intermediate in age. But supposing for an instant, in this and other such cases,
that the record of the first appearance and disappearance of the species was complete, which is far from the case,
we have no reason to believe that forms successively produced necessarily endure for corresponding lengths of
time. A very ancient form may occasionally have lasted much longer than a form elsewhere subsequently
produced, especially in the case of terrestrial productions inhabiting separated districts. To compare small things
with great; if the principal living and extinct races of the domestic pigeon were arranged in serial affinity, this
arrangement would not closely accord with the order in time of their production, and even less with the order of
their disappearance; for the parent rock−pigeon still lives; and many varieties between the rock−pigeon and the
carrier have become extinct; and carriers which are extreme in the important character of length of beak
originated earlier than short−beaked tumblers, which are at the opposite end of the series in this respect.
Closely connected with the statement, that the organic remains from an intermediate formation are in some
degree intermediate in character, is the fact, insisted on by all palaeontologists, that fossils from two consecutive
formations are far more closely related to each other, than are the fossils from two remote formations. Pictet gives
as a well−known instance, the general resemblance of the organic remains from the several stages of the Chalk
formation, though the species are distinct in each stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his belief in the immutability of species. He who is acquainted with the distribution of existing
species over the globe, will not attempt to account for the close resemblance of distinct species in closely
consecutive formations, by the physical conditions of the ancient areas having remained nearly the same. Let it be
remembered that the forms of life, at least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different climates and conditions. Consider the prodigious
vicissitudes of climate during the pleistocene period, which includes the whole glacial epoch, and note how little
the specific forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fossil remains from closely consecutive formations, being
closely related, though ranked as distinct species, is obvious. As the accumulation of each formation has often
been interrupted, and as long blank intervals have intervened between successive formations, we ought not to
expect to find, as I attempted to show in the last chapter, in any one or in any two formations, all the intermediate
varieties between the species which appeared at the commencement and close of these periods: but we ought to
find after intervals, very long as measured by years, but only moderately long as measured geologically, closely
allied forms, or, as they have been called by some authors, representative species; and these assuredly we do find.
We find, in short, such evidence of the slow and scarcely sensible mutations of specific forms, as we have the
right to expect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.
We have seen in the fourth chapter that the degree of differentiation and specialisation of the parts in organic
beings, when arrived at maturity, is the best standard, as yet suggested, of their degree of perfection or highness.
We have also seen that, as the specialisation of parts is an advantage to each being, so natural selection will tend
to render the organisation of each being more specialised and perfect, and in this sense higher; not but that it may
leave many creatures with simple and unimproved structures fitted for simple conditions of life, and in some cases
will even degrade or simplify the organisation, yet leaving such degraded beings better fitted for their new walks
of life. In another and more general manner, new species become superior to their predecessors; for they have to
beat in the struggle for life all the older forms, with which they come into close competition. We may therefore
conclude that if under a nearly similar climate the eocene inhabitants of the world could be put into competition
with the existing inhabitants, the former would be beaten and exterminated by the latter, as would the secondary
by the eocene, and the palaeozoic by the secondary forms. So that by this fundamental test of victory in the battle
for life, as well as by the standard of the specialisation of organs, modern forms ought, on the theory of natural
selection, to stand higher than ancient forms. Is this the case? A large majority of palaeontologists would answer
in the affirmative; and it seems that this answer must be admitted as true, though difficult of proof.
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It is no valid objection to this conclusion, that certain Brachiopods have been but slightly modified from an
extremely remote geological epoch; and that certain land and fresh−water shells have remained nearly the same,
from the time when, as far as is known, they first appeared. It is not an insuperable difficulty that Foraminifera
have not, as insisted on by Dr. Carpenter, progressed in organisation since even the Laurentian epoch; for some
organisms would have to remain fitted for simple conditions of life, and what could be better fitted for this end
than these lowly organised Protozoa? Such objections as the above would be fatal to my view, if it included
advance in organisation as a necessary contingent. They would likewise be fatal, if the above Foraminifera, for
instance, could be proved to have first come into existence during the Laurentian epoch, or the above Brachiopods
during the Cambrian formation; for in this case, there would not have been time sufficient for the development of
these organisms up to the standard which they had then reached. When advanced up to any given point, there is
no necessity, on the theory of natural selection, for their further continued process; though they will, during each
successive age, have to be slightly modified, so as to hold their places in relation to slight changes in their
conditions. The foregoing objections hinge on the question whether we really know how old the world is, and at
what period the various forms of life first appeared; and this may well be disputed.
The problem whether organisation on the whole has advanced is in many ways excessively intricate. The
geological record, at all times imperfect, does not extend far enough back to show with unmistakable clearness
that within the known history of the world organisation has largely advanced. Even at the present day, looking to
members of the same class, naturalists are not unanimous which forms ought to be ranked as highest: thus, some
look at the selaceans or sharks, from their approach in some important points of structure to reptiles, as the highest
fish; others look at the teleosteans as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in number; but formerly selaceans and ganoids
alone existed; and in this case, according to the standard of highness chosen, so will it be said that fishes have
advanced or retrograded in organisation. To attempt to compare members of distinct types in the scale of highness
seems hopeless; who will decide whether a cuttle−fish be higher than a bee−−that insect which the great Von Baer
believed to be "in fact more highly organised than a fish, although upon another type?" In the complex struggle
for life it is quite credible that crustaceans, not very high in their own class, might beat cephalopods, the highest
molluscs; and such crustaceans, though not highly developed, would stand very high in the scale of invertebrate
animals, if judged by the most decisive of all trials−−the law of battle. Beside these inherent difficulties in
deciding which forms are the most advanced in organisation, we ought not solely to compare the highest members
of a class at any two periods−−though undoubtedly this is one and perhaps the most important element in striking
a balance−−but we ought to compare all the members, high and low, at two periods. At an ancient epoch the
highest and lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed in numbers; at the
present time both groups are greatly reduced, while others, intermediate in organisation, have largely increased;
consequently some naturalists maintain that molluscs were formerly more highly developed than at present; but a
stronger case can be made out on the opposite side, by considering the vast reduction of brachiopods, and the fact
that our existing cephalopods, though few in number, are more highly organised than their ancient representatives.
We ought also to compare the relative proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand kinds of vertebrate animals exist, and if we
knew that at some former period only ten thousand kinds existed, we ought to look at this increase in number in
the highest class, which implies a great displacement of lower forms, as a decided advance in the organisation of
the world. We thus see how hopelessly difficult it is to compare with perfect fairness, under such extremely
complex relations, the standard of organisation of the imperfectly−known faunas of successive periods.
We shall appreciate this difficulty more clearly by looking to certain existing faunas and floras. From the
extraordinary manner in which European productions have recently spread over New Zealand, and have seized on
places which must have been previously occupied by the indigenes, we must believe, that if all the animals and
plants of Great Britain were set free in New Zealand, a multitude of British forms would in the course of time
become thoroughly naturalized there, and would exterminate many of the natives. On the other hand, from the
fact that hardly a single inhabitant of the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in Great Britain, any considerable number
would be enabled to seize on places now occupied by our native plants and animals. Under this point of view, the
productions of Great Britain stand much higher in the scale than those of New Zealand. Yet the most skilful
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The Origin of Species by means of Natural Selection
naturalist, from an examination of the species of the two countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist that ancient animals resemble to a certain extent the
embryos of recent animals belonging to the same classes; and that the geological succession of extinct forms is
nearly parallel with the embryological development of existing forms. This view accords admirably well with our
theory. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations
having supervened at a not early age, and having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more
difference to the adult. Thus the embryo comes to be left as a sort of picture, preserved by nature, of the former
and less modified condition of the species. This view may be true, and yet may never be capable of proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fishes strictly belong to their proper classes, though
some of these old forms are in a slight degree less distinct from each other than are the typical members of the
same groups at the present day, it would be vain to look for animals having the common embryological character
of the Vertebrata, until beds rich in fossils are discovered far beneath the lowest Cambrian strata−−a discovery of
which the chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER
TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the
living marsupials of that continent. In South America, a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armour, like those of the armadillo, found in several parts of La Plata; and Professor
Owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are
related to South American types. This relationship is even more clearly seen in the wonderful collection of fossil
bones made by MM. Lund and Clausen in the caves of Brazil. I was so much impressed with these facts that I
strongly insisted, in 1839 and 1845, on this "law of the succession of types,"−−on "this wonderful relationship in
the same continent between the dead and the living." Professor Owen has subsequently extended the same
generalisation to the mammals of the Old World. We see the same law in this author's restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the caves of Brazil. Mr. Woodward has shown
that the same law holds good with sea−shells, but, from the wide distribution of most molluscs, it is not well
displayed by them. Other cases could be added, as the relation between the extinct and living land−shells of
Madeira; and between the extinct and living brackish water−shells of the Aralo−Caspian Sea.
Now, what does this remarkable law of the succession of the same types within the same areas mean? He
would be a bold man who, after comparing the present climate of Australia and of parts of South America, under
the same latitude, would attempt to account, on the one hand through dissimilar physical conditions, for the
dissimilarity of the inhabitants of these two continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent during the later tertiary periods. Nor can it be pretended that it
is an immutable law that marsupials should have been chiefly or solely produced in Australia; or that Edentata
and other American types should have been solely produced in South America. For we know that Europe in
ancient times was peopled by numerous marsupials; and I have shown in the publications above alluded to, that in
America the law of distribution of terrestrial mammals was formerly different from what it now is. North America
formerly partook strongly of the present character of the southern half of the continent; and the southern half was
formerly more closely allied, than it is at present, to the northern half. In a similar manner we know, from
Falconer and Cautley's discoveries, that Northern India was formerly more closely related in its mammals to
Africa than it is at the present time. Analogous facts could be given in relation to the distribution of marine
animals.
On the theory of descent with modification, the great law of the long enduring, but not immutable, succession
of the same types within the same areas, is at once explained; for the inhabitants of each quarter of the world will
obviously tend to leave in that quarter, during the next succeeding period of time, closely allied though in some
degree modified descendants. If the inhabitants of one continent formerly differed greatly from those of another
continent, so will their modified descendants still differ in nearly the same manner and degree. But after very long
intervals of time, and after great geographical changes, permitting much intermigration, the feebler will yield to
the more dominant forms, and there will be nothing immutable in the distribution of organic beings.
It may be asked in ridicule whether I suppose that the megatherium and other allied huge monsters, which
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The Origin of Species by means of Natural Selection
formerly lived in South America, have left behind them the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals have become wholly extinct, and have
left no progeny. But in the caves of Brazil there are many extinct species which are closely allied in size and in all
other characters to the species still living in South America; and some of these fossils may have been the actual
progenitors of the living species. It must not be forgotten that, on our theory, all the species of the same genus are
the descendants of some one species; so that, if six genera, each having eight species, be found in one geological
formation, and in a succeeding formation there be six other allied or representative genera, each with the same
number of species, then we may conclude that generally only one species of each of the older genera has left
modified descendants, which constitute the new genera containing the several species; the other seven species of
each old genus having died out and left no progeny. Or, and this will be a far commoner case, two or three species
in two or three alone of the six older genera will be the parents of the new genera: the other species and the other
old genera having become utterly extinct. In failing orders, with the genera and species decreasing in numbers as
is the case with the Edentata of South America, still fewer genera and species will leave modified
blood−descendants.
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The Origin of Species by means of Natural Selection
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely imperfect; that only a small portion of the
globe has been geologically explored with care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of species, preserved in our museums, is
absolutely as nothing compared with the number of generations which must have passed away even during a
single formation; that, owing to subsidence being almost necessary for the accumulation of deposits rich in fossil
species of many kinds, and thick enough to outlast future degradation, great intervals of time must have elapsed
between most of our successive formations; that there has probably been more extinction during the periods of
subsidence, and more variation during the periods of elevation, and during the latter the record will have been
least perfectly kept; that each single formation has not been continuously deposited; that the duration of each
formation is probably short compared with the average duration of specific forms; that migration has played an
important part in the first appearance of new forms in any one area and formation; that widely ranging species are
those which have varied most frequently, and have oftenest given rise to new species; that varieties have at first
been local; and lastly, although each species must have passed through numerous transitional stages, it is probable
that the periods, during which each underwent modification, though many and long as measured by years, have
been short in comparison with the periods during which each remained in an unchanged condition. These causes,
taken conjointly, will to a large extent explain why−−though we do find many links−−we do not find interminable
varieties, connecting together all extinct and existing forms by the finest graduated steps. It should also be
constantly borne in mind that any linking variety between two forms, which might be found, would be ranked,
unless the whole chain could be perfectly restored, as a new and distinct species; for it is not pretended that we
have any sure criterion by which species and varieties can be discriminated.
He who rejects this view of the imperfection of the geological record, will rightly reject the whole theory. For
he may ask in vain where are the numberless transitional links which must formerly have connected the closely
allied or representative species, found in the successive stages of the same great formation? He may disbelieve in
the immense intervals of time which must have elapsed between our consecutive formations; he may overlook
how important a part migration has played, when the formations of any one great region, as those of Europe, are
considered; he may urge the apparent, but often falsely apparent, sudden coming in of whole groups of species.
He may ask where are the remains of those infinitely numerous organisms which must have existed long before
the Cambrian system was deposited? We now know that at least one animal did then exist; but I can answer this
last question only by supposing that where our oceans now extend they have extended for an enormous period,
and where our oscillating continents now stand they have stood since the commencement of the Cambrian system;
but that, long before that epoch, the world presented a widely different aspect; and that the older continents,
formed of formations older than any known to us, exist now only as remnants in a metamorphosed condition, or
lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in palaeontology agree admirably with the theory
of descent with modification through variation and natural selection. We can thus understand how it is that new
species come in slowly and successively; how species of different classes do not necessarily change together, or at
the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. The
extinction of old forms is the almost inevitable consequence of the production of new forms. We can understand
why, when a species has once disappeared, it never reappears. Groups of species increase in numbers slowly, and
endure for unequal periods of time; for the process of modification is necessarily slow, and depends on many
complex contingencies. The dominant species belonging to large and dominant groups tend to leave many
modified descendants, which form new sub−groups and groups. As these are formed, the species of the less
vigorous groups, from their inferiority inherited from a common progenitor, tend to become extinct together, and
to leave no modified offspring on the face of the earth. But the utter extinction of a whole group of species has
sometimes been a slow process, from the survival of a few descendants, lingering in protected and isolated
situations. When a group has once wholly disappeared, it does not reappear; for the link of generation has been
broken.
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The Origin of Species by means of Natural Selection
We can understand how it is that dominant forms which spread widely and yield the greatest number of
varieties tend to people the world with allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for existence. Hence, after long intervals of time, the
productions of the world appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent, make together a few grand classes.
We can understand, from the continued tendency to divergence of character, why the more ancient a form is, the
more it generally differs from those now living. Why ancient and extinct forms often tend to fill up gaps between
existing forms, sometimes blending two groups, previously classed as distinct into one; but more commonly
bringing them only a little closer together. The more ancient a form is, the more often it stands in some degree
intermediate between groups now distinct; for the more ancient a form is, the more nearly it will be related to, and
consequently resemble, the common progenitor of groups, since become widely divergent. Extinct forms are
seldom directly intermediate between existing forms; but are intermediate only by a long and circuitous course
through other extinct and different forms. We can clearly see why the organic remains of closely consecutive
formations are closely allied; for they are closely linked together by generation. We can clearly see why the
remains of an intermediate formation are intermediate in character.
The inhabitants of the world at each successive period in its history have beaten their predecessors in the race
for life, and are, in so far, higher in the scale, and their structure has generally become more specialised; and this
may account for the common belief held by so many palaeontologists, that organisation on the whole has
progressed. Extinct and ancient animals resemble to a certain extent the embryos of the more recent animals
belonging to the same classes, and this wonderful fact receives a simple explanation according to our views. The
succession of the same types of structure within the same areas during the later geological periods ceases to be
mysterious, and is intelligible on the principle of inheritance.
If, then, the geological record be as imperfect as many believe, and it may at least be asserted that the record
cannot be proved to be much more perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of palaeontology plainly proclaim, as it seems to
me, that species have been produced by ordinary generation: old forms having been supplanted by new and
improved forms of life, the products of variation and the survival of the fittest.
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The Origin of Species by means of Natural Selection
CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical conditions −− Importance of barriers
−− Affinity of the productions of the same continent −− Centres of creation −− Means of dispersal by changes of
climate and of the level of the land, and by occasional means −− Dispersal during the Glacial period −− Alternate
Glacial periods in the North and South.
In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us
is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be wholly accounted for
by climatal and other physical conditions. Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to prove its truth; for if we exclude the arctic and
northern temperate parts, all authors agree that one of the most fundamental divisions in geographical distribution
is that between the New and Old Worlds; yet if we travel over the vast American continent, from the central parts
of the United States to its extreme southern point, we meet with the most diversified conditions; humid districts,
arid deserts, lofty mountains, grassy plains, forests, marshes, lakes and great rivers, under almost every
temperature. There is hardly a climate or condition in the Old World which cannot be paralleled in the New−−at
least so closely as the same species generally require. No doubt small areas can be pointed out in the Old World
hotter than any in the New World; but these are not inhabited by a fauna different from that of the surrounding
districts; for it is rare to find a group of organisms confined to a small area, of which the conditions are peculiar in
only a slight degree. Notwithstanding this general parallelism in the conditions of Old and New Worlds, how
widely different are their living productions!
In the southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western South
America, between latitudes 25 and 35 degrees, we shall find parts extremely similar in all their conditions, yet it
would not be possible to point out three faunas and floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35 degrees with those north of 25 degrees, which consequently are
separated by a space of ten degrees of latitude, and are exposed to considerably different conditions; yet they are
incomparably more closely related to each other than they are to the productions of Australia or Africa under
nearly the same climate. Analogous facts could be given with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free
migration, are related in a close and important manner to the differences between the productions of various
regions. We see this in the great difference in nearly all the terrestrial productions of the New and Old Worlds,
excepting in the northern parts, where the land almost joins, and where, under a slightly different climate, there
might have been free migration for the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the inhabitants of Australia, Africa, and South
America under the same latitude; for these countries are almost as much isolated from each other as is possible.
On each continent, also, we see the same fact; for on the opposite sides of lofty and continuous mountain−ranges,
and of great deserts and even of large rivers, we find different productions; though as mountain chains, deserts,
etc., are not as impassable, or likely to have endured so long, as the oceans separating continents, the differences
are very inferior in degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants of the eastern and western shores of South
America are very distinct, with extremely few shells, crustacea, or echinodermata in common; but Dr. Gunther
has recently shown that about thirty per cent of the fishes are the same on the opposite sides of the isthmus of
Panama; and this fact has led naturalists to believe that the isthmus was formerly open. Westward of the shores of
America, a wide space of open ocean extends, with not an island as a halting−place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the eastern islands of the Pacific with another and
totally distinct fauna. So that three marine faunas range northward and southward in parallel lines not far from
each other, under corresponding climate; but from being separated from each other by impassable barriers, either
of land or open sea, they are almost wholly distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no impassable barriers, and we have innumerable
islands as halting−places, or continuous coasts, until, after travelling over a hemisphere, we come to the shores of
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The Origin of Species by means of Natural Selection
Africa; and over this vast space we meet with no well−defined and distinct marine faunas. Although so few
marine animals are common to the above−named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific into the Indian Ocean, and many shells are
common to the eastern islands of the Pacific and the eastern shores of Africa on almost exactly opposite meridians
of longitude.
A third great fact, partly included in the foregoing statement, is the affinity of the productions of the same
continent or of the same sea, though the species themselves are distinct at different points and stations. It is a law
of the widest generality, and every continent offers innumerable instances. Nevertheless, the naturalist, in
travelling, for instance, from north to south, never fails to be struck by the manner in which successive groups of
beings, specifically distinct, though nearly related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea
(American ostrich), and northward the plains of La Plata by another species of the same genus; and not by a true
ostrich or emu, like those inhabiting Africa and Australia under the same latitude. On these same plains of La
Plata we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits, and
belonging to the same order of Rodents, but they plainly display an American type of structure. We ascend the
lofty peaks of the Cordillera, and we find an alpine species of bizcacha; we look to the waters, and we do not find
the beaver or muskrat, but the coypu and capybara, rodents of the South American type. Innumerable other
instances could be given. If we look to the islands off the American shore, however much they may differ in
geological structure, the inhabitants are essentially American, though they may be all peculiar species. We may
look back to past ages, as shown in the last chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep organic bond, throughout space and time,
over the same areas of land and water, independently of physical conditions. The naturalist must be dull who is
not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far as we positively know, produces organisms
quite like each other, or, as we see in the case of varieties, nearly alike. The dissimilarity of the inhabitants of
different regions may be attributed to modification through variation and natural selection, and probably in a
subordinate degree to the definite influence of different physical conditions. The degrees of dissimilarity will
depend on the migration of the more dominant forms of life from one region into another having been more or
less effectually prevented, at periods more or less remote−−on the nature and number of the former
immigrants−−and on the action of the inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for life being, as I have already often
remarked, the most important of all relations. Thus the high importance of barriers comes into play by checking
migration; as does time for the slow process of modification through natural selection. Widely−ranging species,
abounding in individuals, which have already triumphed over many competitors in their own widely−extended
homes, will have the best chance of seizing on new places, when they spread out into new countries. In their new
homes they will be exposed to new conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and will produce groups of modified descendants.
On this principle of inheritance with modification we can understand how it is that sections of genera, whole
genera, and even families, are confined to the same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the existence of any law of necessary
development. As the variability of each species is an independent property, and will be taken advantage of by
natural selection, only so far as it profits each individual in its complex struggle for life, so the amount of
modification in different species will be no uniform quantity. If a number of species, after having long competed
with each other in their old home, were to migrate in a body into a new and afterwards isolated country, they
would be little liable to modification; for neither migration nor isolation in themselves effect anything. These
principles come into play only by bringing organisms into new relations with each other and in a lesser degree
with the surrounding physical conditions. As we have seen in the last chapter that some forms have retained
nearly the same character from an enormously remote geological period, so certain species have migrated over
vast spaces, and have not become greatly or at all modified.
According to these views, it is obvious that the several species of the same genus, though inhabiting the most
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distant quarters of the world, must originally have proceeded from the same source, as they are descended from
the same progenitor. In the case of those species which have undergone, during whole geological periods, little
modification, there is not much difficulty in believing that they have migrated from the same region; for during
the vast geographical and climatical changes which have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have reason to believe that the species of a genus
have been produced within comparatively recent times, there is great difficulty on this head. It is also obvious that
the individuals of the same species, though now inhabiting distant and isolated regions, must have proceeded from
one spot, where their parents were first produced: for, as has been explained, it is incredible that individuals
identically the same should have been produced from parents specifically distinct.
SINGLE CENTRES OF SUPPOSED CREATION.
We are thus brought to the question which has been largely discussed by naturalists, namely, whether species
have been created at one or more points of the earth's surface. Undoubtedly there are many cases of extreme
difficulty in understanding how the same species could possibly have migrated from some one point to the several
distant and isolated points, where now found. Nevertheless the simplicity of the view that each species was first
produced within a single region captivates the mind. He who rejects it, rejects the vera causa of ordinary
generation with subsequent migration, and calls in the agency of a miracle. It is universally admitted, that in most
cases the area inhabited by a species is continuous; and that when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space could not have been easily passed over by
migration, the fact is given as something remarkable and exceptional. The incapacity of migrating across a wide
sea is more clear in the case of terrestrial mammals than perhaps with any other organic beings; and, accordingly,
we find no inexplicable instances of the same mammals inhabiting distant points of the world. No geologist feels
any difficulty in Great Britain possessing the same quadrupeds with the rest of Europe, for they were no doubt
once united. But if the same species can be produced at two separate points, why do we not find a single mammal
common to Europe and Australia or South America? The conditions of life are nearly the same, so that a
multitude of European animals and plants have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of the northern and southern hemispheres? The
answer, as I believe, is, that mammals have not been able to migrate, whereas some plants, from their varied
means of dispersal, have migrated across the wide and broken interspaces. The great and striking influence of
barriers of all kinds, is intelligible only on the view that the great majority of species have been produced on one
side, and have not been able to migrate to the opposite side. Some few families, many subfamilies, very many
genera, a still greater number of sections of genera, are confined to a single region; and it has been observed by
several naturalists that the most natural genera, or those genera in which the species are most closely related to
each other, are generally confined to the same country, or if they have a wide range that their range is continuous.
What a strange anomaly it would be if a directly opposite rule were to prevail when we go down one step lower in
the series, namely to the individuals of the same species, and these had not been, at least at first, confined to some
one region!
Hence, it seems to me, as it has to many other naturalists, that the view of each species having been produced
in one area alone, and having subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most probable. Undoubtedly many cases occur in
which we cannot explain how the same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within recent geological times, must have
rendered discontinuous the formerly continuous range of many species. So that we are reduced to consider
whether the exceptions to continuity of range are so numerous, and of so grave a nature, that we ought to give up
the belief, rendered probable by general considerations, that each species has been produced within one area, and
has migrated thence as far as it could. It would be hopelessly tedious to discuss all the exceptional cases of the
same species, now living at distant and separated points; nor do I for a moment pretend that any explanation could
be offered of many instances. But, after some preliminary remarks, I will discuss a few of the most striking
classes of facts, namely, the existence of the same species on the summits of distant mountain ranges, and at
distant points in the Arctic and Antarctic regions; and secondly (in the following chapter), the wide distribution of
fresh water productions; and thirdly, the occurrence of the same terrestrial species on islands and on the nearest
mainland, though separated by hundreds of miles of open sea. If the existence of the same species at distant and
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isolated points of the earth's surface can in many instances be explained on the view of each species having
migrated from a single birthplace; then, considering our ignorance with respect to former climatical and
geographical changes, and to the various occasional means of transport, the belief that a single birthplace is the
law seems to me incomparably the safest.
In discussing this subject we shall be enabled at the same time to consider a point equally important for us,
namely, whether the several species of a genus which must on our theory all be descended from a common
progenitor, can have migrated, undergoing modification during their migration from some one area. If, when most
of the species inhabiting one region are different from those of another region, though closely allied to them, it
can be shown that migration from the one region to the other has probably occurred at some former period, our
general view will be much strengthened; for the explanation is obvious on the principle of descent with
modification. A volcanic island, for instance, upheaved and formed at the distance of a few hundreds of miles
from a continent, would probably receive from it in the course of time a few colonists, and their descendants,
though modified, would still be related by inheritance to the inhabitants of that continent. Cases of this nature are
common, and are, as we shall hereafter see, inexplicable on the theory of independent creation. This view of the
relation of the species of one region to those of another, does not differ much from that advanced by Mr. Wallace,
who concludes that "every species has come into existence coincident both in space and time with a pre−existing
closely allied species." And it is now well known that he attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another though allied question, namely,
whether all the individuals of the same species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously created. With organic beings which
never intercross, if such exist, each species, must be descended from a succession of modified varieties, that have
supplanted each other, but have never blended with other individuals or varieties of the same species, so that, at
each successive stage of modification, all the individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which habitually unite for each birth, or which
occasionally intercross, the individuals of the same species inhabiting the same area will be kept nearly uniform
by intercrossing; so that many individuals will go on simultaneously changing, and the whole amount of
modification at each stage will not be due to descent from a single parent. To illustrate what I mean: our English
race−horses differ from the horses of every other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting and training of many individuals during each
generation.
Before discussing the three classes of facts, which I have selected as presenting the greatest amount of
difficulty on the theory of "single centres of creation," I must say a few words on the means of dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can give here only the briefest abstract of the
more important facts. Change of climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have been a high road for migration, when
the climate was different. I shall, however, presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine
faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend together, or may
formerly have blended. Where the sea now extends, land may at a former period have connected islands or
possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. No
geologist disputes that great mutations of level have occurred within the period of existing organisms. Edward
Forbes insisted that all the islands in the Atlantic must have been recently connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically bridged over every ocean, and united
almost every island with some mainland. If, indeed, the arguments used by Forbes are to be trusted, it must be
admitted that scarcely a single island exists which has not recently been united to some continent. This view cuts
the Gordian knot of the dispersal of the same species to the most distant points, and removes many a difficulty;
but to the best of my judgment we are not authorized in admitting such enormous geographical changes within the
period of existing species. It seems to me that we have abundant evidence of great oscillations in the level of the
land or sea; but not of such vast changes in the position and extension of our continents, as to have united them
within the recent period to each other and to the several intervening oceanic islands. I freely admit the former
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existence of many islands, now buried beneath the sea, which may have served as halting places for plants and for
many animals during their migration. In the coral−producing oceans such sunken islands are now marked by rings
of coral or atolls standing over them. Whenever it is fully admitted, as it will some day be, that each species has
proceeded from a single birthplace, and when in the course of time we know something definite about the means
of distribution, we shall be enabled to speculate with security on the former extension of the land. But I do not
believe that it will ever be proved that within the recent period most of our continents which now stand quite
separate, have been continuously, or almost continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution−−such as the great difference in the marine faunas on the opposite
sides of almost every continent−−the close relation of the tertiary inhabitants of several lands and even seas to
their present inhabitants−−the degree of affinity between the mammals inhabiting islands with those of the nearest
continent, being in part determined (as we shall hereafter see) by the depth of the intervening ocean−−these and
other such facts are opposed to the admission of such prodigious geographical revolutions within the recent
period, as are necessary on the view advanced by Forbes and admitted by his followers. The nature and relative
proportions of the inhabitants of oceanic islands are likewise opposed to the belief of their former continuity of
continents. Nor does the almost universally volcanic composition of such islands favour the admission that they
are the wrecks of sunken continents; if they had originally existed as continental mountain ranges, some at least of
the islands would have been formed, like other mountain summits, of granite, metamorphic schists, old
fossiliferous and other rocks, instead of consisting of mere piles of volcanic matter.
I must now say a few words on what are called accidental means, but which more properly should be called
occasional means of distribution. I shall here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or less facilities for transport across the sea
may be said to be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was not
even known how far seeds could resist the injurious action of sea−water. To my surprise I found that out of
eighty−seven kinds, sixty− four germinated after an immersion of twenty−eight days, and a few survived an
immersion of 137 days. It deserves notice that certain orders were far more injured than others: nine Leguminosae
were tried, and, with one exception, they resisted the salt−water badly; seven species of the allied orders,
Hydrophyllaceae and Polemoniaceae, were all killed by a month's immersion. For convenience sake I chiefly tried
small seeds without the capsules or fruit; and as all of these sank in a few days, they could not have been floated
across wide spaces of the sea, whether or not they were injured by salt water. Afterwards I tried some larger fruits,
capsules, etc., and some of these floated for a long time. It is well known what a difference there is in the
buoyancy of green and seasoned timber; and it occurred to me that floods would often wash into the sea dried
plants or branches with seed−capsules or fruit attached to them. Hence I was led to dry the stems and branches of
ninety−four plants with ripe fruit, and to place them on sea−water. The majority sank quickly, but some which,
whilst green, floated for a very short time, when dried floated much longer; for instance, ripe hazel−nuts sank
immediately, but when dried they floated for ninety days, and afterwards when planted germinated; an asparagus
plant with ripe berries floated for twenty−three days, when dried it floated for eighty−five days, and the seeds
afterwards germinated: the ripe seeds of Helosciadium sank in two days, when dried they floated for above ninety
days, and afterwards germinated. Altogether, out of the ninety−four dried plants, eighteen floated for above
twenty−eight days; and some of the eighteen floated for a very much longer period. So that as 64/87 kinds of
seeds germinated after an immersion of twenty−eight days; and as 18/94 distinct species with ripe fruit (but not all
the same species as in the foregoing experiment) floated, after being dried, for above twenty−eight days, we may
conclude, as far as anything can be inferred from these scanty facts, that the seeds of 14/100 kinds of plants of any
country might be floated by sea−currents during twenty−eight days, and would retain their power of germination.
In Johnston's Physical Atlas, the average rate of the several Atlantic currents is thirty−three miles per diem (some
currents running at the rate of sixty miles per diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country; and when stranded, if blown by an inland
gale to a favourable spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a much better manner, for he placed
the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating
plants. He tried ninety−eight seeds, mostly different from mine, but he chose many large fruits, and likewise
seeds, from plants which live near the sea; and this would have favoured both the average length of their flotation
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and their resistance to the injurious action of the salt−water. On the other hand, he did not previously dry the
plants or branches with the fruit; and this, as we have seen, would have caused some of them to have floated much
longer. The result was that 18/98 of his seeds of different kinds floated for forty−two days, and were then capable
of germination. But I do not doubt that plants exposed to the waves would float for a less time than those
protected from violent movement as in our experiments. Therefore, it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried, could be floated across a space of sea 900 miles in
width, and would then germinate. The fact of the larger fruits often floating longer than the small, is interesting;
as plants with large seeds or fruit which, as Alph. de Candolle has shown, generally have restricted ranges, could
hardly be transported by any other means.
Seeds may be occasionally transported in another manner. Drift timber is thrown up on most islands, even on
those in the midst of the widest oceans; and the natives of the coral islands in the Pacific procure stones for their
tools, solely from the roots of drifted trees, these stones being a valuable royal tax. I find that when irregularly
shaped stones are embedded in the roots of trees, small parcels of earth are very frequently enclosed in their
interstices and behind them, so perfectly that not a particle could be washed away during the longest transport: out
of one small portion of earth thus COMPLETELY enclosed by the roots of an oak about fifty years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this observation. Again, I can show that the
carcasses of birds, when floating on the sea, sometimes escape being immediately devoured; and many kinds of
seeds in the crops of floating birds long retain their vitality: peas and vetches, for instance, are killed by even a
few days' immersion in sea−water; but some taken out of the crop of a pigeon, which had floated on artificial
sea−water for thirty days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the transportation of seeds. I could give many facts
showing how frequently birds of many kinds are blown by gales to vast distances across the ocean. We may
safely assume that under such circumstances their rate of flight would often be thirty−five miles an hour; and
some authors have given a far higher estimate. I have never seen an instance of nutritious seeds passing through
the intestines of a bird; but hard seeds of fruit pass uninjured through even the digestive organs of a turkey. In the
course of two months, I picked up in my garden twelve kinds of seeds, out of the excrement of small birds, and
these seemed perfect, and some of them, which were tried, germinated. But the following fact is more important:
the crops of birds do not secrete gastric juice, and do not, as I know by trial, injure in the least the germination of
seeds; now, after a bird has found and devoured a large supply of food, it is positively asserted that all the grains
do not pass into the gizzard for twelve or even eighteen hours. A bird in this interval might easily be blown to the
distance of five hundred miles, and hawks are known to look out for tired birds, and the contents of their torn
crops might thus readily get scattered. Some hawks and owls bolt their prey whole, and after an interval of from
twelve to twenty hours, disgorge pellets, which, as I know from experiments made in the Zoological Gardens,
include seeds capable of germination. Some seeds of the oat, wheat, millet, canary, hemp, clover, and beet
germinated after having been from twelve to twenty−one hours in the stomachs of different birds of prey; and two
seeds of beet grew after having been thus retained for two days and fourteen hours. Fresh−water fish, I find, eat
seeds of many land and water plants; fish are frequently devoured by birds, and thus the seeds might be
transported from place to place. I forced many kinds of seeds into the stomachs of dead fish, and then gave their
bodies to fishing−eagles, storks, and pelicans; these birds, after an interval of many hours, either rejected the
seeds in pellets or passed them in their excrement; and several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself caught one 370 miles from the coast
of Africa, and have heard of others caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that in
November, 1844, swarms of locusts visited the island of Madeira. They were in countless numbers, as thick as the
flakes of snow in the heaviest snowstorm, and extended upward as far as could be seen with a telescope. During
two or three days they slowly careered round and round in an immense ellipse, at least five or six miles in
diameter, and at night alighted on the taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have not since visited the island. Now, in parts of
Natal it is believed by some farmers, though on insufficient evidence, that injurious seeds are introduced into their
grass−land in the dung left by the great flights of locusts which often visit that country. In consequence of this
belief Mr. Weale sent me in a letter a small packet of the dried pellets, out of which I extracted under the
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microscope several seeds, and raised from them seven grass plants, belonging to two species, of two genera.
Hence a swarm of locusts, such as that which visited Madeira, might readily be the means of introducing several
kinds of plants into an island lying far from the mainland.
Although the beaks and feet of birds are generally clean, earth sometimes adheres to them: in one case I
removed sixty−one grains, and in another case twenty−two grains of dry argillaceous earth from the foot of a
partridge, and in the earth there was a pebble as large as the seed of a vetch. Here is a better case: the leg of a
woodcock was sent to me by a friend, with a little cake of dry earth attached to the shank, weighing only nine
grains; and this contained a seed of the toad−rush (Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years has paid close attention to our migratory birds, informs
me that he has often shot wagtails (Motacillae), wheatears, and whinchats (Saxicolae), on their first arrival on our
shores, before they had alighted; and he has several times noticed little cakes of earth attached to their feet. Many
facts could be given showing how generally soil is charged with seeds. For instance, Professor Newton sent me
the leg of a red−legged partridge (Caccabis rufa) which had been wounded and could not fly, with a ball of hard
earth adhering to it, and weighing six and a half ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty−two plants sprung from it: these consisted of twelve
monocotyledons, including the common oat, and at least one kind of grass, and of seventy dicotyledons, which
consisted, judging from the young leaves, of at least three distinct species. With such facts before us, can we
doubt that the many birds which are annually blown by gales across great spaces of ocean, and which annually
migrate−−for instance, the millions of quails across the Mediterranean−−must occasionally transport a few seeds
embedded in dirt adhering to their feet or beaks? But I shall have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and have even carried brushwood,
bones, and the nest of a land−bird, it can hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the arctic and antarctic regions; and during the Glacial period
from one part of the now temperate regions to another. In the Azores, from the large number of plants common to
Europe, in comparison with the species on the other islands of the Atlantic, which stand nearer to the mainland,
and (as remarked by Mr. H.C. Watson) from their somewhat northern character, in comparison with the latitude, I
suspected that these islands had been partly stocked by ice−borne seeds during the Glacial epoch. At my request
Sir C. Lyell wrote to M. Hartung to inquire whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks, which do not occur in the archipelago.
Hence we may safely infer that icebergs formerly landed their rocky burdens on the shores of these mid−ocean
islands, and it is at least possible that they may have brought thither the seeds of northern plants.
Considering that these several means of transport, and that other means, which without doubt remain to be
discovered, have been in action year after year for tens of thousands of years, it would, I think, be a marvellous
fact if many plants had not thus become widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the sea are not accidental, nor is the direction of prevalent
gales of wind. It should be observed that scarcely any means of transport would carry seeds for very great
distances; for seeds do not retain their vitality when exposed for a great length of time to the action of sea water;
nor could they be long carried in the crops or intestines of birds. These means, however, would suffice for
occasional transport across tracts of sea some hundred miles in breadth, or from island to island, or from a
continent to a neighbouring island, but not from one distant continent to another. The floras of distant continents
would not by such means become mingled; but would remain as distinct as they now are. The currents, from their
course, would never bring seeds from North America to Britain, though they might and do bring seeds from the
West Indies to our western shores, where, if not killed by their very long immersion in salt water, they could not
endure our climate. Almost every year, one or two land−birds are blown across the whole Atlantic Ocean, from
North America to the western shores of Ireland and England; but seeds could be transported by these rare
wanderers only by one means, namely, by dirt adhering to their feet or beaks, which is in itself a rare accident.
Even in this case, how small would be the chance of a seed falling on favourable soil, and coming to maturity!
But it would be a great error to argue that because a well−stocked island, like Great Britain, has not, as far as is
known (and it would be very difficult to prove this), received within the last few centuries, through occasional
means of transport, immigrants from Europe or any other continent, that a poorly−stocked island, though standing
more remote from the mainland, would not receive colonists by similar means. Out of a hundred kinds of seeds or
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animals transported to an island, even if far less well−stocked than Britain, perhaps not more than one would be
so well fitted to its new home, as to become naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of geological time, whilst the island was being
upheaved, and before it had become fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed which chanced to arrive, if fitted for the climate, would
germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain−summits, separated from each other by hundreds of
miles of lowlands, where Alpine species could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility of their having migrated from one point to
the other. It is indeed a remarkable fact to see so many plants of the same species living on the snowy regions of
the Alps or Pyrenees, and in the extreme northern parts of Europe; but it is far more remarkable, that the plants on
the White Mountains, in the United States of America, are all the same with those of Labrador, and nearly all the
same, as we hear from Asa Gray, with those on the loftiest mountains of Europe. Even as long ago as 1747, such
facts led Gmelin to conclude that the same species must have been independently created at many distinct points;
and we might have remained in this same belief, had not Agassiz and others called vivid attention to the Glacial
period, which, as we shall immediately see, affords a simple explanation of these facts. We have evidence of
almost every conceivable kind, organic and inorganic, that, within a very recent geological period, central Europe
and North America suffered under an Arctic climate. The ruins of a house burnt by fire do not tell their tale more
plainly than do the mountains of Scotland and Wales, with their scored flanks, polished surfaces, and perched
boulders, of the icy streams with which their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders and scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the inhabitants of Europe, as explained by
Edward Forbes, is substantially as follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly occurred. As the cold came on, and as each
more southern zone became fitted for the inhabitants of the north, these would take the places of the former
inhabitants of the temperate regions. The latter, at the same time would travel further and further southward,
unless they were stopped by barriers, in which case they would perish. The mountains would become covered
with snow and ice, and their former Alpine inhabitants would descend to the plains. By the time that the cold had
reached its maximum, we should have an arctic fauna and flora, covering the central parts of Europe, as far south
as the Alps and Pyrenees, and even stretching into Spain. The now temperate regions of the United States would
likewise be covered by arctic plants and animals and these would be nearly the same with those of Europe; for the
present circumpolar inhabitants, which we suppose to have everywhere travelled southward, are remarkably
uniform round the world.
As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the
productions of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending, as the warmth increased and the snow
still further disappeared, higher and higher, whilst their brethren were pursuing their northern journey. Hence,
when the warmth had fully returned, the same species, which had lately lived together on the European and North
American lowlands, would again be found in the arctic regions of the Old and New Worlds, and on many isolated
mountain−summits far distant from each other.
Thus we can understand the identity of many plants at points so immensely remote as the mountains of the
United States and those of Europe. We can thus also understand the fact that the Alpine plants of each
mountain−range are more especially related to the arctic forms living due north or nearly due north of them: for
the first migration when the cold came on, and the re−migration on the returning warmth, would generally have
been due south and north. The Alpine plants, for example, of Scotland, as remarked by Mr. H.C. Watson, and
those of the Pyrenees, as remarked by Ramond, are more especially allied to the plants of northern Scandinavia;
those of the United States to Labrador; those of the mountains of Siberia to the arctic regions of that country.
These views, grounded as they are on the perfectly well−ascertained occurrence of a former Glacial period, seem
to me to explain in so satisfactory a manner the present distribution of the Alpine and Arctic productions of
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The Origin of Species by means of Natural Selection
Europe and America, that when in other regions we find the same species on distant mountain−summits, we may
almost conclude, without other evidence, that a colder climate formerly permitted their migration across the
intervening lowlands, now become too warm for their existence.
As the arctic forms moved first southward and afterwards backward to the north, in unison with the changing
climate, they will not have been exposed during their long migrations to any great diversity of temperature; and as
they all migrated in a body together, their mutual relations will not have been much disturbed. Hence, in
accordance with the principles inculcated in this volume, these forms will not have been liable to much
modification. But with the Alpine productions, left isolated from the moment of the returning warmth, first at the
bases and ultimately on the summits of the mountains, the case will have been somewhat different; for it is not
likely that all the same arctic species will have been left on mountain ranges far distant from each other, and have
survived there ever since; they will also, in all probability, have become mingled with ancient Alpine species,
which must have existed on the mountains before the commencement of the Glacial epoch, and which during the
coldest period will have been temporarily driven down to the plains; they will, also, have been subsequently
exposed to somewhat different climatical influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification; and they have been modified; for if we
compare the present Alpine plants and animals of the several great European mountain ranges, one with another,
though many of the species remain identically the same, some exist as varieties, some as doubtful forms or
sub−species and some as distinct yet closely allied species representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement of our imaginary Glacial period, the
arctic productions were as uniform round the polar regions as they are at the present day. But it is also necessary
to assume that many sub−arctic and some few temperate forms were the same round the world, for some of the
species which now exist on the lower mountain slopes and on the plains of North America and Europe are the
same; and it may be asked how I account for this degree of uniformity of the sub−arctic and temperate forms
round the world, at the commencement of the real Glacial period. At the present day, the sub−arctic and northern
temperate productions of the Old and New Worlds are separated from each other by the whole Atlantic Ocean and
by the northern part of the Pacific. During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been still more completely separated from each
other by wider spaces of ocean; so that it may well be asked how the same species could then or previously have
entered the two continents. The explanation, I believe, lies in the nature of the climate before the commencement
of the Glacial period. At this, the newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that the climate was warmer than at the present
day. Hence, we may suppose that the organisms which now live under latitude 60 degrees, lived during the
Pliocene period further north, under the Polar Circle, in latitude 66−67 degrees; and that the present arctic
productions then lived on the broken land still nearer to the pole. Now, if we look at a terrestrial globe, we see
under the Polar Circle that there is almost continuous land from western Europe through Siberia, to eastern
America. And this continuity of the circumpolar land, with the consequent freedom under a more favourable
climate for intermigration, will account for the supposed uniformity of the sub−arctic and temperate productions
of the Old and New Worlds, at a period anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long remained in nearly the same relative
position, though subjected to great oscillations of level, I am strongly inclined to extend the above view, and to
infer that during some earlier and still warmer period, such as the older Pliocene period, a large number of the
same plants and animals inhabited the almost continuous circumpolar land; and that these plants and animals, both
in the Old and New Worlds, began slowly to migrate southwards as the climate became less warm, long before
the commencement of the Glacial period. We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On this view we can understand the relationship
with very little identity, between the productions of North America and Europe−−a relationship which is highly
remarkable, considering the distance of the two areas, and their separation by the whole Atlantic Ocean. We can
further understand the singular fact remarked on by several observers that the productions of Europe and America
during the later tertiary stages were more closely related to each other than they are at the present time; for during
these warmer periods the northern parts of the Old and New Worlds will have been almost continuously united by
land, serving as a bridge, since rendered impassable by cold, for the intermigration of their inhabitants.
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During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which
inhabited the New and Old Worlds, migrated south of the Polar Circle, they will have been completely cut off
from each other. This separation, as far as the more temperate productions are concerned, must have taken place
long ages ago. As the plants and animals migrated southward, they will have become mingled in the one great
region with the native American productions, and would have had to compete with them; and in the other great
region, with those of the Old World. Consequently we have here everything favourable for much
modification−−for far more modification than with the Alpine productions, left isolated, within a much more
recent period, on the several mountain ranges and on the arctic lands of Europe and North America. Hence, it has
come, that when we compare the now living productions of the temperate regions of the New and Old Worlds, we
find very few identical species (though Asa Gray has lately shown that more plants are identical than was
formerly supposed), but we find in every great class many forms, which some naturalists rank as geographical
races, and others as distinct species; and a host of closely allied or representative forms which are ranked by all
naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which, during the
Pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the Polar Circle,
will account, on the theory of modification, for many closely allied forms now living in marine areas completely
sundered. Thus, I think, we can understand the presence of some closely allied, still existing and extinct tertiary
forms, on the eastern and western shores of temperate North America; and the still more striking fact of many
closely allied crustaceans (as described in Dana's admirable work), some fish and other marine animals, inhabiting
the Mediterranean and the seas of Japan−−these two areas being now completely separated by the breadth of a
whole continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly inhabiting the seas on the eastern and
western shores of North America, the Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain that such species have been created alike,
in correspondence with the nearly similar physical conditions of the areas; for if we compare, for instance, certain
parts of South America with parts of South Africa or Australia, we see countries closely similar in all their
physical conditions, with their inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I am convinced that Forbes's view may be largely
extended. In Europe we meet with the plainest evidence of the Glacial period, from the western shores of Britain
to the Ural range, and southward to the Pyrenees. We may infer from the frozen mammals and nature of the
mountain vegetation, that Siberia was similarly affected. In the Lebanon, according to Dr. Hooker, perpetual snow
formerly covered the central axis, and fed glaciers which rolled 4,000 feet down the valleys. The same observer
has recently found great moraines at a low level on the Atlas range in North Africa. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent; and in Sikkim, Dr. Hooker saw maize
growing on ancient and gigantic moraines. Southward of the Asiatic continent, on the opposite side of the equator,
we know, from the excellent researches of Dr. J. Haast and Dr. Hector, that in New Zealand immense glaciers
formerly descended to a low level; and the same plants, found by Dr. Hooker on widely separated mountains in
this island tell the same story of a former cold period. From facts communicated to me by the Rev. W.B. Clarke, it
appears also that there are traces of former glacial action on the mountains of the south− eastern corner of
Australia.
Looking to America: in the northern half, ice−borne fragments of rock have been observed on the eastern side
of the continent, as far south as latitude 36 and 37 degrees, and on the shores of the Pacific, where the climate is
now so different, as far south as latitude 46 degrees. Erratic boulders have, also, been noticed on the Rocky
Mountains. In the Cordillera of South America, nearly under the equator, glaciers once extended far below their
present level. In central Chile I examined a vast mound of detritus with great boulders, crossing the Portillo
valley, which, there can hardly be a doubt, once formed a huge moraine; and Mr. D. Forbes informs me that he
found in various parts of the Cordillera, from latitude 13 to 30 degrees south, at about the height of 12,000 feet,
deeply−furrowed rocks, resembling those with which he was familiar in Norway, and likewise great masses of
detritus, including grooved pebbles. Along this whole space of the Cordillera true glaciers do not now exist even
at much more considerable heights. Further south, on both sides of the continent, from latitude 41 degrees to the
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The Origin of Species by means of Natural Selection
southernmost extremity, we have the clearest evidence of former glacial action, in numerous immense boulders
transported far from their parent source.
>From these several facts, namely, from the glacial action having extended all round the northern and
southern hemispheres−−from the period having been in a geological sense recent in both hemispheres−−from its
having lasted in both during a great length of time, as may be inferred from the amount of work effected−−and
lastly, from glaciers having recently descended to a low level along the whole line of the Cordillera, it at one time
appeared to me that we could not avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll, in a series of admirable memoirs, has
attempted to show that a glacial condition of climate is the result of various physical causes, brought into
operation by an increase in the eccentricity of the earth's orbit. All these causes tend towards the same end; but the
most powerful appears to be the indirect influence of the eccentricity of the orbit upon oceanic currents.
According to Mr. Croll, cold periods regularly recur every ten or fifteen thousand years; and these at long
intervals are extremely severe, owing to certain contingencies, of which the most important, as Sir C. Lyell has
shown, is the relative position of the land and water. Mr. Croll believes that the last great glacial period occurred
about 240,000 years ago, and endured, with slight alterations of climate, for about 160,000 years. With respect to
more ancient glacial periods, several geologists are convinced, from direct evidence, that such occurred during the
miocene and eocene formations, not to mention still more ancient formations. But the most important result for
us, arrived at by Mr. Croll, is that whenever the northern hemisphere passes through a cold period the temperature
of the southern hemisphere is actually raised, with the winters rendered much milder, chiefly through changes in
the direction of the ocean currents. So conversely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much light on geographical distribution that I am
strongly inclined to trust in it; but I will first give the facts which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied species, between forty and fifty of
the flowering plants of Tierra del Fuego, forming no inconsiderable part of its scanty flora, are common to North
America and Europe, enormously remote as these areas in opposite hemispheres are from each other. On the lofty
mountains of equatorial America a host of peculiar species belonging to European genera occur. On the Organ
Mountains of Brazil some few temperate European, some Antarctic and some Andean genera were found by
Gardner which do not exist in the low intervening hot countries. On the Silla of Caraccas the illustrious Humboldt
long ago found species belonging to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and some few representatives of the flora of the Cape of
Good Hope, occur on the mountains of Abyssinia. At the Cape of Good Hope a very few European species,
believed not to have been introduced by man, and on the mountains several representative European forms are
found which have not been discovered in the intertropical parts of Africa. Dr. Hooker has also lately shown that
several of the plants living on the upper parts of the lofty island of Fernando Po, and on the neighbouring
Cameroon Mountains, in the Gulf of Guinea, are closely related to those on the mountains of Abyssinia, and
likewise to those of temperate Europe. It now also appears, as I hear from Dr. Hooker, that some of these same
temperate plants have been discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde Islands. This
extension of the same temperate forms, almost under the equator, across the whole continent of Africa and to the
mountains of the Cape Verde archipelago, is one of the most astonishing facts ever recorded in the distribution of
plants.
On the Himalaya, and on the isolated mountain ranges of the peninsula of India, on the heights of Ceylon, and
on the volcanic cones of Java, many plants occur either identically the same or representing each other, and at the
same time representing plants of Europe not found in the intervening hot lowlands. A list of the genera of plants
collected on the loftier peaks of Java, raises a picture of a collection made on a hillock in Europe. Still more
striking is the fact that peculiar Australian forms are represented by certain plants growing on the summits of the
mountains of Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend along the heights of the
peninsula of Malacca, and are thinly scattered on the one hand over India, and on the other hand as far north as
Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered several European species; other species,
not introduced by man, occur on the lowlands; and a long list can be given, as I am informed by Dr. Hooker, of
European genera, found in Australia, but not in the intermediate torrid regions. In the admirable "Introduction to
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The Origin of Species by means of Natural Selection
the Flora of New Zealand," by Dr. Hooker, analogous and striking facts are given in regard to the plants of that
large island. Hence, we see that certain plants growing on the more lofty mountains of the tropics in all parts of
the world, and on the temperate plains of the north and south, are either the same species or varieties of the same
species. It should, however, be observed that these plants are not strictly arctic forms; for, as Mr. H.C. Watson has
remarked, "in receding from polar toward equatorial latitudes, the Alpine or mountain flora really become less
and less Arctic." Besides these identical and closely allied forms, many species inhabiting the same widely
sundered areas, belong to genera not now found in the intermediate tropical lowlands.
These brief remarks apply to plants alone; but some few analogous facts could be given in regard to terrestrial
animals. In marine productions, similar cases likewise occur; as an example, I may quote a statement by the
highest authority, Prof. Dana, that "it is certainly a wonderful fact that New Zealand should have a closer
resemblance in its crustacea to Great Britain, its antipode, than to any other part of the world." Sir J. Richardson,
also, speaks of the reappearance on the shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr.
Hooker informs me that twenty−five species of Algae are common to New Zealand and to Europe, but have not
been found in the intermediate tropical seas.
>From the foregoing facts, namely, the presence of temperate forms on the highlands across the whole of
equatorial Africa, and along the Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well−marked manner across the wide expanse of tropical South America, it appears almost certain that at some
former period, no doubt during the most severe part of a Glacial period, the lowlands of these great continents
were everywhere tenanted under the equator by a considerable number of temperate forms. At this period the
equatorial climate at the level of the sea was probably about the same with that now experienced at the height of
from five to six thousand feet under the same latitude, or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been clothed with a mingled tropical and temperate vegetation,
like that described by Hooker as growing luxuriantly at the height of from four to five thousand feet on the lower
slopes of the Himalaya, but with perhaps a still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found temperate European forms beginning
to appear at the height of about five thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with forms of the torrid zone harmoniously
blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern hemisphere suffered from the extreme
cold of the great Glacial period, the southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the temperate parts of both hemispheres, and
on the mountains of the tropics. The Glacial period, as measured by years, must have been very long; and when
we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this
period will have been ample for any amount of migration. As the cold became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just given, there can hardly be a doubt that
some of the more vigorous, dominant and widest−spreading temperate forms invaded the equatorial lowlands.
The inhabitants of these hot lowlands would at the same time have migrated to the tropical and subtropical
regions of the south, for the southern hemisphere was at this period warmer. On the decline of the Glacial period,
as both hemispheres gradually recovered their former temperature, the northern temperate forms living on the
lowlands under the equator, would have been driven to their former homes or have been destroyed, being replaced
by the equatorial forms returning from the south. Some, however, of the northern temperate forms would almost
certainly have ascended any adjoining high land, where, if sufficiently lofty, they would have long survived like
the Arctic forms on the mountains of Europe. They might have survived, even if the climate was not perfectly
fitted for them, for the change of temperature must have been very slow, and plants undoubtedly possess a certain
capacity for acclimatisation, as shown by their transmitting to their offspring different constitutional powers of
resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn be subjected to a severe Glacial
period, with the northern hemisphere rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the mountains would now descend and
mingle with the southern forms. These latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with them some of the northern temperate
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The Origin of Species by means of Natural Selection
forms which had descended from their mountain fastnesses. Thus, we should have some few species identically
the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions.
But the species left during a long time on these mountains, or in opposite hemispheres, would have to compete
with many new forms and would be exposed to somewhat different physical conditions; hence, they would be
eminently liable to modification, and would generally now exist as varieties or as representative species; and this
is the case. We must, also, bear in mind the occurrence in both hemispheres of former Glacial periods; for these
will account, in accordance with the same principles, for the many quite distinct species inhabiting the same
widely separated areas, and belonging to genera not now found in the intermediate torrid zones.
It is a remarkable fact, strongly insisted on by Hooker in regard to America, and by Alph. de Candolle in
regard to Australia, that many more identical or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from the north to the south is due to the greater extent of
land in the north, and to the northern forms having existed in their own homes in greater numbers, and having
consequently been advanced through natural selection and competition to a higher stage of perfection, or
dominating power, than the southern forms. And thus, when the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods, the northern forms were the more powerful and were able
to hold their places on the mountains, and afterwards migrate southward with the southern forms; but not so the
southern in regard to the northern forms. In the same manner, at the present day, we see that very many European
productions cover the ground in La Plata, New Zealand, and to a lesser degree in Australia, and have beaten the
natives; whereas extremely few southern forms have become naturalised in any part of the northern hemisphere,
though hides, wool, and other objects likely to carry seeds have been largely imported into Europe during the last
two or three centuries from La Plata and during the last forty or fifty years from Australia. The Neilgherrie
Mountains in India, however, offer a partial exception; for here, as I hear from Dr. Hooker, Australian forms are
rapidly sowing themselves and becoming naturalised. Before the last great Glacial period, no doubt the
intertropical mountains were stocked with endemic Alpine forms; but these have almost everywhere yielded to the
more dominant forms generated in the larger areas and more efficient workshops of the north. In many islands the
native productions are nearly equalled, or even outnumbered, by those which have become naturalised; and this is
the first stage towards their extinction. Mountains are islands on the land; and their inhabitants have yielded to
those produced within the larger areas of the north, just in the same way as the inhabitants of real islands have
everywhere yielded and are still yielding to continental forms naturalised through man's agency.
The same principles apply to the distribution of terrestrial animals and of marine productions, in the northern
and southern temperate zones, and on the intertropical mountains. When, during the height of the Glacial period,
the ocean−currents were widely different to what they now are, some of the inhabitants of the temperate seas
might have reached the equator; of these a few would perhaps at once be able to migrate southwards, by keeping
to the cooler currents, while others might remain and survive in the colder depths until the southern hemisphere
was in its turn subjected to a glacial climate and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by Arctic productions exist to the present day in the deeper parts of
the northern temperate seas.
I am far from supposing that all the difficulties in regard to the distribution and affinities of the identical and
allied species, which now live so widely separated in the north and south, and sometimes on the intermediate
mountain ranges, are removed on the views above given. The exact lines of migration cannot be indicated. We
cannot say why certain species and not others have migrated; why certain species have been modified and have
given rise to new forms, while others have remained unaltered. We cannot hope to explain such facts, until we can
say why one species and not another becomes naturalised by man's agency in a foreign land; why one species
ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance, the occurrence, as shown by Dr. Hooker, of
the same plants at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs, as
suggested by Lyell, may have been concerned in their dispersal. The existence at these and other distant points of
the southern hemisphere, of species, which, though distinct, belong to genera exclusively confined to the south, is
a more remarkable case. Some of these species are so distinct, that we cannot suppose that there has been time
since the commencement of the last Glacial period for their migration and subsequent modification to the
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The Origin of Species by means of Natural Selection
necessary degree. The facts seem to indicate that distinct species belonging to the same genera have migrated in
radiating lines from a common centre; and I am inclined to look in the southern, as in the northern hemisphere, to
a former and warmer period, before the commencement of the last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It may be suspected that before this flora was
exterminated during the last Glacial epoch, a few forms had been already widely dispersed to various points of the
southern hemisphere by occasional means of transport, and by the aid, as halting−places, of now sunken islands.
Thus the southern shores of America, Australia, and New Zealand may have become slightly tinted by the same
peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of
great alternations of climate throughout the world on geographical distribution. And we have now seen that Mr.
Croll's conclusion that successive Glacial periods in the one hemisphere coincide with warmer periods in the
opposite hemisphere, together with the admission of the slow modification of species, explains a multitude of
facts in the distribution of the same and of the allied forms of life in all parts of the globe. The living waters have
flowed during one period from the north and during another from the south, and in both cases have reached the
equator; but the stream of life has flowed with greater force from the north than in the opposite direction, and has
consequently more freely inundated the south. As the tide leaves its drift in horizontal lines, rising higher on the
shores where the tide rises highest, so have the living waters left their living drift on our mountain summits, in a
line gently rising from the Arctic lowlands to a great latitude under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up and surviving in the mountain fastnesses of
almost every land, which serves as a record, full of interest to us, of the former inhabitants of the surrounding
lowlands.
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The Origin of Species by means of Natural Selection
CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION−−continued.
Distribution of fresh−water productions −− On the inhabitants of oceanic islands −− Absence of Batrachians
and of terrestrial Mammals −− On the relation of the inhabitants of islands to those of the nearest mainland −− On
colonisation from the nearest source with subsequent modification −− Summary of the last and present chapters.
FRESH−WATER PRODUCTIONS.
As lakes and river−systems are separated from each other by barriers of land, it might have been thought that
fresh−water productions would not have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to distant countries. But the case is exactly the
reverse. Not only have many fresh−water species, belonging to different classes, an enormous range, but allied
species prevail in a remarkable manner throughout the world. When first collecting in the fresh waters of Brazil, I
well remember feeling much surprise at the similarity of the fresh−water insects, shells, etc., and at the
dissimilarity of the surrounding terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh−water productions can, I think, in most cases be explained by their
having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or
from stream to stream, within their own countries; and liability to wide dispersal would follow from this capacity
as an almost necessary consequence. We can here consider only a few cases; of these, some of the most difficult
to explain are presented by fish. It was formerly believed that the same fresh−water species never existed on two
continents distant from each other. But Dr. Gunther has lately shown that the Galaxias attenuatus inhabits
Tasmania, New Zealand, the Falkland Islands and the mainland of South America. This is a wonderful case, and
probably indicates dispersal from an Antarctic centre during a former warm period. This case, however, is
rendered in some degree less surprising by the species of this genus having the power of crossing by some
unknown means considerable spaces of open ocean: thus there is one species common to New Zealand and to the
Auckland Islands, though separated by a distance of about 230 miles. On the same continent fresh−water fish
often range widely, and as if capriciously; for in two adjoining river systems some of the species may be the same
and some wholly different.
It is probable that they are occasionally transported by what may be called accidental means. Thus fishes still
alive are not very rarely dropped at distant points by whirlwinds; and it is known that the ova retain their vitality
for a considerable time after removal from the water. Their dispersal may, however, be mainly attributed to
changes in the level of the land within the recent period, causing rivers to flow into each other. Instances, also,
could be given of this having occurred during floods, without any change of level. The wide differences of the
fish on the opposite sides of most mountain−ranges, which are continuous and consequently must, from an early
period, have completely prevented the inosculation of the river systems on the two sides, leads to the same
conclusion. Some fresh−water fish belong to very ancient forms, and in such cases there will have been ample
time for great geographical changes, and consequently time and means for much migration. Moreover, Dr.
Gunther has recently been led by several considerations to infer that with fishes the same forms have a long
endurance. Salt−water fish can with care be slowly accustomed to live in fresh water; and, according to
Valenciennes, there is hardly a single group of which all the members are confined to fresh water, so that a marine
species belonging to a fresh−water group might travel far along the shores of the sea, and could, it is probable,
become adapted without much difficulty to the fresh waters of a distant land.
Some species of fresh−water shells have very wide ranges, and allied species which, on our theory, are
descended from a common parent, and must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not likely to be transported by birds; and the ova, as
well as the adults, are immediately killed by sea−water. I could not even understand how some naturalised species
have spread rapidly throughout the same country. But two facts, which I have observed−−and many others no
doubt will be discovered−−throw some light on this subject. When ducks suddenly emerge from a pond covered
with duck−weed, I have twice seen these little plants adhering to their backs; and it has happened to me, in
removing a little duck−weed from one aquarium to another, that I have unintentionally stocked the one with
fresh−water shells from the other. But another agency is perhaps more effectual: I suspended the feet of a duck in
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The Origin of Species by means of Natural Selection
an aquarium, where many ova of fresh−water shells were hatching; and I found that numbers of the extremely
minute and just−hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water
they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These
just−hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to
twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown
across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet. Sir
Charles Lyell informs me that a Dyticus has been caught with an Ancylus (a fresh−water shell like a limpet)
firmly adhering to it; and a water−beetle of the same family, a Colymbetes, once flew on board the "Beagle,"
when forty−five miles distant from the nearest land: how much farther it might have been blown by a favouring
gale no one can tell.
With respect to plants, it has long been known what enormous ranges many fresh−water, and even
marsh−species, have, both over continents and to the most remote oceanic islands. This is strikingly illustrated,
according to Alph. de Candolle, in those large groups of terrestrial plants, which have very few aquatic members;
for the latter seem immediately to acquire, as if in consequence, a wide range. I think favourable means of
dispersal explain this fact. I have before mentioned that earth occasionally adheres in some quantity to the feet
and beaks of birds. Wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the
most likely to have muddy feet. Birds of this order wander more than those of any other; and are occasionally
found on the most remote and barren islands of the open ocean; they would not be likely to alight on the surface
of the sea, so that any dirt on their feet would not be washed off; and when gaining the land, they would be sure to
fly to their natural fresh−water haunts. I do not believe that botanists are aware how charged the mud of ponds is
with seeds: I have tried several little experiments, but will here give only the most striking case: I took in
February three tablespoonfuls of mud from three different points, beneath water, on the edge of a little pond; this
mud when dry weighed only 6 and 3/4 ounces; I kept it covered up in my study for six months, pulling up and
counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the
viscid mud was all contained in a breakfast cup! Considering these facts, I think it would be an inexplicable
circumstance if water−birds did not transport the seeds of fresh−water plants to unstocked ponds and streams,
situated at very distant points. The same agency may have come into play with the eggs of some of the smaller
fresh−water animals.
Other and unknown agencies probably have also played a part. I have stated that fresh−water fish eat some
kinds of seeds, though they reject many other kinds after having swallowed them; even small fish swallow seeds
of moderate size, as of the yellow water−lily and Potamogeton. Herons and other birds, century after century,
have gone on daily devouring fish; they then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected many hours afterwards in pellets or in
the excrement. When I saw the great size of the seeds of that fine water−lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on the distribution of this plant, I thought that the means of its dispersal must remain
inexplicable; but Audubon states that he found the seeds of the great southern water−lily (probably according to
Dr. Hooker, the Nelumbium luteum) in a heron's stomach. Now this bird must often have flown with its stomach
thus well stocked to distant ponds, and, then getting a hearty meal of fish, analogy makes me believe that it would
have rejected the seeds in the pellet in a fit state for germination.
In considering these several means of distribution, it should be remembered that when a pond or stream is first
formed, for instance on a rising islet, it will be unoccupied; and a single seed or egg will have a good chance of
succeeding. Although there will always be a struggle for life between the inhabitants of the same pond, however
few in kind, yet as the number even in a well−stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will probably be less severe than between
terrestrial species; consequently an intruder from the waters of a foreign country would have a better chance of
seizing on a new place, than in the case of terrestrial colonists. We should also remember that many fresh−water
productions are low in the scale of nature, and we have reason to believe that such beings become modified more
slowly than the high; and this will give time for the migration of aquatic species. We should not forget the
probability of many fresh−water forms having formerly ranged continuously over immense areas, and then having
become extinct at intermediate points. But the wide distribution of fresh−water plants, and of the lower animals,
whether retaining the same identical form, or in some degree modified, apparently depends in main part on the
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wide dispersal of their seeds and eggs by animals, more especially by fresh−water birds, which have great powers
of flight, and naturally travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have selected as presenting the greatest amount
of difficulty with respect to distribution, on the view that not only all the individuals of the same species have
migrated from some one area, but that allied species, although now inhabiting the most distant points, have
proceeded from a single area, the birthplace of their early progenitors. I have already given my reasons for
disbelieving in continental extensions within the period of existing species on so enormous a scale that all the
many islands of the several oceans were thus stocked with their present terrestrial inhabitants. This view removes
many difficulties, but it does not accord with all the facts in regard to the productions of islands. In the following
remarks I shall not confine myself to the mere question of dispersal, but shall consider some other cases bearing
on the truth of the two theories of independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number compared with those on equal
continental areas: Alph. de Candolle admits this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780 miles of latitude, together with the outlying
islands of Auckland, Campbell and Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal areas in Southwestern Australia or at the
Cape of Good Hope, we must admit that some cause, independently of different physical conditions, has given
rise to so great a difference in number. Even the uniform county of Cambridge has 847 plants, and the little island
of Anglesea 764, but a few ferns and a few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the barren island of Ascension aboriginally
possessed less than half−a−dozen flowering plants; yet many species have now become naturalised on it, as they
have in New Zealand and on every other oceanic island which can be named. In St. Helena there is reason to
believe that the naturalised plants and animals have nearly or quite exterminated many native productions. He
who admits the doctrine of the creation of each separate species, will have to admit that a sufficient number of the
best adapted plants and animals were not created for oceanic islands; for man has unintentionally stocked them far
more fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the proportion of endemic kinds (i.e. those found
nowhere else in the world) is often extremely large. If we compare, for instance, the number of endemic
land−shells in Madeira, or of endemic birds in the Galapagos Archipelago, with the number found on any
continent, and then compare the area of the island with that of the continent, we shall see that this is true. This fact
might have been theoretically expected, for, as already explained, species occasionally arriving, after long
intervals of time in the new and isolated district, and having to compete with new associates, would be eminently
liable to modification, and would often produce groups of modified descendants. But it by no means follows that,
because in an island nearly all the species of one class are peculiar, those of another class, or of another section of
the same class, are peculiar; and this difference seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations have not been much disturbed; and partly on the
frequent arrival of unmodified immigrants from the mother−country, with which the insular forms have
intercrossed. It should be borne in mind that the offspring of such crosses would certainly gain in vigour; so that
even an occasional cross would produce more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there are twenty−six land birds; of these
twenty−one (or perhaps twenty−three) are peculiar; whereas of the eleven marine birds only two are peculiar; and
it is obvious that marine birds could arrive at these islands much more easily and frequently than land−birds.
Bermuda, on the other hand, which lies at about the same distance from North America as the Galapagos Islands
do from South America, and which has a very peculiar soil, does not possess a single endemic land bird; and we
know from Mr. J.M. Jones's admirable account of Bermuda, that very many North American birds occasionally or
even frequently visit this island. Almost every year, as I am informed by Mr. E.V. Harcourt, many European and
African birds are blown to Madeira; this island is inhabited by ninety−nine kinds, of which one alone is peculiar,
though very closely related to a European form; and three or four other species are confined to this island and to
the Canaries. So that the islands of Bermuda and Madeira have been stocked from the neighbouring continents
with birds, which for long ages have there struggled together, and have become mutually co−adapted. Hence,
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when settled in their new homes, each kind will have been kept by the others to its proper place and habits, and
will consequently have been but little liable to modification. Any tendency to modification will also have been
checked by intercrossing with the unmodified immigrants, often arriving from the mother−country. Madeira again
is inhabited by a wonderful number of peculiar land−shells, whereas not one species of sea−shell is peculiar to its
shores: now, though we do not know how sea−shells are dispersed, yet we can see that their eggs or larvae,
perhaps attached to seaweed or floating timber, or to the feet of wading birds, might be transported across three or
four hundred miles of open sea far more easily than land−shells. The different orders of insects inhabiting
Madeira present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole classes, and their places are occupied by
other classes; thus in the Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, take, or recently
took, the place of mammals. Although New Zealand is here spoken of as an oceanic island, it is in some degree
doubtful whether it should be so ranked; it is of large size, and is not separated from Australia by a profoundly
deep sea; from its geological character and the direction of its mountain ranges, the Rev. W.B. Clarke has lately
maintained that this island, as well as New Caledonia, should be considered as appurtenances of Australia.
Turning to plants, Dr. Hooker has shown that in the Galapagos Islands the proportional numbers of the different
orders are very different from what they are elsewhere. All such differences in number, and the absence of certain
whole groups of animals and plants, are generally accounted for by supposed differences in the physical
conditions of the islands; but this explanation is not a little doubtful. Facility of immigration seems to have been
fully as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the inhabitants of oceanic islands. For instance, in
certain islands not tenanted by a single mammal, some of the endemic plants have beautifully hooked seeds; yet
few relations are more manifest than that hooks serve for the transportal of seeds in the wool or fur of quadrupeds.
But a hooked seed might be carried to an island by other means; and the plant then becoming modified would
form an endemic species, still retaining its hooks, which would form a useless appendage, like the shrivelled
wings under the soldered wing−covers of many insular beetles. Again, islands often possess trees or bushes
belonging to orders which elsewhere include only herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence trees would be little likely to reach distant
oceanic islands; and an herbaceous plant, which had no chance of successfully competing with the many fully
developed trees growing on a continent, might, when established on an island, gain an advantage over other
herbaceous plants by growing taller and taller and overtopping them. In this case, natural selection would tend to
add to the stature of the plant, to whatever order it belonged, and thus first convert it into a bush and then into a
tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on oceanic islands, Bory St. Vincent long ago
remarked that Batrachians (frogs, toads, newts) are never found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this assertion, and have found it true, with the exception of New
Zealand, New Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the Seychelles. But I have
already remarked that it is doubtful whether New Zealand and New Caledonia ought to be classed as oceanic
islands; and this is still more doubtful with respect to the Andaman and Solomon groups and the Seychelles. This
general absence of frogs, toads and newts on so many true oceanic islands cannot be accounted for by their
physical conditions; indeed it seems that islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance. But as these
animals and their spawn are immediately killed (with the exception, as far as known, of one Indian species) by
sea−water, there would be great difficulty in their transportal across the sea, and therefore we can see why they do
not exist on strictly oceanic islands. But why, on the theory of creation, they should not have been created there, it
would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the oldest voyages, and have not found a
single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great continental island; and many islands
situated at a much less distance are equally barren. The Falkland Islands, which are inhabited by a wolf−like fox,
come nearest to an exception; but this group cannot be considered as oceanic, as it lies on a bank in connection
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with the mainland at a distance of about 280 miles; moreover, icebergs formerly brought boulders to its western
shores, and they may have formerly transported foxes, as now frequently happens in the arctic regions. Yet it
cannot be said that small islands will not support at least small mammals, for they occur in many parts of the
world on very small islands, when lying close to a continent; and hardly an island can be named on which our
smaller quadrupeds have not become naturalised and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals; many volcanic islands are sufficiently ancient,
as shown by the stupendous degradation which they have suffered, and by their tertiary strata: there has also been
time for the production of endemic species belonging to other classes; and on continents it is known that new
species of mammals appear and disappear at a quicker rate than other and lower animals. Although terrestrial
mammals do not occur on oceanic islands, aerial mammals do occur on almost every island. New Zealand
possesses two bats found nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin Islands, the
Caroline and Marianne Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it may be asked, has
the supposed creative force produced bats and no other mammals on remote islands? On my view this question
can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly
across. Bats have been seen wandering by day far over the Atlantic Ocean; and two North American species,
either regularly or occasionally, visit Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
Tomes, who has specially studied this family, that many species have enormous ranges, and are found on
continents and on far distant islands. Hence, we have only to suppose that such wandering species have been
modified in their new homes in relation to their new position, and we can understand the presence of endemic bats
on oceanic islands, with the absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of the sea separating islands from each other, or
from the nearest continent, and the degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has made
some striking observations on this head, since greatly extended by Mr. Wallace's admirable researches, in regard
to the great Malay Archipelago, which is traversed near Celebes by a space of deep ocean, and this separates two
widely distinct mammalian faunas. On either side, the islands stand on a moderately shallow submarine bank, and
these islands are inhabited by the same or by closely allied quadrupeds. I have not as yet had time to follow up
this subject in all quarters of the world; but as far as I have gone, the relation holds good. For instance, Britain is
separated by a shallow channel from Europe, and the mammals are the same on both sides; and so it is with all the
islands near the shores of Australia. The West Indian Islands, on the other hand, stand on a deeply submerged
bank, nearly one thousand fathoms in depth, and here we find American forms, but the species and even the
genera are quite distinct. As the amount of modification which animals of all kinds undergo partly depends on the
lapse of time, and as the islands which are separated from each other, or from the mainland, by shallow channels,
are more likely to have been continuously united within a recent period than the islands separated by deeper
channels, we can understand how it is that a relation exists between the depth of the sea separating two
mammalian faunas, and the degree of their affinity, a relation which is quite inexplicable on the theory of
independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic islands, namely, the fewness of the species,
with a large proportion consisting of endemic forms−−the members of certain groups, but not those of other
groups in the same class, having been modified−−the absence of certain whole orders, as of batrachians and of
terrestrial mammals, notwithstanding the presence of aerial bats, the singular proportions of certain orders of
plants, herbaceous forms having been developed into trees, etc., seem to me to accord better with the belief in the
efficiency of occasional means of transport, carried on during a long course of time, than with the belief in the
former connection of all oceanic islands with the nearest continent; for on this latter view it is probable that the
various classes would have immigrated more uniformly, and from the species having entered in a body, their
mutual relations would not have been much disturbed, and consequently, they would either have not been
modified, or all the species in a more equable manner.
I do not deny that there are many and serious difficulties in understanding how many of the inhabitants of the
more remote islands, whether still retaining the same specific form or subsequently modified, have reached their
present homes. But the probability of other islands having once existed as halting−places, of which not a wreck
now remains, must not be overlooked. I will specify one difficult case. Almost all oceanic islands, even the most
isolated and smallest, are inhabited by land−shells, generally by endemic species, but sometimes by species found
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elsewhere striking instances of which have been given by Dr. A.A. Gould in relation to the Pacific. Now it is
notorious that land−shells are easily killed by sea−water; their eggs, at least such as I have tried, sink in it and are
killed. Yet there must be some unknown, but occasionally efficient means for their transportal. Would the
just−hatched young sometimes adhere to the feet of birds roosting on the ground and thus get transported? It
occurred to me that land−shells, when hybernating and having a membranous diaphragm over the mouth of the
shell, might be floated in chinks of drifted timber across moderately wide arms of the sea. And I find that several
species in this state withstand uninjured an immersion in sea−water during seven days. One shell, the Helix
pomatia, after having been thus treated, and again hybernating, was put into sea−water for twenty days and
perfectly recovered. During this length of time the shell might have been carried by a marine country of average
swiftness to a distance of 660 geographical miles. As this Helix has a thick calcareous operculum I removed it,
and when it had formed a new membranous one, I again immersed it for fourteen days in sea−water, and again it
recovered and crawled away. Baron Aucapitaine has since tried similar experiments. He placed 100 land− shells,
belonging to ten species, in a box pierced with holes, and immersed it for a fortnight in the sea. Out of the
hundred shells twenty−seven recovered. The presence of an operculum seems to have been of importance, as out
of twelve specimens of Cyclostoma elegans, which is thus furnished, eleven revived. It is remarkable, seeing how
well the Helix pomatia resisted with me the salt−water, that not one of fifty−four specimens belonging to four
other species of Helix tried by Aucapitaine recovered. It is, however, not at all probable that land−shells have
often been thus transported; the feet of birds offer a more probable method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.
The most striking and important fact for us is the affinity of the species which inhabit islands to those of the
nearest mainland, without being actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a distance of between 500 and 600 miles from the shores of South
America. Here almost every product of the land and of the water bears the unmistakable stamp of the American
continent. There are twenty−six land birds. Of these twenty−one, or perhaps twenty−three, are ranked as distinct
species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds
to American species is manifest in every character in their habits, gestures, and tones of voice. So it is with the
other animals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable Flora of this
archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several
hundred miles from the continent, feels that he is standing on American land. Why should this be so? Why should
the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so
plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the proportions in which the several classes are
associated together, which closely resembles the conditions of the South American coast. In fact, there is a
considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in
the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape
Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape
Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts, such as these, admit of
no sort of explanation on the ordinary view of independent creation; whereas, on the view here maintained, it is
obvious that the Galapagos Islands would be likely to receive colonists from America, whether by occasional
means of transport or (though I do not believe in this doctrine) by formerly continuous land, and the Cape Verde
Islands from Africa; such colonists would be liable to modification−−the principle of inheritance still betraying
their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule that the endemic productions of
islands are related to those of the nearest continent, or of the nearest large island. The exceptions are few, and
most of them can be explained. Thus, although Kerguelen Land stands nearer to Africa than to America, the
plants are related, and that very closely, as we know from Dr. Hooker's account, to those of America: but on the
view that this island has been mainly stocked by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its endemic plants is much more closely related to
Australia, the nearest mainland, than to any other region: and this is what might have been expected; but it is also
plainly related to South America, which, although the next nearest continent, is so enormously remote, that the
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fact becomes an anomaly. But this difficulty partially
disappears on the view that New Zealand, South