The diagram below illustrates the historical development of the lineages of several animal species. If each of the vertical lines in the diagram above represents a lineage, what is being shown about the number of living species present over time? a. A constant and gradual decrease in the number of species and body forms. b. The number of living species is mostly constant and stable and has experienced little change over time. c. A constant and gradual increase in the number of species and body forms. d. Increases and decreases in the number of species present over time..