The passage discusses how civilizations throughout history, though attaining high levels of development, were often confined to small groups of people and did not last. They were isolated "oases of civilization" surrounded by "deserts of barbarism." While having a civilized society was somewhat beneficial, it was also risky, as the barbarians surrounding them could easily overrun the civilized peoples due to their greater numbers and strength, scattering the civilization. Time and again in history, civilized urban societies were conquered in this manner by barbarians invading from nearby hills and destroying whatever they found.