Chapter 11: Geological Succession of Organic Beings

Evidence suggests that new species have appeared very slowly, one after another, and that various groups, genera, and classes change at unequal rates; for instance, productions of the land often modify faster than those of the sea. A key tenet is that once a specific form disappears from the earth, it is not believed to ever reappear. Extinction is presented not as the result of sudden catastrophes, but as an intimate and necessary consequence of the production of new, improved, and more competitive forms. Rarity typically precedes extinction, driven by complex, often unseen, hostile agencies that check population growth. The disappearance and appearance of groups follow the same general rules as single species, tending to increase gradually in number until they reach a maximum, followed by a slower process of extermination. A striking observation is that major forms of life, particularly marine species, change almost simultaneously throughout the world in a broad geological sense. This global parallelism is explained by dominant forms spreading widely, varying, and subsequently generating new dominant species that displace older, weaker groups. Analysis of extinct forms reveals that the more ancient a species is, the more it tends to differ from living forms, yet extinct species often serve as intermediate or connecting links, helping to fill the gaps between currently distinct living genera or families. These ancient forms resemble common progenitors before the maximal divergence of character took place. Furthermore, organisms today are generally considered higher in the scale of perfection and more specialized than ancient forms, reflecting the victory of modern forms in the competitive struggle for life. Finally, the law governing the succession of the same types within the same geographical areas during later geological periods—such as the close relationship between extinct and living South American mammals—is explained directly by the principle of inheritance, where modified descendants remain in the same region as their ancestors. This entire framework relies on acknowledging the extreme imperfection and brevity of the geological record.