In Chapter 10, Darwin sets out to demonstrate that the patterns we see in the fossil record, despite its imperfections, are consistent with his theory of descent with modification. If species have evolved gradually, we should expect to find some trace of this evolutionary history in the succession of fossils across geological strata. But instead of a perfectly continuous sequence, what we see is a patchwork: abrupt appearances, long absences, and mysterious disappearances. Darwin argues that these patterns are not only compatible with evolution but are exactly what one should expect given the geological and biological realities.
He begins by asking the central question: Do the known fossils support the immutability of species, or do they better fit the idea of slow and gradual modification through descent with variation and natural selection? The answer, he argues, becomes clearer the more we understand how the geological record forms and what it can and cannot preserve.
Darwin emphasizes that evolution is not governed by fixed or goal-driven laws. Some species change significantly over time, others very little. He coins the term living fossils to describe species that have remained morphologically stable for tens or even hundreds of millions of years. Horseshoe crabs are a key example. Despite their ancient origins, they persist today in a form almost identical to that found in the fossil record. This stability is not mysterious; it simply reflects that under certain environmental conditions, little change is favored.
To capture the nature of geological change, Darwin uses the metaphor of a play: each geological formation is not a complete act of creation, but a randomly captured scene in an ongoing drama. Most scenes are missing, some are fragmented, but taken together, they reveal a continuous process of transformation.
One of Darwin’s core claims in this chapter is that evolutionary change tends to be gradual, even if it sometimes appears abrupt in the fossil record. He admits that these apparent leaps could be fatal to my views if they were real, but he insists they are only apparent. What looks like sudden change is typically the result of missing data or the migration of faunas from one region to another. Most fossil evidence, when examined in sequence, reveals a pattern of slow expansion, diversification, and eventual decline, like the branching of a tree from a single stem.
Darwin devotes special attention to extinction. He distinguishes between two kinds. Gradual extinction happens all the time as a result of ongoing competition and environmental change. But there are also rare catastrophic events, such as those later identified at the ends of the Permian and Cretaceous periods, that cause massive die-offs. These mass extinctions are often triggered by nonbiological factors like asteroid impacts or volcanic eruptions.
One puzzle that lingered for Darwin was why some species disappear despite apparently favorable conditions. On his Beagle voyage, he discovered the fossil remains of an extinct horse in South America: Where horses had later become abundant after their reintroduction by the Spanish. Why had the native horse disappeared? Darwin had no clear answer, but he emphasized that extinction is not always tied to visible causes. Sometimes, the replacement of old forms by newer ones is simply a result of better adaptation and competition.
Darwin explains that extinction and species emergence are linked. New forms arise and supplant the less improved varieties in the same neighborhood. As new forms increase in number, they edge out those that are less suited to the current conditions. This steady state between extinction and origination gives rise to the evolutionary tree: species branch, diversify, and occasionally die out, shaping the overall structure of life.
He illustrates this with the story of trilobites and ammonites. Trilobites first appeared in the mid-Cambrian and thrived until the end-Permian extinction. Ammonoids, which appeared around 409 million years ago, were highly successful until their extinction at the end of the Cretaceous. Both groups underwent adaptive radiation, developed specialized ecologies, and were wiped out by catastrophic events. The fate of these species, Darwin argues, is consistent with gradual evolution punctuated by occasional upheaval.
The global pattern of evolutionary change is also addressed. Darwin notes that marine faunas found in chalk formations (now called the Cretaceous) are found all over the world. They don’t consist of identical species everywhere, but they are composed of related families and genera. This global distribution supports the idea that evolution proceeds gradually and divergently, influenced by both isolation and environmental continuity.
A key part of Darwin’s argument is the affinity between extinct and living species. If species descend from common ancestors, then extinct forms should be more closely related to modern species found in the same geographic region. And indeed, this is what the fossil record shows. Darwin observed that the extinct megafauna of South America, like giant ground sloths and armadillo-like glyptodonts, were clearly related to modern edentates like sloths and armadillos. This continuity between fossil and living forms was one of the first observations that convinced him of common descent.
Darwin also engages with taxonomy. He argues that extinct species must be given equal weight as living ones in any natural classification system. When both are considered, the fossil record reveals a more complete picture of evolutionary history. Extinct organisms help us trace the branching paths that connect modern groups, and in many cases, they fill gaps that would otherwise be difficult to explain.
He cautions against misinterpreting ancient species as simple intermediates between modern forms. Because lineages evolve at unequal rates, an ancient form may be more different from its descendants than the descendants are from each other. Still, he observes, the more ancient a form is, by so much the more it tends to connect groups now widely separated from each other.
Throughout this chapter, Darwin returns again and again to the idea of branching. He reuses the tree metaphor from Chapter 4 to show how lineages expand, diversify, and sometimes vanish. Older forms are generally more like each other than they are to more recent ones, because they represent earlier stages in the branching process.
When discussing progression, Darwin is careful. He acknowledges that some readers believe evolution leads inevitably to greater perfection. But he refuses to accept such value-laden interpretations. High and low are subjective categories. In one of his earlier notebooks, he had written:
It is absurd to talk of one animal being higher than another. We consider those, where the intellectual faculties are most developed, as highest. A bee doubtless would instead choose instincts.
Still, he concedes one sense in which recent species may be higher: they have won the competition for survival. They are better adapted to the present world, which is why they are here while others are gone.
In the final section, Darwin emphasizes what he calls the law of succession of types. In any given region, the species of one geological period are succeeded by similar forms in the next. The extinct and living species of South America, for example, form a continuous lineage. This pattern, Darwin writes, ceases to be mysterious once we accept common descent. It is simply the result of inheritance and adaptation over time.
Darwin ends the chapter by summarizing the cumulative case. Despite the many gaps in the geological record, what we do observe supports his theory. Extinct and living forms follow each other in a branching, adaptive sequence. The succession of species across time and space is best explained by descent with modification, not special creation or fixed laws of development. Fossils, when understood properly, do not contradict evolution. They are its silent witnesses.