Geology plays a key role in Darwin’s arguments for evolution by natural selection in The Origin. He devotes two whole chapters to geological problems, geological evidence is routinely called upon throughout the book and Darwin refers to Lyell’s gradualist geology (which was by then accepted) to demonstrate that slow processes can have big effects.
The two chapters that deal solely with geology are titled On the Imperfection of the Geological Record and On the Geological Succession of Organic Remains. In the former, Darwin explains why the fossil record does not document the huge number of intermediates that Darwin’s theory suggests would have existed. Darwin’s explanation is inspired by his observation of erosion of rock and fossils by geological processes and by Lyell’s analogy of the fossil record as a book. Darwin writes:
‘I look at the natural geological record, as 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. Each word of the slowly changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated, formations.’
Darwin claims that all the transitional forms between two living species will not be found. Darwin later adds to this a discussion on the huge amounts of time involved and the various processes that will destroy fossils to argue that the apparent lack of fossil evidence for his theory is not a death blow. He also discusses why groups of animals and plants appear suddenly in the fossil record, invoking migration and range expansion and that the earlier forms may just not have been found yet; a compelling argument when we consider what amount of the earth’s crust has been investigated and that new fossils are frequently found pushing the origin of a group back by tens of millions of years.
In the chapter On the Geological Succession of Organic Remains Darwin discusses how his view of a slow and gradual process of evolution through descent and natural selection explains why related species replace each other in the fossil record and why there appears to be a succession from lower to higher groups of species. These observations are exactly what would be predicted by a theory of evolution through common descent.
Darwin argued that what evidence could be extracted from the fossil record supported his theories. The fact that few intermediates are known is not a problem for his ideas, as it is expected based on our understanding of geological processes. In later editions however, Darwin did include discussions on new fossil finds, including the link between birds and reptiles through fossils such as Archaeopteryx. In the 150 years since The Origin was published many more examples of fossils which link distinct groups have been found; as the fossil record becomes more complete it only adds to the evidence for evolution.