This is a geologist's favorite chapter because the data (or lack of data?!), that is, fossils and the rocks they are found in, play the starring role; they are the source of the "most obvious and gravest objection which can be urged against my theory" as Darwin wrote, because of the apparent lack of transitional lifeforms predicted by his theory of incremental, gradual change.
Darwin was well aware of this "problem" with his theory and used this chapter to head off anticipated objections.
The apparent lack of transitional forms was not the only problem Darwin addressed in this chapter. Communicating to the general public the immensity of geological time was another challenge. Darwin's scenario of slow, gradual change required vast expanses of time and he advised his readers that if they did not acknowledge "how incomprehensibly vast have been the past periods of time" they should "at once close this volume."
Always the experimentalist, Darwin suggested a line of inquiry/observation one could pursue to illustrate the phenomenon: "A man must for years examine for himself great piles of superimposed strata, and watch the sea at work grinding down old rocks and making fresh sediment, before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us."
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