Sudden disappearances of many species, now called mass extinctions, did not actually occur. Although the Cretaceous–Tertiary (K–T) event was well known in Darwin's day (Lyell, 1833, p. 328), Darwin was convinced that sudden disappearances of species from the fossil record were due solely to unrecognized gaps in the temporal record.

With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the close of the palaeozoic period [Permian mass extinction] and of Ammonites at the close of the secondary period [K–T mass extinction], we must remember what has already been said on the probable wide intervals of time between our consecutive formations; and in these intervals there may have been much slower extermination. (Darwin, 1859, pp. 321–322)

Like his geologist colleague Charles Lyell, Darwin was contemptuous of those who thought extinctions were caused by great catastrophes.

… so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life! (p. 73)

Species extinction is usually, though not always, caused by the failure of a species in competition with other species. That is, causes of extinction are generally biological, not physical.

The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature. … (p. 345)

If … the eocene inhabitants … were put into competition with the existing inhabitants, … the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary [Mesozoic] fauna by an eocene, and a palaeozoic fauna by a secondary fauna. (p. 337)

… each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of the less-favoured forms almost inevitably follows. (p. 320)

The extinction of species (and larger groups) is closely tied to the process of natural selection and is thus a major component of progressive evolution. In some passages of the Origin, Darwin seems to have seen extinction as part of natural selection; in others, as an inevitable outcome.

… extinction and natural selection … go hand in hand. (p. 172)

The extinction of species and of whole groups of species, which has played so