In the Origin (1), Darwin made his view of extinction, and its role in evolution, quite clear. He saw four essential features.
(i) Extinctions of species have occurred gradually and continuously throughout the history of life.
... species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world. (ref. 1, pp. 317-318)
... the complete extinction of the species of a group is generally a slower process than their production: if the appearance and disappearance be represented ... by a vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks the progress of extermination.... (p. 218)
(ii) 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 (ref. 2, 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. (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)
(iii) 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)
(iv) The extinction of species (and larger groups) is closely tied to the process of natural selection and is thus a major component ofprogressive 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 conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms. (p. 475)
Thus, as it seems to me, the manner in which single species and whole groups of species become extinct accords well with the theory of natural selection. (p. 322)
In his final summary of the Origin (ref. 1, pp. 489-490), Darwin listed the fundamental components ("laws") of the evolutionary process: reproduction, inheritance, variability, strugglefor life, and natural selection, with its "consequences" divergence of character and the extinction of lessimproved forms. Despite Darwin's obvious concern for the role of extinction, the word does not appear in the index to the Origin, nor have biologists paid much attention to the phenomenon until the past decade. Mayr (3) published an expanded and modernized index to the Origin, but even this contains only a small fraction of the possible citations to the word extinction. For some reason or reasons not entirely clear, extinction largely dropped out of the consciousness of evolutionary biologists and paleobiologists. Only with the advent ofvigorous controversy (4) over the causes ofthe K-T event, and with the development of concern for presently endangered species, has the role of extinction been confronted in modern terms.