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 (pp. 489–490), Darwin listed the fundamental components (''laws") of the evolutionary process: reproduction, inheritance, variability, struggle for life, and natural selection , with its "consequences" divergence of character and the extinction of less-improved 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 (1964) 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 of vigorous controversy (Alvarez et al., 1980) over the causes of the K–T event, and with the development of concern for presently endangered species, has the role of extinction been confronted in modern terms.
In Tempo and Mode (1944), Simpson detailed what he considered to be the most important determinants of evolution. These were (Chapter II) variability, mutation rate, character of mutations, generation length, population size, and natural selection. But missing from this chapter is any indication that extinction plays an important role in evolution. To be sure, Chapter II includes occasional mention of specific extinctions, but not as significant drivers of evolution. For example, Simpson suggests that mammals with long generation times (equated with large body size) suffered greater extinction in the latest Pleistocene because natural selection could not operate quickly enough for adaptation to changing climatic conditions. But the implications of this are not developed, and Simpson clearly did not share Darwin's view that extinction is a vital part of the evolutionary process. Elsewhere in Tempo and Mode, however, Simpson noted that major extinctions provide opportunities (space, ecological niches, etc.) for later diversification by the survivors.
In sharp contrast to Darwin's view, Simpson saw interspecies competition as only rarely the cause of extinction of species or larger groups. He thought replacement of one group by another was generally passive.