Eugene M. McCarthy, Ph.D.
Modern biology texts emphasize the word species should be defined in terms of populations of individuals. However, Cuvier, and his paleontologist successors, in recent years most prominently Stephen J. Gould and Niles Eldredge, have provided extensive empirical support for the idea that the entities biologists typically treat as species are, morphologically, 1) separated from each other by sharp discontinuities; 2) constant over time; and 3) each composed of individuals that differ relatively little from one to the next. These are some of the very features creationists emphasize in arguing that evolution does not occur. In fact, although they are all biological notions and have an easily interpreted meaning, they come close to summarizing the ancient essentialist conception of species. Essentialism posited the existence of a parallel world of ideas occupied by "essences" that determined the forms of all the various types of organisms in the visible world (see Section 1). The essentialist outlook, then, was typological. Individual variation was irrelevant under that view.
Many present-day biologists disapprove of thinking in terms of types. They emphasize that individual variation is the sine qua non of natural selection. And yet, there was an element of truth in the essentialist outlook: Despite individual variation, a tiger can be distinguished from a lion, a sand dollar, from a sea urchin. A particular type of bird can be identified from a single picture of an individual representing the type. There are types. But what is the essence of each type? Obviously, the essence of a type is the set of traits distinguishing it from other types. The thing each lion has in common with every other lion is a particular set of traits differentiating it from other related types of animals. The idea that each form of life has an essence, in the sense of a characteristic set of traits, is not, however, necessarily inconsistent with the gradualist outlook. An evolving population, as described in the typical gradualistic scenario, actually is composed of individuals sharing a characteristic set of traits. It is the idea of constancy that does not, and a priori cannot, find a place in gradualistic accounts of evolution. Mayr (1982: 38–39) states this clearly:
Constancy and discontinuity are the points of special emphasis for the essentialists. Variation is attributed to the imperfect manifestation of the underlying essences … Genuine change, according to essentialism, is possible only through the saltational origin of new essences. Because evolution as explained by Darwin is by necessity gradual, it is quite incompatible with essentialism.
Certainly, constancy and saltation are inconsistent with gradual change. But, as we have seen, fossils show that one fossil form does not usually change gradually into another. Darwin's explanation of evolution is largely inconsistent with observation.
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