In the whole field of taxonomy no identification is as important as that of the species of an animal. Higher categories, such as the genus, family, order, and so on, are subject to argument and revision, and lower categories, the subspecies and local race, are also more difficult to establish. The species, however, is the pivot of the entire structure because it is the unit of evolutionary change.
In the early days of taxonomy, a collector would shoot a bird or animal, keep its skin and skull, compare it with others in existing collections to determine whether it was something new, and « it was, he would write up a detailed description, giving the bird or animal a new name. It thus became the type specimen, or holotype, of its species, and future collectors would compare their iscoveries with it. This practice was applied to the anthropologi-ai held. Blumenbach, whose classification of mankind in the miliar fivefold skin-color system is still used in some school geography books, selected a particularly handsome skull from a European collection as the type specimen of the white race, and as it had belonged in life to a native of the Caucasus Mountains, white people came to be called Caucasians, or Caucasoids, and still are. As late as 1912 Boule selected the skeleton of La Chapelle aux Saints as the type specimen of Neanderthal man, which he compared to the skeletons of one Frenchman and three anthropoid apes.
As early as Darwin, however, it was recognized that a species is not just the specimen that happened to be killed or unearthed first, and others later found to resemble it, but a population. Indeed, Darwin based his theory of natural selection on his observation that individuals of a species are variable, and that one need not be more typical than another. As time went on, it became clear that a species is a breeding unit or population, which has a gene pool of its own, and not just a collection of individuals, and that each population is a separate entity, living in two related states of dynamic equilibrium. The first regulates the balance between the individuals that compose the population. The second governs its relations with the other species in its environment.
Another early observation was that members of different species do not interbreed, at least in a state of nature. It was first thought that this was not for lack of trying but simply because each species was incapable of fertility with any other. However, early in the twentieth century the rising science of genetics made it clear that some animals of different species could produce felt tile offspring if they could be made to come together. Sterile hybrids like the mule were known from antiquity, and tiger-lion mixtures have been produced in zoos, but hvbridization, it was found, is not a common or important mechanism of evolutionary change in the higher animals, as it is in plants. Furthermore, as each species is in genetic equilibrium with its environment, the addition of new genes from an animal with a different kind of equilibrium could be expected to produce offspring less viable than either parent.
The important distinction is that members of potentially interfertile species do not ordinarily interbreed eitherbreeding periods fall at different seasons or because they simply do not attract each other: they do not recognize each other's mating symbols—visual, olfactory, auditory, or whatever.
In any case, whether or not unconfined animals of different populations interbreed when given the opportunity is the critical test of a zoological species. Paleontologists, of course, cannot use this test, which may be another reason why they prefer to deal in the more readily identified unit of the genus. In the case of living human populations, we can confirm Linnaeus's decision that all men belong to the same species, not only because all races are in-terfertile but also because some individuals among them interbreed, although others oppose mixture. In the case of early human populations unearthed by archaeologists, we cannot be sure whether interbreeding has or has not taken place; and at only one site, the Mt. Carmel caves of Palestine, is there any evidence—a high degree of individual variability combined with a mingling of tool forms—to suggest that the races were mixing, but even that is inconclusive. Therefore, the statements commonly made that Pithecanthropus, Sinanthropus, Neanderthal man, or a member of any other ancient population was unable to interbreed with his neighbors, if he had any, is speculative and cannot be demonstrated.
These statements are based on the old idea that if in some characteristic the ranges of variability of two populations fail to overlap, then these populations are different species. If this were true, then the Pygmies and Watusi of Ruanda-Urundi in Central Africa, who live near each other, would be different species on the basis of stature, and the black-skinned and white-skinned races of the world would also be different species.
This obsolete concept of single-character taxonomy has long since been abandoned. Zoologists now base their decisions on all the characteristics they can identify and measure, characteristics which together give the animal its essential nature, its (to borrow a psychological term) gestalt. The determination of species cannot be made by feeding figures into a computer. It is in a sense an art, practiced by men of experience who know, first of all, how species are formed.