Studies in the variation and limitations of modern species with large comparative series of specimens have led some of our ablest zoologists to regard each species as having its special “ecologic niche” (Grinnell). We do not, in fact, find two or more distinct species or subspecies of a genus occupying the same area and habitat at the same time. We do very frequently find a considerable range of variation among the individuals, but it is shown by comparison to be within the limits of a single species or subspecies. They are variants, the result of interbreeding of the many, different strains which compose any natural species. Usually, but by no means always, the differences intergrade.

The range of such individual variation is sometimes very considerable. It is apt to be greater in such characters as horns, tusks, proportions of skull and of skeletal parts, than in teeth. Variation is . . .

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