t, which is also darker in color. Now, it is an interesting fact that
a fossil bison skull from the lower Pliocene of India resembles the
present European species, and in later geological times very similar
bisons closely allied to each other, if not identical, inhabited all
northern regions, including America. These were large animals with wide
skulls, and there is little doubt that from this circumpolar form came
both of the bisons now inhabiting Europe and America. Out of some half
dozen fossil bison which have been described from America, none earlier
than the latest Tertiary, _Bison latifrons_ from the Pleistocene
seems likely to have been the immediate ancestor of recent American
species, and as the one skull of the woodland bison which has been
examined resembles both _latifrons_ and the European species more
than the plains species does, it seems probable that these two more
nearly represent the primitive bison, of which the former inhabitant of
the prairies is a more modified descendant.

The process of elimination has at last led to this outline definition of
a bison, but among the ungulates we have passed over, there are certain
others which concern us because they are American.

Sheep and goats agree together and differ from oxen in being usually of
smaller size; the tail is shorter, the horns of females are much smaller
than those of males, they lack the accessory column on the inner side of
the upper molars, and the cannon bone is longer and more slender; but
when it comes to a comparison of the one with the other, it is by no
means always easy to tell the difference. It is true that the early
Greeks seem to have had a rough and ready rule under which mistakes were
not easy, for Aristotle tells us "Alcmaeon is mistaken when he says that
goats breathe through their ears," but the severely practical methods of
our own day leave us little but some very minute points of
difference. One of the best of these lies in the shape of the
basi-occipital bone, but naturally this can be observed only in the
prepared skull. The terms often employed to denote difference in the
horns can have only a general application, for they break down in
certain species in which the two groups approach each other. The
following table expresses some fairly definite points of separation:

SHEEP (_Ovis_). GOAT (_Capra_).

1. Muzzle hairy except between 1. Muzzle entirely hairy.
and just above the
nostril