d, good
cutting instruments and powerful and lasting crushers are
needful. Accordingly, the twelve cutting teeth of a horse are
close-set and concentrated in the forepart of its mouth, like so
many adzes or chisels. The grinders or molars are large, and
have an extremely complicated structure, being composed of a
number of different substances of unequal hardness. The
consequence of this is that they wear away at different rates;
and, hence, the surface of each grinder is always as uneven as
that of a good millstone."[186]

We thus see that the Equidae differ very widely in structure from most
other mammals. Assuming the truth of the theory of evolution, we should
expect to find traces among extinct animals of the steps by which this
great modification has been effected; and we do really find traces of
these steps, imperfectly among European fossils, but far more completely
among those of America.

It is a singular fact that, although no horse inhabited America when
discovered by Europeans, yet abundance of remains of extinct horses have
been found both in North and South America in Post-Tertiary and Upper
Pliocene deposits; and from these an almost continuous series of
modified forms can be traced in the Tertiary formation, till we reach,
at the very base of the series, a primitive form so unlike our perfected
animal, that, had we not the intermediate links, few persons would
believe that the one was the ancestor of the other. The tracing out of
this marvellous history we owe chiefly to Professor Marsh of Yale
College, who has himself discovered no less than thirty species of
fossil Equidae; and we will allow him to tell the story of the
development of the horse from a humble progenitor in his own words.

"The oldest representative of the horse at present known is the
diminutive Eohippus from the Lower Eocene. Several species have
been found, all about the size of a fox. Like most of the early
mammals, these ungulates had forty-four teeth, the molars with
short crowns and quite distinct in form from the premolars. The
ulna and fibula were entire and distinct, and there were four
well-developed toes and a rudiment of another on the forefeet,
and three toes behind. In the structure of the feet and teeth,
the Eohippus unmistakably indicates that the direct ancestral
line to the modern horse has already separated from the other
perissod