386 
DARWINISM 
CHAP. 
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 hard¬ 
ness. 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.” 1 
We thus see that the Equidse 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 Equidse; 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 rudi¬ 
ment of another on the forefeet, and three toes behind. In 
the structure of the feet and teeth, the Eohippus unmistak- 
1 American Addresses, pp. 73-76. 
