380 UNGULATA 
and endurance surpassing that of almost any other animal. When 
surprised, however, they are by no means helpless, both fore and 
hind feet becoming at need powerful weapons of defence. 
If we were not so habituated to the sight of the Horse as hardly 
ever to consider its structure, we should greatly marvel at being 
told of a mammal so strangely constructed that it had but a single 
toe on each extremity, on the end of the nail of which it walked or 
galloped. Such a conformation is without a parallel in the vertebrate 
series, and is one of the most remarkable instances of specialisation, 
or deviation from the usual type, in accordance with particular 
conditions of life. It is clear, both from the structure of the foot 
itself, and also by an examination of the intermediate forms, that 
this toe corresponds to the middle or third digit of the complete 
typical or pentadactyle foot; and there is very strong evidence to 
show that by a gradual concentration of all the power of the limb 
upon this toe, and the concurrent dwindling away and final dis- 
appearance of all the others, the present condition of the Horse’s 
foot has been produced. 
Protohippus.1—tIn this Lower Pliocene North American genus 
(also described as Merychippus) the cheek-teeth resemble those of 
the generalised species of Equus, but have shorter crowns; while 
the milk-molars approximate to the permanent molars of Anchi- 
theriwm. Each foot has three digits. 
Hipparion.2— Upper cheek-teeth (Fig. 159), with the antero- 
Fic. 159.—Three right upper cheek-teeth of Hipparion. «a, Antero-external column ; b, 
postero-external column; ¢, postero-internal column, or posterior pillar; d, antero-internal 
column, or anterior pillar; f, posterior intermediate column; i, anterior intermediate column. 
(From the Palcontologia Indica.) 
internal column, or anterior pillar as it may be conveniently termed 
in this family, detached throughout the greater part of its height 
from the adjacent column. Lither a single or three digits in each foot. 
First upper premolar large and persistent. This genus was very 
widely distributed in the Pliocene, occurring in Europe, Asia, and 
North America. In the typical European forms, and also in those 
1 Leidy, Proc. Ac. Nat. Sci. Philad. 1858, p. 26. 
* Christol, Ann. Set. Indust. Mid. France, vol. i. p. 180 (1832). 
