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.* In 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- 

 therium. Each foot has three digits. 



Hipparion? Upper cheek-teeth (Fig. 159), with the antero- 



FIG. 159. Three right upper cheek-teeth of Hipparion. a, Antero-external column ; 6, 

 postero-external column ; c, postero-internal column, or posterior pillar ; d, antero-internal 

 column, or anterior pillar ; /, posterior intermediate column ; i, anterior intermediate column. 

 (From the Palceontologia 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. Either 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. 

 2 Christol, Ann. Sci. Indust. Mid. France, vol. i. p. 180 (1832). 



