HISTORY OF THE PERISSODACTYLA 295 



ports and is enclosed in the characteristic hoof, unlike that of 

 any other mammal. 



In brief, the whole structure of the horses is pre-eminently 

 adapted to swift running; they are admirable "cursorial 

 machines," as they have been called, and every part of the 

 skeleton has been modified and specialized to that end; the 

 narrow, rigid hoofs fit them for walking on firm^ound and 

 they speedily are made helpless in quicksand or bog. Did we 

 know nothing of their mode of life, we might confidently infer 

 from their teeth that the horses were grazers, feeding prin- 

 cipally upon grass. A long-legged, grazing animal must needs 

 have a neck of sufficient length to enable the mouth to reach 

 the groimd easily, imless a long proboscis is developed ; and so 

 we shall find in the history of the horses that the elongation 

 of the head and neck kept pace with the lengthening of the 

 legs and feet. 



Though it can hardly be doubted that the horses passed 

 through most of their development in North America, yet the 

 immediate ancestry of all the existing species must be sought 

 in the Old World, none of the many Pleistocene species of the 

 western hemisphere having left any descendants. In North 

 America aU of the known Pleistocene forms belonged to the 

 genus Equus, but the True Horse, E. caballus, was not among 

 them. The more abundant and important of these species 

 have been sufficiently described in Chapter VII (p. 199) ; it 

 need only be recalled that there were ten or more distinct 

 forms, ranging in size from the great E. fgigantetis of Texas 

 to the minute E. \tau of Mexico, while the plains and forests 

 were the feeding grounds of moderate-sized species, about 

 14 hands high. 



In the latest Pliocene, and no doubt earUer, species of the 

 modern genus Eqwus had already come into existence ; and in 

 association with these, at least in Florida, were the last sur- 

 vivors of the three-toed horses which were so characteristic of 

 the early Pliocene and the Miocene. However, Uttle is known 



