692 EVOLUTION OF THE HORSE. 



Pliohippus (Fig. 643) was about 14 hands high, 

 and its fossil remains are found in the Phocene of 

 America. It was very equine in type, and had only 

 one hoof on each leg ; the 2nd and 4th digits being in 

 a vestigial condition under the skin. The splint bones 

 were a little longer than those of the horse, but did not 

 come quite down to the fetlock joint. All the incisors 

 were well cupped, and were also partly filled with cement. 

 The canine teeth, like those of the horse, were compara- 

 tively small. The 2nd, 3rd and 4th premolars were 

 identical with the molars ; and the length of the crowns 

 of both premolars and molars was about intermediate 

 between those of Protohippus and the horse. 



Pleistocene Equidae. — Horses '' made their first 

 appearance in the Upper Pliocene ; they extended over 

 North and South America, Asia, Europe, and North 

 Africa, during the Pleistocene ; and became extinct in 

 America before the beginning of the Historic period " 

 (von Zittel). No satisfactory reason has been given 

 for their extinction in America. All available evi- 

 dence points to North America as the area in which 

 the ancestors of the horse were one by one evolved, 

 and from which three-toed and, later on, one-toed 

 forms came by natural migration from America into 

 Asia. This migration must have occurred in pre-his- 

 toric ages, when North America was united to Northern 

 Asia. The subsidence of the land which is now covered 

 by Behring Strait and by the Sea of Kamtchatka, severed 

 this connection, apparently during the continuance of 

 the conditions which had destroyed equine life in North 

 America ; because no natural return of horses from Asia 

 to America seems to have taken place, although horses 

 bred freely and prospered all over the American Continent, 

 after they had been brought over from Europe by the 

 Spaniards, long before which time, the agency that had 



