THE EXTINCT HORSES. 409 



The animals belonging to this order being usually of larger size, and 

 accustomed to travel in herds, are liable to wholesale destruction by 

 floods, bogs, precipices, drought, or hunger. Hence their remains are 

 exceedingly numerous in the older geological strata. Such fossil 

 remains are especially abundant in America. The horse is pecujiarly 

 interesting. When Columbus and the Spaniards landed, it was entirely 

 unknown on this continent ; but in the earlier ages of the world's histor}^ 

 horses of all kinds must have roamed over our plains. Among these was 

 the Protohippus, only two feet and a half high, with the lateral toes not 

 externally developed, the Mcsohippus and Ancliitlicriiini, about the size of 

 a sheep with three toes used ibr locomotion, but still unmistakably 

 equine ; and in the deposits of Utah and Wyoming, species have been 

 discovered about the size of a fox, with four toes in front and three 

 behind. These form the genus Oroliippus, the oldest ancestral horse 

 known. America thus possesses a perfect series of forms which, begin- 

 ning with this minute ancient type, is gradually modified by gaining 

 increased size and increased speed, by concentration of the limb-bones, 

 by elongation of the head and neck, by the canine teeth decreasing m 

 size, and by the molars becoming larger and being coated with cement, 

 till we at last come to animals hardly distinguishable, specifically, from 

 the living horse v^ith which we are all acquainted. 



The family Equid.^, as befits the beauty and utility of the animals 

 embraced in it, stands at the head of the order. It contains one genus, 

 which is divided into eight species ; among which are four species of 

 Asses, and tJiree of Zebras. 



GENUS EQUUS. 



The members of this genus are often styled Solipcdcs or "solid- 

 footed," as they have only one apparent toe on each foot, which is 

 enclosed in a single hoof. They have, however, under the skin, the 

 rudiments of lateral toes. They have six incisor teeth in each jaw, 

 small canines exist in the males, but are wanting in the females. Be- 

 tween the canines and the molars there is a space where the bit is 

 placed, an arrangement by which alone man has been able to subdue 

 these vigorous animals. They all attain a moderate size, and possess a 

 graceful figure with strong limbs; their head is long, the eyes vivacious, 

 the nostrils expanded, the ear small, and the hair is short and thick, 

 forming a mane on the neck. 



