302 CAUSES OF VARIATION 



abrupt end. The modern horse, however, after his long and 

 tortuous evolution, seems destined for a prolonged and notable 

 existence. During the progressive changes in the feet the 

 leg has been greatly lengthened, the joints modified from the 

 loose ball and socket to the firmer hinge joint, and the teeth 

 have become exceedingly serviceable. It is a noticeable fact 

 that the power of a species to withstand the vicissitudes of ex- 

 treme lapses of time depends very largely upon the ability of 

 its feet, its legs, and its teeth to undergo modification. It has 

 been said that the elephant has succeeded in maintaining him- 

 self to the present in spite of his feet, and by virtue of his excel- 

 lent teeth and his remarkable proboscis. 



It is noticeable that the larger part of this evolutionary his- 

 tory of the horse has been worked out from specimens found in 

 western America, 1 but no one believes that the modern horse 

 is an American animal. This evolution seems to have proceeded 

 upon substantially parallel lines in the eastern and the western 

 continents, which were, during its progress, united by a broad 

 strip of land in the region of Alaska; but something seems to 

 have happened to the American branch, and it is believed that 

 we are indebted to the European and Asiatic branch for the horse 

 of the present. 



Indeed, South America is represented by a fossil form (Hip- 

 pidium) whose feet resembled Equus, except that they were 

 short and stout. Its teeth resembled those of Pliohippus (Museo 

 Nacional, Buenos Ayres). This form had evidently advanced 

 nearly to that of the present, but perished in the general disaster, 

 whatever it was, that overtook the American horse, for it left 

 no descendants that persisted until historic times. 



Causes of the evolution of the horse. As is remarked by 

 Matthew, " the evolution of the horse, adapting it to live on the 

 dry plains, probably went hand in hand with the evolution of 

 the plains themselves." At the commencement of mammalian 



1 Our knowledge of the evolution of the horse is largely due to the indefat- 

 igable labors of Professor Henry F. Osborn, Director of the American Museum 

 of Natural History, and to the magnificent generosity of the late William C. 

 Whitney, through which extensive explorations and significant discoveries were 

 made in Wyoming and other regions of western America. The student of this 

 subject will eagerly await Professor Osborn's forthcoming full report. 



