EVOLUTION OF THE HORSE 309 



It is generally admitted that the Equidae originated from the 

 Condylarthra, a group of primitive, five-toed, ungulate mammals 

 which made its appearance in early Eocene times, and the best- 

 known representative of which is Phenacodus (Fig. 152). The 

 evolution of the horses appears to have taken place chiefly in 

 America, though occasionally representatives of the group seem to 

 have migrated to or from Europe, doubtless by a former land 

 connection in the neighbourhood of Behring Strait. In com- 

 paratively recent times, however, the family became confined to 

 the old world and was only re-introduced to America by human 

 agency. 



The course of their evolution has evidently been determined by 

 the development of extensive, dry, grass-covered, open plains 

 on the American continent. In adaptation to life on such areas 

 structural modification has proceeded chiefly in two directions. 

 The limbs have become greatly elongated and the foot uplifted 

 from the ground, and thus adapted for rapid flight from pursuing 

 enemies, while the middle digit has become more and more 

 important and the others, together with the ulna and the fibula, 

 have gradually disappeared or become reduced to mere vestiges. 

 At the same time the grazing mechanism has been gradually 

 perfected. The neck and head have become elongated so 

 that the animal is able to reach the ground without bending its 

 legs, and the cheek teeth have acquired complex grinding surfaces 

 and have greatly increased in length to compensate for the 

 increased rate of wear. As in so many other groups, the evolution 

 of these special characters has been accompanied by gradual 

 increase in size (Fig. 153). Thus Eohippus, of lower Eocene 

 times, appears to have been not more than 11 inches high at the 

 shoulder, while existing horses measure about 64 inches, and the 

 numerous intermediate genera for the most part show a regular 

 progress in this respect. 



All these changes have taken place very gradually, and a 

 beautiful series of intermediate forms indicating the different 

 stages from Eohippus to the modern horse (Equus) have been 

 discovered. The sequence of these stages in geological time 

 exactly fits in with the theory that each one has been derived 

 from the one next below it by more perfect adaptation to the 

 conditions of life. Numerous genera have been described, but it 

 is not necessary to mention more than a few. 



The first indisputably horse-like animal appears to have been 



