72 THE HOESE. 



on the direct line of descent of the modern horses, 

 but that it was a form which, having attained a con- 

 siderable degree of specialization in some particulars, 

 a wide geographical distribution and great abun- 

 dance of individuals, became, as has so often hap- 

 pened in similar cases, extinct without direct descend- 

 ants from causes which we at present cannot divine. 

 Perhaps an inability to lose the useless outer toes 

 may have given it a disadvantage in a severe com- 

 petition for existence with otherwise closely allied 

 forms, which had already adopted the style of foot 

 which clearly shows itself the best for the existing 

 requirements of the race. 



Judging from tooth-structure alone, a very per- 

 fect series of modifications from Anchitherium to the 

 modern horses can be shown through various species 

 of the American genera called Merychippus and 

 Protohippus, without the intervention of the special 

 characteristics of hipparion ; but, unfortunately, of 

 many of these forms, the bones, and especially those 

 of the limbs, are known very imperfectly or not at 

 all. There is, however, already enough to show that 

 it is by no means impossible that America may have 

 been the cradle of all the existing Equidce, as it seems 

 to have been of such apparently typical Old World 

 forms as rhinoceroses and camels, and that they 

 spread westward by means of the former free com- 



