182 EVOLUTION AND GENETICS 



country. It was of about the same size as the other Miocene 

 species. 



Pliohippus, a North American genus of the early PHocene, was 

 the last progenitor of the modern genus Equus. The included 

 species were no larger than the few immediate ancestors, i.e., 

 about forty inches high. They difTered conspicuously, however, 

 in the reduction of the second and fourth digits on all feet to 

 splints; thus Pliohippus is the earHest known one-toed horse. 

 The third digit was highly developed and ])ore a well formed hoof. 

 This genus gave rise to Plesippus, from which it was only a step to 

 Equus. 



South American Horses. Either Pliohippus or Protohippus gave 

 rise to the genus Hippidion of South American horses which ex- 

 isted during the Pliocene. This genus, its derivate Onohippidion 

 of the Pleistocene, and some migrants from North America be- 

 longing to the genus Equus are all of the horses known to have 

 occurred in South America. All were extinct by the end of the 

 latter epoch. (See diagram, page 183.) 



The Genus Equus in North America. Equus of North America 

 is not well known. Scott's statement quoted below, gives the 

 paleontologists' opinion of these species. 



"In the latest Pliocene, and no doubt earlier, species of the 

 modern genus Equus had already come into existence ; and in asso- 

 ciation with these, at least in Florida, were the last survivors of 

 the three-toed horses which were so characteristic of the early 

 Pliocene and the Miocene. However, little is known about these 

 earliest recorded American species of Equus, for the material so 

 far obtained is very fragmentary. In the absence of any richly 

 fossiliferous beds of the upper Pliocene generally, there is a pain- 

 fully felt hiatus in the genealogy of the horses ; and it is impossible 

 to say from present knowledge, whether all of the many species of 

 horses which inhabited North America in the Pleistocene were 

 autochthonous, derived from a purely American ancestry, or how 

 large a proportion of them were migrants from the Old World, 

 coming in when so many of the Pleistocene immigrants of other 

 groups arrived. It is even possible, though not in the least 

 hkely, that all of the native American stocks became extinct in 

 the upper Pliocene and that the Pleistocene species were all 

 immigrants from the eastern hemisphere ; or the slightly modified 

 descendants of such immigrants; but, on the other hand, it is 



