296 LAND MAMMALS IN THE WESTERN HEMISPHERE 



about those earliest recorded American species of Equus, for 

 the material so far obtained is very fragmentary. In the ab- 

 sence of any richly fossiliferous beds of the upper Pliocene 

 generally, there is a painfully 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 likely, 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 altogether prob- 

 able that some of these numerous species were intruders. Un- 

 fortunately we are in no position yet to distinguish the native 

 from the foreign stocks. 



In the middle Pliocene, which also has preserved but a meagre 

 and scanty record of its mammalian life, we again meet with 

 horses in relative abundance, but of a far more primitive type. 

 They are still incompletely known, but it is clear that they 

 belonged to three parallel series, or phyla, of three-toed grazing 

 horses, with teeth which, though high-crowned, had not at- 

 tained to the extreme degree of hypsodontism seen in the 

 species of Equus and had a somewhat less complex pattern 

 of the grinding surface, though distinctly foreshadowing the 

 modern degree of complication. One of the genera {'fPlio- 

 hippus) was not improbably the ancestor of a very peculiar 

 horse {^Hippidion) of the South American Pleistocene. These 

 middle Pliocene genera were much smaller animals than the 

 Pleistocene horses, aside from the pygmy species of the latter, 

 of light and more deer-like proportions, and with three func- 

 tional toes or digits. The median digit (3d of the original 

 five) was much the largest and carried most of the weight, on 



