244 THE HORSE AND ITS RELATIVES 



generally believed that the same thing took place in 

 South America. 



Why this sudden disappearance of a dominant 

 and thriving group occurred was long a puzzle. It 

 was not that the country had become unsuited to 

 these animals, for when domesticated horses were 

 introduced and escaped from captivity, they ran 

 wild and increased amazingly in both halves of the 

 New World. This suggests that the extinction 

 was probably brought about either by bacterial 

 infection or by a disease analogous to that pro- 

 duced by the agency of tsetse flies in certain parts 

 of Africa at the present day. In connection with 

 the latter part of this suggestion, it is especially 

 noteworthy that remains of extinct tsetses have 

 been discovered in the Miocene formation of Floris- 

 sant, Colorado. 



The existing genus Equus, which, as shown in 

 the preceding chapters of this volume, includes all 

 the living members of the family, extends down- 

 wards through the Pleistocene into the upper 

 portion of the Pliocene period alike in North 

 America, Asia, and Europe. Possibly it may go 

 as low down as the Lower Pliocene in the Siwalik 

 Hills of India, although this is uncertain, as the 

 higher beds of the Siwaliks, which contain remains 

 of true horses, may prove to belong to the upper 

 part of the Pliocene epoch. 



Here it may be well to recapitulate a few of 



