42 THE HORSE. 



power as organs of mastication, and enabled their 

 possessors to find their sustenance among the com- 

 paratively dry and harsh herbage of the open plains, 

 instead of being limited to the more succulent vege- 

 table productions of the marshes and forests in 

 which their predecessors mainly dwelt. 



The structural transitions from the diminutive 

 Hyracotherium of the early Eocene period to the 

 modern horse have been accompanied by a gradual 

 increase of dimensions. The Miocene Anchitherium 

 was of the size of a sheep. The Pliocene Hipparion 

 and its allies were as large as modern donkeys j and 

 it is only in the Pleistocene period that Equidce ap- 

 peared that approached in size the existing horse, 

 the largest races of which are all the products of 

 good feeding and selective breeding since it has be- 

 come a domesticated animal. 



It will be seen from what has been already said 

 that the history of the Perissodactyles as a group 

 offers many points of interest to the naturalist. 

 Among these are its rapid extension and separation 

 into various modifications, all containing numerous 

 minor variations j the complete extinction of many 

 of these, and the survival of three branches only, all 

 of which (except the two domesticated species of the 

 equine branch, which have been largely multiplied 

 and diffused by man's agency) are poor in genera 



