10 THE HORSE AND ITS RELATIVES 



poverty of the odd-toed group at the present day, 

 for the three surviving families are remarkable for 

 the small number of their existing representatives. 

 The horse tribe, for instance, includes at the present 

 day only about eight or nine species, while the 

 rhinoceroses comprise five, and the tapirs another five, 

 or possibly six, specific types. The whole number 

 of living perissodactyles is thus well under a score. 



In this poverty of families, genera, and species 

 the Perissodactyla present a remarkable contrast to 

 the Artiodactyla of the existing epoch, whose 

 specific representatives are between one and two 

 hundred in number, and are classed in no less than 

 nine or ten separate families, with a collectively 

 world-wide distribution — exclusive, of course, of 

 Australia. Despite the fact of its having lost a 

 large number of generic and family types, the 

 Artiodactyla may be regarded as a dominant type 

 at the present day, whereas the Perissodactyla are 

 as distinctly a waning group, so far, at least, as the 

 numerical abundance of genera and species is con- 

 cerned. What may have been the cause of this 

 difference in the two groups cannot yet be 

 determined. 



As regards the characters by which the family 

 Equidce is distinguished from other groups of odd- 

 toed ungulates, there is no difficulty at all when 

 the existing representatives of the sub-order are 

 alone taken into consideration, since the horse and 



