10 



THE HOESE 



As will be seen from the sequel, the various forms of 

 asses and zebras only differ from the horse in slight 

 details of their organisation, and with it form a group 

 entirely apart from all other existing animals, a group 

 constituting the genus Equus and the family JEquidce, 

 but no longer considered so isolated as to form a 

 distinct order. In much of what follows the term 

 £ Horse,' unless the contrary is especially stated, must 

 be understood to include the other members of the 

 family. 



To understand the natural place of the horse in the 

 zoological system it will be necessary to take a wide 

 glance at the whole great group to which it belongs. 

 That it is a vertebrate animal, and that it occupies a place 

 in the class Mammalia, no one will doubt. Within that 

 class there can also be no doubt about its taking its place 

 in the great division of JEutheria, which includes all 

 existing mammals except the Marsupials and Mono- 

 tremes. In treating only of existing mammals, a division 

 of the class into distinctly circumscribed groups is per- 

 fectly easy. The so-called orders generally accepted are 

 separated from each other by well-marked breaks of 

 continuity. Many extinct forms can also be contained 

 within the definitions of these orders. But the rapid 

 advance of palaeontology is disclosing to view an 

 enormous number of long-buried animal forms, which 

 are gradually filling up all the spaces left between the 



