12 THE HORSE AND ITS RELATIVES 



families, of which all the members are extinct : to 

 these fuller reference is made in the chapter on 

 the extinct relatives of the horse. It will of course 

 be obvious that if we had before us the whole of 

 the ancestral series of the horse, we should find 

 an absolutely complete gradation from types with 

 tall-crowned to those with short-crowned cheek- 

 teeth ; and it would consequently be impossible 

 to draw a hard-and-fast line between the two. In 

 that case it would be necessary to make an arbi- 

 trary line of division, as it is sufficiently obvious that 

 it is quite impossible to include all the ancestors 

 of a given mammal in a single family group ; as, if 

 this were attempted, we should find reptiles included 

 in the horse family, mammals being undoubtedly 

 descended from certain extinct, and to some extent 

 primitive, groups of reptiles. 



The horse family, or Equidce, may, then, be 

 briefly defined as including such odd-toed or perisso- 

 dactyle ungulates as have tall-crowned cheek-teeth 

 of a peculiar and characteristic pattern (described 

 later on), and each foot terminating in a single 

 large functional toe, which, in the case of some 

 extinct species, may be flanked by a pair of much 

 smaller functionless toes. In the course of this 

 chapter other characters are noticed which may 

 likewise be used in the definition of the family, 

 although the two mentioned above are amply 

 sufficient for the purpose. 



