ITS ANCESTORS AND RELATIONS 



39 



of smell, give them ample means of becoming aware of 

 the approach of enemies ; while the length of their limbs, 

 the angles the different segments form with each other, 

 and especially the combination of firmness, stability, and 

 lightness in the reduction of all the toes to a single one, 

 upon which the whole weight of the body and all the 

 muscular power are concentrated, give them speed and 

 endurance surpassing that of almost any other animal. 



Remarkable changes in the structure and mode of 

 growth of the teeth, which will be described in detail 

 later on, have taken place pari passu with the modifica- 

 tions of the limbs and added greatly to their power as 

 organs of mastication, and enabled their possessors to find 

 their sustenance among the comparatively dry and harsh 

 herbage of the open plains, instead of being limited to 

 the more succulent vegetable productions of the marshes 

 and forests in which their predecessors mainly dwelt. 



The structural transitions from the diminutive Hyra- 

 cotherium of the early Eocene period to the modern horse 

 have been accompanied by a gradual increase of dimen- 

 sions. The Miocene Anchitherium was of the size of a 

 sheep. The Pliocene Hipparion and its allies were as 

 large as modern donkeys ; and it is only in the Pleisto- 

 cene period that Equidce appeared 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 become a domesticated animal. 



