ITS ANCESTORS AND RELATIONS. 41 



ing apart from each other when the weight of the 

 creature is borne on them, are sufficiently well 

 adapted for plodding deliberately over marshy and 

 yielding surfaces, and the tapir and the rhinoceros, 

 which in the structure of the limbs have altered but 

 little from the primitive Eocene forms, still haunt 

 the borders of streams and lakes and the shady 

 depths of forests, as was probably the habit of their 

 ancient representatives ; while the horses are all in- 

 habitants of the open plains, for life upon which 

 their whole organization is in the most eminent de- 

 gree adapted. The length and mobility of the neck, 

 position of the eye and ear, and great development 

 of the organ 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 bodv 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 

 modifications of the limbs and added greatly to their 



