POSITION AND STRUCTURE OF HORSE 39 



of their crowns ; in which respect, as will be shown 

 in the sequel, they differ from the corresponding 

 teeth of the horse's early ancestors, which had 

 quite short crowns. Now, if surrounding con- 

 ditions be the same, tall-crowned teeth indicate 

 the potentiality of much longer life on the part of 

 their owner than is afforded by low-crowned ones ; 

 as it is obvious that a tall tooth will take much 

 longer to wear down than will a low one. In this 

 particular instance it has, however, to be borne 

 in mind that the early ancestors of the horse were 

 swamp-dwelling animals living on soft, luscious 

 vegetation which could be masticated without having 

 much effect on their teeth. The horse and its 

 relatives, on the other hand, when in a state of 

 nature, live on open plains where the grass is often 

 more or less hard and wiry, and thus calculated 

 to wear away the teeth at a relatively rapid rate. 

 In the case of domesticated horses the rate of wear 

 is probably still further accelerated by the nature 

 of the food. 



Still, after making due allowance for all this, 

 there can be little doubt that the existing members 

 of the horse tribe are longer-lived animals than 

 were their early forerunners. And, as animals 

 go, the domesticated horse may be considered to 

 have a considerable pre-eminence in the matter 

 of longevity, although in this respect it does not 

 equal its distant cousins the rhinoceroses, some of 



