HI LECTURES ON EVOLUTION 121 



shall have to say by and by. Thus the crowns of 

 the cutting teeth have a peculiar deep pit, which 

 gives rise to the well-known " mark " of the horse. 

 There is a large space between the outer incisors 

 and the front grinder. In this space the adult 

 male horse presents, near the incisors on each 

 side, above and below, a canine or " tush," which 

 is commonly absent in mares. In a young horse, 

 moreover, there is not unfrequently to be seen in 

 front of the first grinder, a very small tooth, 

 which soon falls out. If this small tooth be 

 counted as one, it will be found that there are 

 seven teeth behind the canine on each side ; 

 namely, the small tooth in question, and the six 

 great grinders, among which, by an unusual 

 peculiarity, the foremost tooth is rather larger 

 than those which follow it. 



I have now enumerated those characteristic 

 structures of the horse which are of most import- 

 ance for the purpose we have in view. 



To any one who is acquainted with the mor- 

 phology of vertebrated animals, they show that 

 the horse deviates widely from the general 

 structure of mammals ; and that the horse type 

 is, in many respects, an extreme modification of 

 the general mammalian plan. The least modified 

 mammals, in fact, have the radius and ulna, the 

 tibia and fibula, distinct and separate. They 

 have five distinct and complete digits on each 

 foot, and no one of these digits is very much 



