100 THE THEORY OF EVOLUTION 



enamel and cement, which wear unequally. The 

 arrangement of the three tooth-substances is in a 

 highly characteristic and very complex pattern, and 

 the anterior grinding teeth (premolars) are similar 

 in form and size and in the pattern of the masticat- 

 ing surface to the posterior teeth (molars). The 

 facial portion of the skull is very long and the jaws 

 high vertically, to provide room for the very deep 

 tooth-sockets. The eye-socket is completely en- 

 circled in bone and displaced backward behind the 

 teeth, which otherwise would invade the socket and 

 compress the eyeball. 



The neck is long and its vertebrae are much special- 

 ized to secure the greatest freedom of movement in 

 combination with strength. The limbs and es- 

 pecially the feet are elongate and their bones have 

 undergone nearly the utmost possible reduction in 

 number, either by complete suppression and loss, or 

 by co-ossification, such concentration being favour- 

 able to rapidity of movement; we find it repeated in 

 other swift runners, such as antelopes and deer. 

 The external bone of the forearm (ulna) is greatly 

 reduced in thickness, the middle portion of its shaft 

 has been lost and its two ends are co-ossified with 

 the enlarged radius, which carries the whole weight 

 imposed upon the fore leg. Similarly, in the hind 

 leg only the shin-bone (tibia) is left in the lower 

 segment, while the fibula seems to have completely 

 disappeared, but if the skeleton of a young colt be 

 examined, it is seen that, while the whole shaft of 



