4 EVOLUTION OF THE HORSE 



bones, one on each side, called splint-bones. These represent the 

 second and fourth digits of other animals, but they do not show 

 on the surface, and there is nothing like a separate toe. So that 

 the horse may be said to be an animal that walks on its middle 

 finger-nail, all the other fingers having disappeared. 



The teeth of the horse are almost equally peculiar. The 

 molars are long, square prisms which grow up from the gums 

 as fast as they wear off on the crowns. Their grinding surface 

 exhibits a peculiar and complicated pattern of edges of hard 

 enamel between which are softer spaces composed of dentine and 

 of a material called "cement," much like the dentine in quality 

 but formed in a different way. The dentine is formed on the in- 

 side surfaces of the enamel while the tooth is still within the jaw- 

 bone; the cement is deposited on the outside surfaces of the 

 enamel after the tooth has broken through the jaw-bone and 

 before it appears above the gums. 



Various other peculiarities distinguish the Horse from most 

 other animals ; some of these are shared by other hoofed animals. 

 The two long bones of the fore-arm {radius and ulna) are separate 

 in the greater number of animals, but in the Horse, and in many 

 other hoofed animals they are consolidated into a single bone. 

 The same consolidation is seen in the bones of the lower leg {tibia 

 and fibula). The lengthening of the foot and stepping on the end 

 of the toe raises the heel in the Horse, as in many other animals, 

 to a considerable height above the ground, where it forms the 

 hock joint, bending backward, as the knee bends forward. In 

 these as in various other ways the legs of the horse are especially 

 fitted for swift running over hard and level ground, just as its teeth 

 are for grinding the wiry grasses which grow on the open plain. 



The Zebra and the Ass have the same peculiar structure of 

 teeth and feet as the Domestic Horse, and differ only in the color 

 of the skin, proportions of various parts of the body etc. 



Fossil Horses of the Age of Man. 



The Age of Man, or Quaternary Period, is the last and by far 

 the shortest of the great divisions of geological time. It includes 

 the Great Ice Age or Glacial epoch (Pleistocene), when heavy 



