466 THE CHANGING GENERATIONS 



tinue to grow at the base until the animal is between five and eight years 

 old; then the roots close, growth stops, and the teeth are thenceforth 

 simply pushed up as fast as they are worn away. When they are used up, 

 the horse dies from inability to chew its food. The fossil record shows that 

 these high-crowned, complex grinders evolved from small, short-crowned 

 simple teeth more like the molar teeth of man. 



The adaptations for swift running are equally striking. The body of 

 the horse is narrow, deep-chested, and powerfully muscled at the shoulder 

 and hip; the neck is long, as is necessary in a long-legged grazing animal. 

 The legs are placed close against the body, and when in rapid movement, 

 the feet are brought almost under the body axis, giving maximum support 

 and the ability to "lean into" a curving course. The movement of the 

 legs is restricted largely to a fore-and-aft swing; the radius and ulna of 

 the foreleg and the tibia and fibula of the hind are fused into single bones 

 so that the foot cannot be rotated. The heel is lifted high off the ground, 

 adding the length of the foot to that of the leg proper; and most remark- 

 able of all, the foot is reduced to a single functional toe, on the toenail 

 of which (the hoof) the horse is perched. From the fossils it is evident 

 that this highly specialized leg and foot have evolved from the primitive 

 five-toed type found among the early ungulates. 



Fossil horses. More than a score of genera and a great many species 

 of fossil horses are now known, representing every age from lower Eocene 

 through Pleistocene time. Many of the species are represented by hun- 

 dreds or thousands of specimens. The genera form an almost unbroken 

 gradient of change, which makes them little more than groupings of 

 convenience; many of the species could equally well be placed among the 

 advanced members of one genus or among the primitive members of the 

 genus next higher in the scale. For full descriptive purposes it is customary 

 to recognize about ten stages in horse evolution, but we shall describe 

 only five. 



Eohippus, the dawn horse, is the first stage to be considered. The 

 earliest known ancestral horses were creatures the size of a cat or small 

 dog, which became numerous in the lower Eocene. There were many 

 species, all very similar and mostly members of the genus Eohippus. From 

 the structure of their teeth it is judged that they fed on leaves and 

 tender shoots, and they probably inhabited the undergrowth of the 

 Eocene forests. These dawn horses were slender-bodied, with pawlike 

 feet, but with tiny hoofs instead of claws on the toes. The front feet had 

 four toes, the hind feet three; the missing "thumb" of the forefoot was 

 gone without trace, but rudiments of the side toes were still present on 

 the hind foot. The radius and ulna and the tibia and fibula were still 

 separate bones. The skull was short, with the eye sockets situated at 

 about mid-length, and open behind. The molar teeth were small and low- 



