176 EVOLUTION AND GENETICS 



high crowned. The worn surfaces show a complex pattern of 

 dentine enclosed by enamel, which is in turn surrounded by 

 cement (Fig. 101). Such a structure presents the same funda- 

 mental characteristics as the teeth of the elephants, but in the 

 horse wear is compensated by the height of the crowns, while in 

 the elephants the teeth are used successively. Growth of horses' 

 teeth continues for a little more than five years. At the end of 

 this time all of the permanent teeth are in use. Subsequently the 

 teeth are pushed farther out of the jaws as the crowns are worn 

 down, and when of no further use, are shed (Fig. 102). The total 

 number of teeth which develop in the horse is forty, but canines 

 are not found in mares, and the first molar is vestigial or lacking. 

 Two incisors, three molariform premolars, and three molars are 

 normally produced in each half-jaw. 



Phylogeny of the Horses. The Direction of Change. The evo- 

 lutionary changes which are made evident by the known fossil 

 horses are summarized by Lull under the following heads: 



1. Increase in size. 



2. Lengthening of the limbs. 



3. Reduction of ulna and fibula, with a consequent limitation 

 of the range of movement. 



4. Change of the foot posture from plantigrade to unguligrade. 



5. Reduction and loss of digits from five to one. 



6. Perfection of the hoof. 



7. Perfection of the dental battery in elongation and complexity 

 of teeth. 



8. Premolars becoming molariform. 



Eohippus, the American genus, and Hyracotherium, the Euro- 

 pean genus, belong to the Eocene epoch and are the oldest known 

 ancestors of the horse. The connection of the two is un- 

 certain. Htjracotherium is more primitive, but no ancestral forms 

 are known to indicate the origin of the closely related forms. 

 While the European genus has left no known descendants, there is 

 a succession of genera from Eohippus to Equus in North America, 

 although the Hne at last became extinct in the Pleistocene, and 

 modern horses were introduced from Europe by the early ex- 

 plorers. From time to time ancient forms migrated from America 

 into other continents, where they either became extinct or gave 

 rise to the limited number of modern species of Equidae. 



