EVOLUTION OF ANIMALS 525 



developed. The length of the tusks possessed by the modern elephants 

 is far exceeded, however, by the tusks of some of the extinct types. This 

 is perhaps an example of excessive specialization. The skull has become 

 very high, a fact which makes it easier for the animal to carry the weight 

 of the large tusks. The molar teeth have become greatly developed and 

 reduced in number, not more than eight being functional at the same time. 

 These teeth, as in all herbivorous animals, are fitted for grinding the food. 

 The most ancient horses, that from Europe known as Hyracotherium 

 and that from western North America as Eohippus, also belong to the 

 Eocene period. Eohippus (Fig. 321) was an animal about the size of a 

 small dog, standing only about 12 inches high. The head was elongated, 

 the legs and neck only moderately long, and there were four digits on the 

 forehmbs and three on the hind ones. There was little resemblance 

 to a modern horse but the geological record fills in the gaps in the series. 



Fig. 321. — A restoration of Eohippus. {From a model prepared by the American Museum, 



of Natural History, with permission.) 



In the intermediate forms, which lived on open plains, is shown a gradual 

 reduction in the number of toes leading to the one highly developed middle 

 toe which the modern horse possesses. There has also been an increase 

 in size and particularly an increase in length of the limbs and the neck, 

 fitting the animal for speed and for reaching the ground when grazing. 

 The teeth have also become larger and better fitted for grinding and the 

 premolars have become like the molars. 



The camels present a third series which parallels that of the horses 

 and apparently illustrates adaptation to similar conditions. The 

 Eocene Protylopus, which was about the size of a jack rabbit, had 

 two accessory toes as well as the two toes which exist in present-day 

 types. Its home was in North America. Evolution in this group has 

 been accompanied by reduction in the number of toes; increased length of 

 the limbs, particularly those parts of the limbs representing the hand and 

 foot; an increase in size of both the animal and its skull; and modification 

 of the teeth for grinding. Other adaptations which are prominent in 



