554 GENERAL CONSIDERATIONS 



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

 forelimbs 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. 

 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 (Fig. 382). 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 

 the modern camels are the development of water storage cavities in 

 connection with the stomach and the development of humps in which 

 fat is stored. 



A fourth evolutionary series is that represented by the anthropoid 

 apes and man, but this has been discussed in a previous chapter (Chap. 

 LX). 



