30 



l.UA'A'/r.t.V .UrSKlM (HIDE LhAFLtrrs 



hard, firm step troiii the single toe allows of more speed overa smooth 

 surface, although it compels the animal to pick its way slowly and with 

 care on rou^h, irre«^ular j^round. 



The change in the chai'acter of the teeth from hrachydont or short- 

 ciowned to hypsodont or lon^-crowned. enables the animal to subsist on 

 the hard. c()mi)aratively innutritions grasses of the dry j)lains, which re- 

 (piire much more thorough mastication before they can Ix' of any use as 

 food than do the softei' tureen foods of the swamps and forests. 



Ail these changes in the evolution of the horse are adaptations to a 

 life in a region of level, smooth and open grassy plains such as are now its 

 natural habitat. At first the race was better fitted for a forest life, but it 

 has become more and more completely a(la])te(l to live and compete with 

 its enemies or rivals under the conditions which prevail in the high dry 

 plains of the interior of the great continents. The great increase in size, 

 which has occurred in almost all races of animals whose evolution we can 

 trace, is dependent on abundance of food. A large annual, as may be 

 shown on ordinary principles of mechanics, requires more food in propor- 

 tion to its size than does a small one, in order to keep up a proper amount 

 of activity. On the other hand a large animal is better able than a small 

 one to defend itself against its enemies and rivals. Consequently as long 

 as food is abundant, the larger animals have the advantage over their 

 smaller brethren, and by the laws of natural selection the race tends to 

 become continually larger until a limit is reached, when sufficient food 

 becomes difficult to obtain, the animal l^eing compelled to devote nearly 

 all its time to getting enough to eat. 



