EVOLUTION OF THE HORSE 



radius) and of the leg (tibia and fibula) into one, the shaft of the 

 smaller bone practically disappearing, while its ends become 

 fused solidly to its larger neighbor. 



The increase in length of limb renders it necessary for the 

 grazing animal that the head and neck should increase in length 

 in order to enable the mouth to reach the ground. An example 

 of these changes is the modern Horse, in which we find the neck 

 and head much elongated when compared with the little Hyra- 

 cotherium and this elongation has taken place pari passu with the 

 elongation of the legs. The reduction and disappearance of the 

 side toes and the concentration of the step on the single central 

 toe serve likewise to increase the speed over smooth ground. The 

 soft yielding surface of the polydactyl foot is able to accommo- 

 date itself to a rough irregular surface, but on smooth ground 

 the yielding step entails a certain loss of speed. A somewhat 

 similar case is seen in the pneumatic tire of a bicycle; a "soft" 

 tire accommodates itself to a rough road and makes easier riding, 

 but a "hard" tire is faster, especially on a smooth road. Simi- 

 larly, the hard, firm step from the single toe allows of more speed 

 over a smooth surface, although it compels the animal to pick its 

 way slowly and with care on rough, irregular ground. 



The change in the character of the teeth from "brachydont" 

 or short-crowned to "hypsodont" or long-crowned enables the 

 animal to subsist on the hard, comparatively innutritions 

 grasses of the dry plains, which require much more thorough 

 mastication before they can be of any use as food than do the 

 softer green foods of the swamps and forests. 



All these changes in the evolution of the Horse are adapta- 

 tions to a life in a region of the level, smooth and open grassy 

 plains which are now its natural habitat. At first the race was 

 better fitted for a forest life, but it has become more and more 

 completely adapted 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 evo- 

 lution we can trace, is dependent on abundance of food. A 

 large animal, as may be shown on ordinary principles of me- 

 chanics, requires more food in proportion to its size than does a 



