EVOLUTION OF THE HORSE 



.mil 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 

 ing 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- 

 m 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 

 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 accomn* dates 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 t< » subsist on the hard, comparatively innutritious 

 grasses of the dry plains, which require much more thorough 



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

 foods of the swamps and fore 



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

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

 plains which are now its natural habitat. At first t : 

 tted for a forest life, but it has 1 



completelv adapted to live and compete with its enemie 

 rivals under the conditions which prevail in the high dry plains 

 the interior of the ntinents. Tb e in 



. which has I in almost all r inimalswh 



lution we ran b ident on abundance of food. A 



•iimal. as mav be shown on ordinary princi] me- 



dia: food in proportion to it 



