EVOLUTION OF THE HORSE 27 
radius) and of the leg (t#bta 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- 
cothertum 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 innutritious 
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 
