IMEANUTC OF THE CIT ‘NGES TN FEET .'.Ni) TEETH. Along with the disappearanee of the side toes in the evolution of the horse there is a consider able increase in the proportionate length of the limbs, and especially of the lower part of the leg and foot. The surfaces of the joints, at first more or less of the ball-and-socket kind, permitting of free motion of the limb in all directions, become keeled and grooved like a pulley, thus permitting free motion forward and backward, but limiting the motion in all other directions, and increasing considerably the strength of the joint. By this means the foot is made more efficient for locomo tion over a smooth, regular surface, but less so over very rough ground, of little use for striking or grasping, or the varied purposes for which the feet of many-toed animals are used.
The increased length in the lower leg and foot increases the length of the stride without decreas ing its quickness, for the heavy muscles of the leg are chiefly in the upper part, and to increase the length of the lower part changes the centre of gravity of the limb very little, and it. consequent ly swings to and fro from the socket nearly as fast: for in an ordinary step the leg swings like a pendulum. and the speed of the swing is regu lated by the distance of the centre of gravity from the attachment, as that of a pendulum is by the height of the bob.
To increase the length of lower leg and foot will therefore give the animal greater speed: but it puts an increased strain on the ankle and toe joints. and these must be strengthened corre spondingly. by converting them from hall-and socket joints to `ginglymoid' or pulley joints. Additional strength. likewise at the expense of flexibility, is obtained also by the consolidation of the two bones of the forearm (ulna and radius) and leg (tibia and fibula) into one, the shaft of the lesser bone practically disappearing while its ends become fused solidly to its larger neighbor.
Corresponding with the increase in length of limb. it is necessary for a grazing animal that the l.ead and neck should increase in length in order to enable the mouth to reach the ground. So in the modern horse we find the neck and head much elongated when compared with the little Hyra eotherium. and this elongation has taken place at equal pace 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 accommodate itself to a rough, irregular surface, hut on smooth ground the yielding step entails a certain loss of speed. An illustration is afforded by the pneu matic 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. Similarly the hard, firm step from the single toe allows of more speed over a smooth surface, although compelling the animal to pick its way slowly and with care on rough, irregular ground.
The change in the character of the teeth from 'braehydont.' or short•erowned, to `hypsodont,' or long-erowned, enables the animal to subsist on the hard innutrit ions grasses of the dry plains, which require much more thorough mastication before they can he 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 adaptations to a life in a region of level. smooth, and open grassy plains, which are the natural habitat of the horse. The race, better fitted at first for a forest life, 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 in terior of the great continents. The great increase in size, which has occurred in almost all races of animals whose evolution we can trace, is de pendent on abundance of food. A larger animal, as may be shown on ordinary principles of me chanics. requires more food in proportion to its size than does a smaller one, in order to keep up a proper amount of activity. On the other hand, a larger animal is better able to defend itself against its enemies and rivals. Consequently, as long as food is abundant, the larger animals will have the advantage over their smaller breth ren, and by the laws of natural selection the race will tend to become continually larger until a limit is reached when sufficient food becomes difficult to obtain, and the animal is compelled to devote nearly all its time to getting enough to eat.