Page:The New International Encyclopædia 1st ed. v. 10.djvu/267

HORSE. 231 HORSE. able increase in the proportionate length of the limbs, and especial!}- 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 iinib in all directions, become keeled and grooved like a puiley, 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 heaA-y 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 veiy 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 l>y 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 lie strengthened corre- spondingly, by converting them from ball-and- socket joints to 'ginglvmoid' 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 head 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- cotherium. and this elongation ha.s taken place at equal pace with the elongation of the less. 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 smootli ground. The soft yielding surface of the polydactyl foot is able to accommodate itself to a rough, irregiilar surface, but on smooth ground the yielding step entails a certain loss of speed. An illustration is afforded by the pneu- Tuatic 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 'brachydont.' or short-crowned, to 'hypsodont,' or long-crowned, enables the animal to subsist on the hard innutritions grasses of the iry 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. A]] these changes in the evolution of the horse are adaptations to a life in a recion of level, smooth, and open grasisy 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- jiendcnt on abundance of food. A larger animal, as may be shown on ordinary principles of me- clianics, 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 band, 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 'nave 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. Cause of the Evolutiox. The evolution of the horse, adapting it to live on the dry plains, probably went hand in hand with the evolution of the plains themselves. At the commencement of the Age of Mammals the western part of the North American continent was by no means so high above sea-level as now. great parts of it had but recently emerged, and the Gulf of ilesico .still stretched far up the valley of the Missis- sippi. The climate at that time was probably very moist, warm, and tropical, as is shown by the tropical forest trees found fossil even as far north as Greenland. Such a climate, with the low elevation of the land, would favor the growth of dense forests all over the country, and to such conditions of life the animals of the beginning of the mammalian period must have been adapted. During the Tertiary the continent was steadily rising above the ocean level, and at the same time other influences were at work to make the climate continually colder and drier. These conditions restricted and thinned the forests, and caused the appearance and extension of open grassy plains. The ancient forest inhabitants must then either retreat and disappear with the forests, or adapt themselves to the new conditions of life. The ancestors of the horse, adopting the latter course, changed with the changing condi- tions, and the race became finally — as we see it to-day — one of the most highly specialized of ani- mals in its adaptation to its peculiar environ- ment. At the end of the Age of Mammals the continents stood at a higher elevation than at present, and there was a broad land connection between Asia and North America, as well as those now existing. At this time the horses be- came cosmopolitan, and inhabited the plains of all the great continents, excepting Australia. It is a question whether the direct ancestry of the modern horse is to be searched for in Western America or in the little Icnown interior plains of Eastern Asia. It is also unknown why the vari- ous species which inhabited North and South America and Europe during the early part of the Age of Man should have become extinct, while those of Asia (horse and wild ass) and of Africa (wild ass and zebra) survive. Man since his appearance has played an important part in the extermination of the larger animals: but there is nothing to show how far he was responsible for