EVOLUTION OF THE HORSE IN NATURE to =a ~~ always present in the earler stages, but progressively smaller, is now generally absent entirely. The crowns of the teeth are much longer, and of the two half-separated inner columns on the upper molars, one has disappeared, the other has increased in size and changed in form. The skull has lengthened and the animal is much larger. It is well represented among the fossil horses by the skeleton and skulls of Equus scottt already noticed and by skulls, jaws and teeth of other species from various localities. Fig. 18. Equus. Pleistocene. Upper teeth, one-half natural size. Very longcrownedteeth,heavilycemented 136. Hippidium, Onohippidium. PLEISTOCENE. SouTH AMERICA. The feet are like those of Equus, except that they are short and stout. The teeth are like those of Pliohippus, from which it is supposed to be descended. The skull is large and long with very long slender nasal bones. A cast of the skeleton of Hippidium presented by the Museo Nacional of Buenos Ayres, Argentine Republic, is on exhibition, also a skull cast of Onohippidium. CONCLUSIONS HESE are the principal stages in the evolution of the modern horse from the little four-toed Eohippus. Intermediate between them are numerous minor stages, the earlier species of each genus being more primitive, the later species more advanced. The series of upper molar teeth shown in Figs. 19-20 show no wide differences from one stage to the next. But between most of them several intermediate species are known. This gradual change is seen not merely in one or a few selected parts, but in every bone, every tooth, every portion of the skeleton. Elsewhere in the hall may be found similar although less complete series leading up from animals very close to Hohippus into the modern tapir and rhinoceros. The conclusion is unavoidable that horse, rhinoceros and tapir, three races widely different to-day, are derived through progressive changes from a common ancestral type. New species may have appeared suddenly, but the race in its broader lines is the product of gradual evolution, and diverse races may be traced back to a more ancient common stock.! Scientific criticisms of “Darwinism” are concerned with the way in which new species have appeared, whether by infinitesimal gradations or by finite “‘ mutations,”’ appreciable although usually small steps.
The broader lines of evolution are not affected by these criticisms.
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Smoll 4-Toed Horses OF ACE or MA Eocene Epoch | E ohippas-}--- - ----( Edrope and arth America.)
ep | | Orohippus - ; (South America)- ---*- \ ‘( North America, Asia, se i and North id — (WorthundSouthAmericaAsia,EuropeqndAfrica).Beae Fig. 16. Geological and geographical range of ancestors of the horse. The position and length of the heavy black lines show the occurrence and range of each genus in the successive geologic epochs, while the thickness of the lines indicates relative abundance. The dotted connecting lines with arrows indicate the genealogy MEANING OF THE CHANGE IN FEET AND TEETH LONG with the disappearance of the side toes in the evolution of the horse there is a considerable 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-andsocket kind which allows free motion of the limb in all directions, become keeled and grooved like a pulley-wheel, 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 locomotion over a smooth regular surface, but less so for traveling over very rough ground, and it becomes of little use for striking or grasping or the varied purposes for which the feet of polydacty] animals are used.
The increased length in the lower leg and foot increases the length of the stride without decreasing its quickness. 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. Consequently the leg swings to and from the socket nearly as fast as before, since in an ordinary step the action of the leg is like that of a pendulum, and the 26
EVOLUTION OF THE HORSE IN NATURE 24 speed of the swing is regulated by the distance of the centre of gravity from the point of attachment, as that of a pendulum is by the height of the bob. To increase the length of lower leg and foot therefore gives the animal greater speed; but it puts an increased strain on the ankles and cr toe-joints, and these must be strengthened correspondingly by converting them from ball-andsocket joints to ‘‘ginglymoid”’ or pulley joints. Additional strength, likewise at the expense of flexibility, is obtained by the consolidation of the two bones of the forearm (ulna and radius) and of the leg (= i (tibia and fibula) into one, the : fi y 7, % Shaft of the smaller bone pracv | ete tically disappearing, while its \" | > fff nds become fused solidly to | I . its larger neighbor.
: | The increase in length of limb renders it necessary for the grazing animal that the head and neck should FIG. 17. EVOLUTION OF THE FORE LEG. PRINCIPAL STAGES Equus, Merychippus, Mesohippus, Eohippus. In the four-toed horses (Eohippus) the radius and ulna are separate and their shafts of about equal size. In the earlier three-toed horses (Mesohippus) the ulna is separate but its shaft considerably reduced. In the later three-toed horses (Merychippus) the ulna is partly consolidated with the radius and its shaft is reduced to a thin thread. In the one-toed horses (Hquus) the ulna is more completely consolidated with the radius and its shaft has entirely disappeared
28 AMERICAN MUSEUM GUIDE LEAFLETS 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 Hyracotherium 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 vielding surface of the polydactyl (several-toed) foot is able to accommodate 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. Similarly, the Fig. 18. EVOLUTION OF THE HIND LEG. PRINCIPAL STAGES Equus,Merychippus,Mesohippus,Eohippus.
In the four-toed horses (Hohippus) the fibula is complete and separate from the tibia. In the earlier three-toed horse (MVesohippus) it is very slender, and fused with the tibia from about the middle of the shaft down. In the later three-toed horses (Merychippus) the shaft of the fibula is incomplete and both ends are fused with the tibia. In the one-toed horses (Hqi/us) the lower end is more thoroughly united so as to be indistinguishable from the tibia.
EVOLUTION OF THE HORSE IN NATURE 29 Fig. 20. EVOLUTION OF THE TEETH. LATER STAGES. Second Upper Molar (Read up). (8) ; 7 Merychippus, ¢ Middle Miocene; (9) Hipas parion, Upper Miocene; (10) L Kquus slenonis, Pliocene; (11) = Equus complicalus,—Pleistocene;(12)Equus caballus, the modern horse Fig. 19. EVOLU- TION OF THE TEETH. EARLIER STAGES. Second UpperMolar(Read up). (1) Hohippus, Lower Eocene; (2) Orohippus,Middle Eocene;(3)HEpihippus,UpperEocene; (4) Mesohippus montanensis, Lower Oligocene; (5) Mesohippusbairdi,Middle Oligocene; (6) Miohippus, Upper Oligocene; (7) Parahippus,LowerMiocene
30 AMERICAN MUSEUM GUIDE LEAFLETS 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 shortcrowned 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 adaptations to a life in a region of level, smooth and open grassy plains such as 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 evolution we can trace, is dependent on abundance of food. A large animal, as may be shown on ordinary principles of mechanics, requires more food in proportion to its size than does a small one, in order to keep up a proper amount of activity. On the other hand a large animal is better able than a small one to defend itself against its enemies and rivals. Consequently as long as food is abundant, the larger animals have the advantage over their smaller brethren, and by the laws of natural selection the race tends to become continually larger until a limit is reached, when sufficient food becomes difficult to obtain, the animal being compelled to devote nearly all its time to getting enough to eat.