HORSE. 230 HORSE. of each jaw; the crowns of these are very much elongated, so that they can be pushed ip in their sockets as fast as they wear away at the grinding surfai*. The grinding surface displays a com- plicated pattern caused by infoldings of the enamel of the tooth, the enamel edges being sup- ported on one side by the dentine of the tooth, on the other by a similar substance called cement, de|)0sited on the outer surface of the unworn enamel before the teeth are extruded from the gums. This arrangement secures at all stages of wear a series of hard enamel ridges projecting a little above the surface of the softer dentine and cement, and makes a remarkably ellicient grinder for the h.ard. dry grasses which are the natural food of the horse (q.v.). In the series of animals which lead up to the modern horse, ■we can trace every step in the evolution of these marked peculiarities of teeth and feet, from an ancestor so little suggesting the horse that when first found it was named by Richard Owen, the greatest comparative anatomist of his time, Hyracotherium, or "coney-like beast.' Its rela- tion to the horse was not at all suspected, and was recognized by Huxley and Marsh only when the series of intermediate stages between Hyra- cotherium and the mmlern horse was discovered. Tliis first ancestor of the horse line is verv much more like the contemporary ancestors of other lines of descent, and indicates how all the mod- ern quadrupeds have diverged from a single type, each becoming adapted to its especial mode of life. Stages in the Evolutiox Series. The Hyra- cotherium (renamed Eohippus) of the Lower Eocene was a small animal, no larger than the domestic cat, with four complete toes on each fore foot, and three on each hind foot. There is reason to believe that the still more ancient ancestors of this and all other mammals had five toes on each foot, and in the Hyracotherium fore foot we find a splint-bone representing the miss- ing first digit or thumb, while in its hind foot there is a splint-bone representing the missing outer or fifth digit, but here no trace is left of the innermost (first) digit. The proportions of the skull, the short neck and arclied back, and the limbs of moderate length, were very little hor.se-likc, recalling some modem carnivorous animals, especially the civets. The teeth, short- crowned, covere<I with low rounded knobs of enamel, suggested those of monkeys and of pigs, but not at all the long-crowned complicated grinders of the horse. Beginning with this small and primitive animal, eleven stages have been recognized from as many successive formations, showing the gradual evolution of the race into its modern form. Each stage is characteristic of its particular geological horizon. Some have been found in several parts of the world, but by far the most complete and best-known series comes from the Tertiary Bad Lands of the Western States. Besides the main line of descent, which has led into the modem horses, asses, and zebras. there were also collateral branches, which have left no descendants. Only the more important stages can be mentioned here. The successors to the Hyracotherium were the Protorohippus and Orohippus of the Middle Eocene. A complete skeleton of the former animal, from the Wind River Valley. Wyoming, is in the American ^luseum of Natural History in Xew York City. It is about the size of the kit-fox. and much like its predecessor, the Hyracothe- rium, except that the splint representing the fifth digit in the hind foot has <lisappeared. In the Upper Eocene the Epihippus occurs, but only fragmentary specimens have been found. These show that the middle toe was Iwcoming more prominent, and the side toes, especially the outer toe of the fore foot, were quite slender. Contemporary with this animal was the very much larger Paheotherium of Europe, related to the horses, but not in the direct line of de- scent. In the Oligocene is found Mesohippus, of which several complete skeletons are known. The vari- ous si)ecies range in size from that of a red fox to a mastiff. There arc but three toes in each foot, the outer digit of the fore foot being now reduced to a splint-bone, and no longer appearing as a separate toe. The central toe in both fore and hind feet is much larger, and the side toes, although they still reach the ground, are quite slender, and can support but a small part of the weight of the animal. The teeth are of the crested or 'lophodont' type, the crests higher and sharj)er than in the preced- ing genera. This constitutes the necessary inter- mediate stage in the conversion of the low. round- knobbed or 'bunodont' crown into the high, sharp- ly crested crown with cement bracing, which characterizes the later horses. See Tooth. In the Miocene is found Protohippus, in which the side toes, although still complete, are ex- tremely small and slender, and do not reach the ground. They can therefore no longer assist in supporting the weight of the animal, and are merely useless rudiments. Various species range from the size of a mastiff to that of a Shetland pony. The teeth in this animal are much more like those of the modern horse: the crown is greatly lengthened, the crests or ridges being higher and more complicated, and the 'valleys' between the ridges are filled up with a material {'cement') approximating the dentine in texture and hardness. A new and very effective method of grinding is thus begun, for when the sharp enamel crests wear down, they form a double ridge of enamel supported within by dentine and without by cement : the two latter are softer than the enamel and wear away more rapidly, leaving it as ,a sharp projecting ridge, continually re- newed with the wear of the tooth. The tooth is pushed up from the jaw as fast as it wears off on the grinding surface, so that it becomes an efficient grinder for those hard, siliceous grasses which would rapidly wear down .a tooth of the old pattern to a useless stump. In the Pliocene lived the Pliohippus. of which very little is known, exeejit that it was either one-toed or had the side toes reduced to extreme- ly small rudiments, and the teeth were much like those of Protohippus, which it a little e.xceeded in size. In the Pleistocene is fojind the modern genus Equus. of larger size, with but one foe on each foot, the lateral digits represented by splint- bones, and with teeth longer-crowned than those of Protohippus. enabling the animal to grind hard grasses still more efficiently. Extinct species have been found in Europe, Asi<a, Africa. North and South .merica. as we have seen .above. Meani:«o of the Cjianoes in Feet and Teeth. Along with the disappearance of the side foes in the evolution of the horse there is a consider-
