STAGES IN THE EVOLUTION SERIES. The Ilyra eotherium (renamed Eohippus) of the Lower Eocene was a small animal, no larger than the domestic eat, with four complete toes on cacti 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 llyracotherium fore foot we tind 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 arched back, and the limbs of moderate length, were very little horse-like, recalling some modern carnivorous especially the civets,. The teeth, short crow nod. eovered with low rounded knob, of enamel, suggested those of monkeys and of pigs, lout 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 suceessive 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 hest-known series comes from the Tertiary 1:ad Lands of the Western Besides the main line of descent, which has led into the nvslern horses, asses. and zebras, there were also collab.ral branches. which have left no descendants. Only the more important stages can he mentioned here.
The successors to the ITyracotherium were the Protorohippns and Orohippus of the Middle Eocene. A complete skeleton of the former animal, from the Wind River Valley, Wyoming, is in the American Museum of Natural History in New York City. it is about the size of the kit-fox, and much like its predecessor, the Hyraeothe Hum, except that the splint representing the fifth digit in the hind foot has Ilisappeared.
In the Upper Eocene the Epihippus occurs, but enly fragmentary specimens have been found. These show that the middle toe was becoming more prominent, and the side toe,, the outer toe of the fore foot. were quite slender. Contemporary with this animal was the very much larger Paheothcrium of Europe, related to the nurses, but not in the direct line of de scent.
In the Oligocene is found Mesohippus, of which several complete skeleton, are known. vari ous species range in size from that of a red fox to a mastiff. There are but three toes in each foot, the outer digit cf the fore foot being now reduced to a splint-bone, and no longer appearing as a separate toe. The central toe in hoth for,.
and hind feet is much larger, and the side toes. although they still leach 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 '101)110(1,1M' type, the crests higher and shari.er than in the preced ing genera. This constitutes the neeessary inter mediate stage in the conversion of the low, round knobbed or 'hunodont' crown into the high. sharp ly ere sted crown with cement bracing. which characterizes the later horses. See TOOTH.
In the :Miocene is found Protohippus, in which the side toes, although -till 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 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 erown is greatly lengthened. the crests or ridges being higher and more eomplicated, 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 began, 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 I'lioluippus. of which very little is known. except 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 which it a little exceeded in size.
In the Pleistocene is found the modern genus Films. of larger size, with hut one toe on each foot.. the lateral digits represented by splint bones, and with teeth longer-erowned than those of Prot oh lupus. enabling the animal to grind hard grasses still more efficiently. Extinct species have been found in Europe. Asia, Africa. North and South America, as we have seen above.