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Show 386 DARWINISM CIIAP. good cutting instruments and powerful a~d hsting crushers arc needful. Accordingly, the twelve cuttmg teeth. of a horse are close-set and concentrated in the forcpa.rt of Its mouth, like so many adzes or chisels. The ~rinders or molars ~rc large, and have an extremely complicated structure, bemg composed of a number of differ~nt. substances of unequal hardness. The consequence of th1s 1s that they wear ~way :~t different rates · a.nd hence, the surface of each gnnder Js always as uJte\')C il as 't hat of ~ goo.d m1'1 1 stone.." I . We thus see th::tt the Eqmdre differ very w1dely m structure from most other mammals. Assuming the truth of the theory of evolution we should expect to find traces among cxtin <;t animals of the steps by which this great modific::ttion has been effected; and we do really find traces of these step!';, imperfectly among European fossils, but far more completely ::tmona those of America. It 0 is a singular fact that, although no horse inhabited America when discovered by Europeans, yet abundance of remains of extinct horses have been found both in North and South America in Post-Tertiary and Upper Pliocene deposits; and from these an almost continuous series of modified forms ca.n be traced in the Tertiary formation, till we r each, at the very base of the series, a primitive form so unlike our perfected animal, that, had we not the intermediate links, few persons would believe that the. one wa. the a~1cestor of the other. The tracing out of th1s marvellous h1story we owe chiefly to Professor Marsh of Yale College, who has him.'clf discovered no less than thirty species of fossil Equidre ; aJH.l we will allow him to tell the story of the development of the horse from a humble progenitor in his own words. "The oldest representative of the horse at present knowll is the diminutive Eohippus from the Lower Eocene. l::leveral species have been found, all about the size of a fox. Like most of the early mammals, these ungulates had forty-fom teeth, the molars with short crowns and quite distinct in form from the premolar.. The ulna and fibula were entire ;mel distinct, and there were four well-developed toes and a nuliment of another on the forefeet, and three toes behind. In the structure of the feet and teeth, the Eohippus unmistnk- 1 American Addresses, pp. 73-76. xrrr THE GEOLOGICAL EVIDENCES OF EVOLUTION 387 ably indicates that the direct ancestral line to the modern horse has already separated from the other perissodactyles, or odd-toed ungulates. . "In the next higher division of the Eocene another genus, Orohippus, makes its appearance, replacing Eohippus, and showing a greater, though still distant, resemblance to the equine type. The rudimentary first digit of the forefoot has disappeared, and the last premolar has gone over to the molar series. Orohippus was but little larger than Eohippus, and in most other respects very similar. Several species have been found, but none occur later than the Upper Eocene. "Near the base of the Miocene, we find a third closely allied genus, Mesohippus, which is about as large as a sheep, and one stage nearer the horse. There are only three toes and a rudimentary splint on the forefeet, and three toes behind. Two of the premolar teeth are quite like the molars. The ulna is no longer distinct or the fibula entire, and other characters show clearly that the transition is advancing. " In the Upper Miocene Meso hippus is not found, but in its place a fourth form, Miohippus, continues the line. This genus is near the Anchitherium of Europe, but presents several important differences. The three toes in each foot are more nearly of a size, and a rudiment of the fifth metacarpal bone is retained. All the known species of this genus are larger than those of Mesohippus, and none of them pass above the Miocene formation. "The genus Protohippus of the Lower Pliocene is yet more equine, and some of its species equalled the ass in size. There are still three toes on each foot, but only the middle one, -corresponding to the single toe of the horse, comes to the ground. This genus resembles most nearly the Hipparion of Europe. "In the Pliocene we have the last stage of the series before reaching the horse, in the genus Pliohippus, which has lost the small hooflets, and in other respects is very equine. Only in the Upper Pliocene does the true Equus appear and complete the genealogy of the horse, which in the Post-Tertiary roamed over the whole of North and South America, and soon after became extinct. This occurred long before the discovery of the continent by Europeans, and no satisfactory |