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Show . 384 DARWINISM CHAP. number of brae hornless rhinoceroses were developed they are founl in the Upper Miocene Pli~cene, and PostPliocene formations-and then became extmct. The true rhinoceroses have three toes on all the feet.1 The Pedigree of the Horse Tribe. Yet more remarkable is the evidence afforded by the ancestral forms of the horse tribe whic~ have ?een discov.e~·ed in the American tertiaries. The family Eqmdre, compnslllg the living horse, asses, and zebras, differ widely from all otl~ er mammals in the peculiar structure of the feet, all of "\VllH:h terminate in a single large toe forming the hoof. They ha"e forty teeth, the molars being formed of hard and s~ft m~tC1~1al in crescentic folds, so as to be a powerful agent m gnndm~ up hard grasses and othe.r ve~etable food. The forme~· peculiarities depend upon modificatiOns of the skeleton, whiCh have been thus described by Professor Huxley:- "Let us turn in the first place to the fore-limb. In most qua,drupeds, as in ourselves, the fore-arm contains eli ti~ ct bones, called the n1.dius and the ulna. The correspo11dmg region in the horse seems at first to posses~ ?ut ?ne. bon?. Careful observation, however, enables us to distmgmsh m tbts bone a part which clearly answers to t~c upper end of the ulna. This is closely united with the chief mass of the bo11e which represents the radius, and runs out into a slender shaft, which may be traced for some ~istance downwa1:ds upon the back of the radius, and then m most cases thms out <.w:I vanishes. It takes still more trouble to make sure of what IS nevertheless the fact, that a small part of the lower end of the bone of a horse's fore-arm, which is only distinct in a very young foal, is really the lower extremity of the ul~a.. . """\Vhat is commonly called the knee of a horse Is Its wn:-;t. The 'cannon bone' answers to the middle bone of the fi\ e metacarpal bones which support the palm of the hand in o\u' selves. The pastern, coronary, and coffin bones of v:etermarians answer to the joints of our middle fingers, while the hoof is simply a greatly enlarged and thickened nail. But if I From a paper by Messrs. Scott and Osborne, "On. ~he Origit~ ::ll!l Development of the Rhinoceros Group," read before the Bnttsh Assomat10n in 1883. XII£ THE GEOLOGICAL EVIDENCES OF EVOLUTION 385 what lies below the horse's 'knee' thus corresponds to the middle finger in ourselves, what has become of the four other fingers or digits ? ""\V c find in the places of the second and fourth digits only two slender splintlike bones, about twothirds as long as the cannon bone, which gradually taper to their lower ends and hoar no finger joints, or, as they are termed, phalanges. Sometimes, small bony or gri -tly nodules are to be found at the bases of these two metacarpal splints, and it is probable that these represent rudiments of the first and fifth toes. Thus, the part of the horse's . keleton which corresponds with that of the human hand, contains one overgrown middle digit, and at least two imperfect lateral digits ; and these answer, respectively, to the third, the second, and the fourth fingers in man. "Corresponding modifications are found in the hind. limb. In ourselves, and in most quadrupeds, the leg contains two distinct bones, a large bone, the tibia, and a smaller and more slender bone, the fibula. But, in the horse, the fibula seems, at first, to be reduced to its upper end ; a short slender bone united wjth the tibia, and ending in a point below, occupying its place. Examination of the lower end of a young foal's shin-bo11e, however, shows a distinct portion of osseous ma.ttcr which is the lower end of the fibula; so that the, apparently single, lower end of the shin-bone is really made up of the coalesced ends of the tibia and fibula, just as the, apparently single, lower end of the forearm bone is composed of the coalesced radius and ulna. "The heel of the horse is the part commonly known as the hock The hinder cannon bone auswcrs to the middle metatarsal bone of the human foot, the pastern, coronary, and collin bones, to the middle toe bones ; the hind hoof to the nail; as in the forefoot. And, as in the forefoot, there are merely two splints to represent the second and the fourth toes. Sometimes a rudiment of a fifth toe appears to be traceable. "The teeth of a horse arc not less peculiar than its limbs. The living engine, like all others, must be well stoked if it is to do its work; and the horse, if it is to make good its wear and tear, and to exert the enormous amount of force required for its propulsion, must be well and rapidly fed. To this end, 2 c |