. Elementary text-book of zoology. 522 CHORD AT A. limbs), the rhinoceros has three and the horse has one. Hence the conclusion is irresistible that the tapir, haunting the soft ground of forests, has remained at the four-toed stage ; the rhinoceros has pro- gressed slightly further and given up its fifth toe ; and the horse, frequenting drier, harder ground and moving more rapidly, has lost all but the third or middle toe. The same lesson is taught by the other series in which the third and fourth toes are of equal size. Here the pig has four toes, all touching the ground, though the second a
. Elementary text-book of zoology. 522 CHORD AT A. limbs), the rhinoceros has three and the horse has one. Hence the conclusion is irresistible that the tapir, haunting the soft ground of forests, has remained at the four-toed stage ; the rhinoceros has pro- gressed slightly further and given up its fifth toe ; and the horse, frequenting drier, harder ground and moving more rapidly, has lost all but the third or middle toe. The same lesson is taught by the other series in which the third and fourth toes are of equal size. Here the pig has four toes, all touching the ground, though the second and fifth are smaller and shorter than the others. The chevrotain and ox show a further reduction of these two toes, and the camel (in this respect the last of the series) has lost all trace of them and has only the third and fourth. (See Fig. 391, page ^11-) We have seen that the same series can be traced in the teeth, the simple bilophodont teeth and nearly complete dentition (frff) of the Fig. 361.—The Foot Skeleton of the Horse and Four of its Ancestors. (From Marsh.). Showing Gradual Reduction of Outer Toes and Increase of the Middle Toe. ;i, Pachynolophus i^oz^-a^\ b, ^«cAzM^r/K;« (Early Miocene) ; c, Anckitherium (Late Miocene); d, Hipfarian (Pliocene); e, Equus (Pleistocene). tapir leading through the rhinoceros to the horse, whilst the simple bunodont molars of the pig, with its full dentition of %\\%, leads through the chevrotains, with no upper incisors but still with canines, to the very specialised condition of the ox. A similar gradation can be made out in other structural features, such as the loss of fibula and ulna and fusion of tarsal bones. Thirdly, there is the direct evidence furnished by fossil forms. In the case of the horse and the ox the series is practically complete. We cannot do more here than merely enumerate the known ancestors of the horse. Fossil remains of the horse itself are found no further back than the Pliocene in Europe, or possibly the
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