522 CHORDATA. 
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 577.) 
We have seen that the same series can be traced in the teeth, the 
simple bilophodont teeth and nearly complete dentition (244%) of the 
Fig. 361.—TuHe Foot SKELETON OF THE HORSE AND 
Four OF ITS ANCESTORS. 
(From Marsn.) 
a b c 
Showing Gradual Reduction of Outer Toes and Increase of the Middle Toe. 
ua, Pachynolophus (Eocene); b, Anchitheriuim (Early Miocene); c, Anchitherium 
(Late Miocene); d, Hipparion (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 $444, 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 Miocene in India. 
Hipparion, as large as a donkey, and with three toes, is found in the 
