522 



CHORD AT A, 



limbs), the rhinoceros has three and the horse has one. Hence the 

 conchision is irresistible that the tapir, haunting the soft ground ol 

 forests, has remained at the four-toed stage ; the rhinoceros has pro- 

 gressed slightly further and given up its fifth toe ; and the horsC; 

 frequenting drier, harder ground and moving more rapidly, has lost all 

 but the third or middle toe. The same lesson is taught by the othei 

 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 (fftf) of the 



Fig. 361. — The Foot Skeleton of the Horse and 

 Four of its Ancestors. 



i» III III III ill 



Showing Gradual Reduction of Outer Toes and Increase of the Middle Toe. 



a, Pachynolo/>hus {'EjOCguq)', b, A nchithen'u 7n (J^^r\y 'M'loc&n^)', c, Anchitheriuni. 



(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 f^|f, 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 



