GENERAL CHARACTERISTICS. 



153 



LEFT FORE-FOOT OF 

 A THREE-TOED 

 HORSE-LIKE 

 ANIMAL. 



appears to have been slided over those of the lower row towards the fifth toe, so 

 that the bone marked I largely overlaps the one lettered u; and it will be obvious 

 that this interlocking of the bones of the wrist produces a joint much more capable 

 of resisting strain than is that of the coryphodon. The hind-foot of the titanothere, 

 as the extinct Ungulate we are now considering is called, exhibits a still further 

 advance, having lost the fifth as well as the first toe, and thus being three-toed. 

 The living tapirs are in a precisely similar condition, being four- 

 tori 1 in front and three-toed behind; but the rhinoceroses have 

 advanced one step still further, having but three toes both in front 

 and behind. 



In the foot of the titanothere, while the bones of the meta- 

 carpus have become longer than in the coryphodon, the toe-bones 

 still remain as short as in the latter; and the same is the case 

 with the rhinoceroses. All these are, indeed, bulky animals, fitted 

 for dwelling in swampy localities, and not specially adapted for 

 speed. In another group, however, as shown in our third figure, 

 the toe-bones themselves have become elongated, while the meta- 

 carpal bones are still longer and more slender. In the feet repre- 

 sented in our third and fourth figures the middle or third toe is 

 very much larger than either of the others ; but whereas in the 

 one the fifth toe still remains, in the other it is represented only 

 by a rudiment of the upper end of its metacarjDal bone. This 

 type of foot leads on to that of the extinct three-toed horse, or 

 hipparion, of the Pliocene Tertiary, shown in our fifth figure, where 

 the two side-toes have become still smaller, and the last trace of 

 the fifth has disappeared. Finally, at the very top of the geological 

 series, we have the horse, where the only remaining toe is the third, 

 now very large ; the metacarpal bones of the second and fourth 

 toes I icing represented solely by the small splints on either side of bones of the left 

 the large metacarpal, now known as the cannon-bone. 



A complete transition has thus been traced from a five-toed 

 Ungulate, walking partly on the soles of its feet, to one provided 

 with but a single toe to each foot, and walking entirely upon the 

 very tip of that one toe, by which means the full extent of the 

 limb comes into play as an aid to speed. Throughout this series it 

 is the third or middle toe which has undergone development at the 

 expense of the others : and since this toe is always symmetrical 

 in itself, the term Odd-Toed Ungulates is applied to the members 

 of the group thus characterised. 

 odd-Toed and The resources of nature are, however, manifold, 



Even-Toed an d instead of this being the only line of evolution of 

 n a es ' the Ungulates, nearly similar results have been reached LEFT i. ore . KO ot of 

 by a totally different series of modifications. Starting once more the hiitarion. 

 from a foot somewhat similar to the one represented in the first 

 figure of this chapter, it will be found that instead of the third toe remaining 

 symmetrical in itself and gradually increasing in size at the expense of the others, 



FORE-F-OOT OF 

 A FOUR -TOED 

 HORSE- LIKE 



ANIMAL. 



