THE HOLLOW-HORNED RUMINANTS 



743 



LIKE ANIMAL. 



row toward the fifth toe, so that the bone marked / 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 toed 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 LEFT FORE -FOOT 

 bones themselves have become elongated, while the metacarpal OF A THR EE- 

 bones are still longer and more slender. In the feet represented 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 metacarpal bone. This type of foot leads on 

 to that of the extinct three-toed horse, or hipparion, of the Plio- 

 cene Tertiary, shown in our fifth figure, where the two side toes 

 have become still smaller, and the last trace of the fifth has disap- 

 peared. 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 being BONES 

 represented solely by the small splints on either side of the large 

 metacarpal, now known as the canon 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 characterized. 



The resources of nature are, however, manifold, 



06 J" 1 an d instead of this being the only line of evolution 

 Even-Toed 



of the Ungulates, nearly similar results have been 



reached by a totally different series of modifications. 

 Starting once more from a foot somewhat similar to the one repre- 

 resented 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 

 LEFT FORE-FOOT 

 OF A FOUR-TOED 

 HORSE- LIKE AN- 

 IMAL. 



LEFT FORE -FOOT 



(")T? TTTP HTP* 



PARION. 



