GENERAL CHARACTERISTICS. 153 
appears to have been slided over those of the lower row towards the fifth toe, so 
that the bone marked / largely overlaps the one lettered w; 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, je9n orE-FooT OF 
the toe-bones themselves have become elongated, while the meta- A THREE-TOED 
carpal bones are still longer and more slender. In the feet repre- ae ame 
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 metacarpal 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 being represented solely by the small splints on either side of Bones or THE LEFT 
the large metacarpal, now known as the cannon-bone. Ap eee 
A complete transition has thus been traced from a five-toed HORSE-LIKE 
Ungulate, walking partly on the soles of its feet, to one provided ae 
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 and instead of this being the only line of evolution of 
Ungulates. 
ANIMAL, 

the Ungulates, nearly similar results have been reached pop porE-FOOT OF 
by a totally different series of modifications. Starting once more THE HIPPaRION, 
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, 
