Page:Popular Science Monthly Volume 52.djvu/353

Rh out of the one bone (*), which remains as the representative of the "sole" of the foot. Ages ago, as is proved by fossil remains, the forefathers of the horse of to-day had three, four, or even five toes. At a, a, on each side of the great single bone two thin bones can be seen. These two bones are the splint bones, the only remains now to be found of the vanished toes.

All the animals we have so far mentioned have four feet more or less alike, all being used for the same purpose, that of running. But some animals came to use their fore feet for one purpose and their hind feet for another, and in consequence of this the fore feet came to be unlike the hind feet.

A striking example of such a difference in the use of the fore and hind feet, leading also to a difference in their structure, is found in the kangaroo, an animal which is seen wild only in Australia, where it hops or leaps over the open country, more or less upright, with extraordinary swiftness by means of its hind feet alone. When it rests, it sits on the long soles of its hind feet, steadied by its thick tail. Fig. 6 A represents the hind foot of a kangaroo in its ordinary position when leaping, and Fig. 6 B shows of what strangely changed bones it is composed. One ray has become very long and thick, and another, though not so long, is also fairly thick, but the other two are quite thin, as if they were dying away from not being used.

The fore feet of the kangaroo (Fig. 6 C) are never used as walking feet except when the animal is hobbling about slowly. Their chief work is grasping and tearing the leaves, grass, or fruit, or digging up the roots which form the food of the kangaroo. They are never needed to support the weight of the body, and so the toes are not large and thick, and four of the five toes are kept. In looking at the fore feet of the kangaroo one is tempted to call them "hands," for this very interesting difference in the use of fore and hind limbs in other animals, such as the monkeys, gave rise gradually to a true grasping hand.

In a second paper we shall deal with the extraordinary transformations of the fore feet into paddles and wings, found in whales, bats, and birds, and shall also see how true hands came to be developed in monkeys and in man.