The hind feet of birds are less changed than the fore feet, but still differ from the lizardlike foot in the number and character of their toes. Some birds have four toes, others only three, and these toes are usually able to curve so as to grasp the branches of trees, and are armed with claws which help in climbing. In the birds which run along the ground instead of flying, like the ostrich, there are usually only two toes, which become very much thickened. Here, as in the horse, we find increased speed in running obtained by reduction of the number of toes. The horse, as we saw, actually has only one toe, and in some ostriches the second toe is so reduced that they practically have only one also.
Great as is the difference between the fore and hind limbs of the bird, that between our own hands and feet is quite as important. The fore feet of all animals have sometimes been called "hands," to mark the fact that they correspond with what in a human being is the hand, but in the animals we have considered we have had nothing like a hand in the true sense of the word. In a true hand, the inner finger is able to move in such a way as to face the other fingers, and the hand is thus able to grasp any object far more firmly than if the movement of all the fingers was similar, as 
Fig. 6 it is in the foot. Such an inner finger is called an opposable thumb, and its presence is absolutely necessary to a real hand.
Let us see if we can understand how such true hands arose. Besides the animals which run about in various ways on the ground, those that have taken to living in water and those that have learned to fly in the air, there are others that live almost entirely in trees, climbing or springing from branch to branch, and only occasionally walking on the ground. These animals, the monkeys, which feed principally on the fruits that grow on the trees they inhabit, need agility in climbing rather than swiftness in running, both for obtaining their food and for escaping from their enemies, and so all their feet have become specially adapted for firmly grasping the branches of trees—i. e., they developed thumbs on all their feet; for this reason they have been called the Quadrumana, or four-handed animals.
If we examine the hand and foot of a gorilla, given in Fig. 6 A and B, we shall notice that the hand (A) differs very little from the hand of man, given in Fig. 7 A. The feet of the ape, however, differ considerably from those of man (cf. Fig. 6 B with Fig. 7 B).
