Page:Popular Science Monthly Volume 52.djvu/540

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IN our last paper we described the feet of some of the chief groups of four-footed animals. We saw that in most of these animals the four feet are very much alike, because they all have the same kind of work to do—that is, walking or running. But when, in consequence of its manner of life, an animal comes to use its fore feet differently from its hind feet, as the kangaroo does, we find that a difference arises in the structure of the two. We now have to trace some of the more marked changes of this kind.

So far, all the animals we have mentioned have been land animals, all needing their feet for moving in one way or another on the ground. But, ages ago, some land animals changed their manner of life and took to living chiefly in the water, only occasionally coming on to the land, or even merely coming to the surface of the water when they were obliged to breathe, for they still had lungs, which needed to breathe air. To such animals (for example, seals and whales) ordinary feet were partially or altogether useless; they had chiefly to paddle or swim, seldom or never to walk, and, to enable them to hold their own against their water enemies, their limbs gradually became very much changed.

Taking first the seal, which even now climbs on to the land, we see all the four feet changed into paddles, but these paddles, being still occasionally needed for use on land, are not very unlike the feet of an ordinary land animal. In Fig. 1 we see that the fore foot of the seal (A) bears a considerable resemblance to his hind foot (B); there is some difference in length and in the position

PSM V52 D540 Seal and whale limbs.png
Fig. 1.Fig. 2.

on the body, for the hind limbs face each other at the back of the body, but all four are paddles.

Taking next the whale, which has entirely given up coming on to the land, we find only one pair of paddles or changed feet. When the land ancestors of the whale took to living in the water, they probably had tails, which, by whipping the water, helped them to swim, and they evidently learned to wave the hinder part of the body