Page:Popular Science Monthly Volume 52.djvu/541

 in the water as a fish does to help to push him along. They did not use their hind limbs at all for this purpose, and these, like all unused parts of the body, diminished in size and strength. In the whales now existing no hind limbs are to be seen, but traces of the skeleton of the hind limbs are still found imbedded in the sides of the fishlike body. How small these remains of the former leg are in comparison with the constantly used front foot, now a paddle, may be seen from Fig. 2 A and B.

Again, ages ago, other land animals managed to raise themselves from the ground into the air, the better, no doubt, on the one hand, to escape from pursuing enemies, and, on the other, to catch flying insects. For supporting themselves in the air, wings of some sort were necessary, and so the front legs in such animals gradually altered.

Although none of these first flying animals are now in existence, we know pretty well what they were like, as impressions of their bodies have been left upon slatelike rocks. They were animals in many ways like reptiles—i. e., belonging to the same group as the crocodile, whose five-toed fore foot was illustrated in our first paper.

In some of these first fliers all the five toes were retained; four were of ordinary length, but the fifth was immensely long—longer, in fact, than the whole body of the animal—and between it and the body a skin was stretched, making a wing.

Here we have somewhat the same method of flying as we now find in the bat. In a bat's outspread wing (Fig. 3 A) the five rays of the fore foot are quite clear. Four of them are joined together by thin skin which stretches back to the leg and the tail. The lower part of the arm and three of the rays which carry the flying skin are greatly lengthened, and so a very large wing is obtained. The first ray, which we might call the thumb, remains short and ends in a strong claw that is of use in climbing.

We thus, in the bat, have an animal with the fore foot extraordinarily changed to suit it for a special manner of life—i. e., for flying. The hind foot (Fig. 3 B), not having any new kind of work to do, is much less changed. It has five ordinary toes ending in claws, with which the bat climbs or hangs on to trees. So well fitted are the hind feet for this kind of work that many bats always sleep wrapped up in their wings as in a mantle, hanging on to the branch of a tree, head downward.

But not all of the first flying reptiles developed wings of stretched skin. Some came to fly by means of feathered wings not unlike those of birds. Fig. 4 shows the bones of the fore foot or wing of a small creature called the Archæopteryx, or first feathered flier. This animal had three distinct toes, each ending in a claw. Two toes had thus already been lost, the fourth and fifth, which also are