Popular Science Monthly/Volume 52/February 1898/Feet and Hands II

1391663Popular Science Monthly Volume 52 February 1898 — Feet and Hands II1898




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

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 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 never found in any bird. The feathers of this creature grew out of the skin of the arm and of the third finger.

In the fowl (Fig. 5) and all other birds the wing is, as in the Archæopteryx, a feathered fore leg and foot, a, b, c, in Fig. 5, are the leg bones, and the rest represents the foot, which retains in part the three rays seen in the Archæopteryx. Two of these rays, however (1 and 3), instead of having several joints, are nothing more than short, pointed bones, u c and r c are the only bones of the "wrist" left, and mc ii and mc iii show all that remains of the bones

Fig. 4.
Fig. 3. Fig. 5.

which in a four-footed sole-walker form the flat of the foot. 1 is the remains of the first toe or digit, 2 is the long second digit, which still has three joints, and 3 is the one remaining joint of the third digit, which is not distinctly separated from the second. All the other parts of the lizardlike fore foot have disappeared, because this was quite enough to serve as a rigid support for the skin which carries the feathers.

The domestic fowl, of course, uses its wings very little; it walks more than it flies. Most birds use their wings much more, and some of these have more of the bones of the original foot left. Other birds fly still less than the fowl. The penguin, for example, uses its wings not for flying but for swimming, and the bones have become much flatter and broader than those of ordinary birds. The feathers that cover them are exceedingly small, almost more like scales than feathers, so that the whole wing is flat and paddlelike, and is very suitable for acting as an oar in the water.

In some of the ostriches, which also never fly, the bones which support the wing are reduced to mere stumps, even the second digit having only one joint.

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).

There is no doubt that the very earliest men spent their lives much as monkeys now do. They probably lived partly in trees before they learned how to build houses in Fig. 7. which they could take shelter from bad weather and when threatened by animals that were much stronger and could run far more swiftly than they. Thus our hands were once developed for climbing about trees, and since that time have changed very litle in shape. As man became civilized, his hands were put to nobler uses; they were his best instruments for accomplishing all he achieved. Our hands are thus much more flexible Fig. 7. and capable of a far greater variety of movements than the hands of the less civilized monkey.

The hind foot of the monkey was not well suited for man's way of life. One great difference between man and the monkeys is that man walks and runs in an upright position on his hind feet alone, while even the most manlike of the apes walk only partly upright, not supported entirely by their feet but helped along by their hands, which, on account of the great length of the arms, can easily reach the ground. In a man, walking upright, the weight of the body is thrown entirely on the hind limbs, which in him become the lower limbs, and the feet are needed to form a firm pedestal for the whole upright body. Thus in man, the once climbing feet having given up climbing, the useless thumb gradually changed back again into a simple great toe, and the first joints of the rays lengthened out to form a firm flat sole. We can still see some signs of the former thumblike condition of the great toe in little children and in some foreign races. In babies, the great toe is often more or less opposable to the others, and among the Japanese it is still so much so that a Japanese workwoman often holds the work she wants to stretch between her toes, which are able to grasp it firmly.

Men and women have, as a rule, five toes and five fingers, the largest number found in animals; but it is by no means certain that this number will always be retained. The foot of a civilized man is always covered by stockings and shoes, and the toes are hardly used at all, and, like the toes not used in the feet of animals, appear to be diminishing in size. The little toe in a comparatively large number of human beings already has only two joints, and some of the other toes also seem to be becoming shorter. It is thus quite possible that at some future time the human little toe will altogether disappear, and that man may have only four toes, or even, in course of time, fewer than four.