Page:Popular Science Monthly Volume 52.djvu/542

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POPULAR SCIENCE MONTHLY.

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

PSM V52 D542 Evolution of limbs into wings.png
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.