When the wing is moved downward through the air, the feathers are pressed together; the air is confined in the concave surface of the wing, and the bird is raised up; but, when the wing is moved up to repeat the stroke, the air rushes down between the feathers. This is the paddling motion popularly supposed to be flight, but really only a small part of it. The air, being gaseous, does not remain passive under the descending wing, but tends to slide out, and as the front is unyielding, while the back is flexible, the air finds this exit, bending up the tips of the feathers, and sliding out backward and upward, while the feathers, and with them the bird, slide forward and downward. The bird can rotate its wings as well as flap them, and, by fixing them at such an angle that the fall occasioned by sliding shall just balance the lift given by the downward flap, it is able to move forward without rising or falling, although the motion of its wings is up and down; and, by changing the inclination of the wings a little, it can go up or down at the same time that it moves forward.
This is only an outline of what is known of the mechanism of flight—and many parts of the process are not yet understood—but we know enough to be able to appreciate the wonderful way in which every part, and every curve and outline, is adapted to its use; and the attention of thoughtful men has long been attracted by these and the countless similar adaptations in Nature, and many of the greatest thinkers have occupied themselves in attempts to understand the way in which they have been produced. Some have decided that adaptation implies design; and hence these adaptations must be the direct work of a personal designer and creator; but adaptation alone does not always imply design. I may go into the woods and find a young tree adapted for a cane, but no one will say that it was designed for a cane; and I once knew a very unskillful amateur carpenter, who was asked, at the close of a very industrious day's work, what he had made. He answered, "Well, I designed it to be a rustic chair, but I think it will answer nicely as a saw-buck." In this case the adaptation was certainly not the fruit of design.
But, even if design can be shown, it does not follow that the adaptation is the fruit of direct creative interposition; and the fact that it is not always perfect—that, perfect as the wing is in most birds, more than one species has become extinct in recent times, on account of the rudimentary and useless state of the wings—has been held by many to be sufficient proof that the adaptation was not produced in this way.
We shall be able to take a more fair view of this question after we have examined the ultimate nature—the homology, as it is called—of the organs of flight.
Feathers evidently take the place occupied by hair in mammals; and, in some birds which do not fly—such as the ostrich—they are very like hair; and examination of the microscopic structure and mode of growth of a feather shows that it is formed in the same way