Popular Science Monthly/Volume 4/April 1874/A Feather
By W. K. BROOKS,
OF THE AGASSIZ MUSEUM.
ALTHOUGH not by a balloon, yet the Atlantic has been crossed in the air, and "what has been can be." There are enough well-authenticated cases of the occurrence of American wild birds on the west coast of Europe to prove that the trip can be made by birds, and it is probable that successful navigation of the air will be the fruit of careful study of that natural flying-machine, a bird's wing.
Every person who has not given more than a passing thought to the mechanism of flight is confident that he understands the whole subject, and tells you, if you ask, that the bird rows through the air with its wings, and that our lack of available force and of a sufficiently strong and light material is the only difficulty in the way of a successful flying-machine.
A very little study of a bird's wing and its action will show that it is not by any means simple, and that every part and every curve and angle has a use, and helps in the performance of the function of the whole, which function is not yet perfectly understood, but does not in the least resemble the action of a paddle or oar. We shall also learn that all attempts to construct flying-machines have been made with an utter disregard of every thing that a wing might have taught. To this sweeping assertion I know of only two exceptions; a boy's kite, and the little circle of cardboard which runs up the kite-string in such a mysterious way, bear a very slight resemblance to a wing, in their mode of action, and may contain the germ of a successful flying-machine.
To point out some of the facts already known about flying is one of the objects of this paper; another is to show how much there is to be learned about any natural object, and the way to set about it; for he who knows all that is to be learned about a wing has a good store of useful information, but he who knows all that may be learned from a wing is a wise man.
Let us examine a feather. When I say "examine a feather," I mean, let "every one take the trouble to pull a quill-feather from an old duster, or find an old quill-pen, or in some way get possession of an actual feather, to see for himself what I wish to show; for, if what I have to say is not worth this trouble, it is not worth reading at all.
Having found your feather, notice, first, the great strength of the shaft, compared with its lightness, and how secured by placing almost all the material on the outer wall of the quill. Notice, too, that the quill, where strength is most necessary, is tubular, while the rest of the shaft has a groove on its lower surface, and tapers toward the tip, so that it may be bent downward very easily by pressure near the tip, but does not bend so easily the other way. Notice, too, that the shaft is not straight, but bent so that the upper surface of the feather is convex, and the lower concave. We shall soon find a meaning in all these properties.
Passing now from the shaft to the vane, we see that the two sides are not alike: one edge, or vane—the one on the right-hand side of every feather in the right wing, that is, the side nearest the tip of the spread wing—is short and stiff, while the vane on the other side of the shaft is broad and flexible; and, by examining the feathers in their places in the wing, we see that the broad inside edge of the first or outside feather passes under the stiff, narrow outside edge of the next feather, which has its own inner edge supported in the same way by the third feather, and so on through the wing. When the wing is flapped downward through the air, the broad edge of each feather is pressed against the inflexible narrow edge of the feather next it, and the whole wing is thus made air-tight; but, when the wing is moved the other way, the broad edges have nothing to support them, and are pressed downward, so that the air can pass between the feathers.
The vane is also made up of separate pieces. If it is carefully examined, separate pieces, or "barbs," will be seen running off from each side of the shaft at a slight angle, and parallel to each other, united in such a way as to form two flat plates, the vanes. These barbs are fastened to each other quite firmly, but, if part of the vane is pulled down toward the quill, the barbs will separate at last with a tearing sound, and if this is repeated in a few places it will give the feather a very draggled appearance, and it will seem torn beyond possibility of restoration; but, if it be drawn gently between the fingers two or three times from base to tip, the broken places will unite so perfectly that it may be quite impossible to find them again. The working of the mechanism by which the attachment is made is so perfect that it need only be noticed to be admired, and careful examination will reveal the simple means by which it is accomplished.
Each barb, when examined with a lens, is seen to bear some resemblance to the whole feather; like the feather, it has a shaft running longitudinally, and a vane on each side of it. These vanes are unequal, as in the whole feather, and they are composed of separate pieces running off from the shaft, and called "barblets," because they are to the barb what the barbs are. to the whole feather. On the side of the barb toward the tip of the feather the barblets run out from the shaft of the barb nearly at right angles, and send off from their lower surfaces little hooks at regular intervals, all pointing downward; on the other side of the barb the barblets have no hooks, and, instead of being set at a large angle with the shaft, they are almost parallel with it, so that where they meet and run under the hooked barblets from the other side of the next barb they cross at right angles, and each hook falls directly over one of the straight barblets and fastens to it.
This is a very beautiful adaptation, but what is the use of it? Why could not the whole vane be made in one piece? It is commonly said that its purpose is the same as that accomplished by the overlapping of the feathers, to form valves which shall allow the air to pass in one direction but to resist its passage in the other. Unfortunately, the hooks are arranged in just the wrong way for this, for pressure from above tightens them, while pressure from below tends to loosen them, although they are too firmly fastened to be easily unfastened. The true use of the separate pieces seems to be to secure that all-important property, the greatest strength, by the least use of material; and it is done in precisely the way that men have employed for securing the same end.
A scantling of wood placed on its edge will support a much greater weight than one placed on its side; so, in laying a floor, instead of laying all the boards on their sides, which would not be strong enough, or placing them all on their edges, which would use too much wood, the plan is to place on edge a sufficient number, and on these to lay the floor of boards on their sides.
The method employed in the feather is still better than this: the shaft of each barb is flattened vertically, but, instead of a separate floor laid on these, the top of each rafter, or barb, is split, and these split portions are bent down and bound to each other by the hooks already spoken of, arching over the spaces between the barbs, in exactly the way that the arches of masonry span the spaces between the iron girders of a fire-proof floor.
Before we shall be prepared to understand the way the feathers act in flight, we must examine the way in which they are placed in the wing. The anterior edge of the wing is a firm rim of bone, and the quills are fastened to this rim, with the flexible end of each feather projecting backward with nothing but adjacent feathers to support it, so that the posterior border of the wing bends very easily. The feathers are of such a shape that the wing is convex on its upper surface, and concave below.
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 with hair, claws, or finger-nails, and is only modified skin. When a section through a piece of skin is examined with the aid of a moderately-powerful microscope, the lower or internal surface is seen to be made up of little, irregularly-rounded cells, or bags, with soft semi-fluid contents; and, while the animal is alive, new cells are constantly forming under the old ones, which are pushed outward and crowded together, and gradually lose their soft contents, and are flattened out into very small scales. The outer layer of the skin is made up of
these scales, which are fastened to each other firmly enough to be separated from the living layer below in a thin sheet, as happens when a blister is raised on the hand by unusual work, but in most parts of the body they are slowly rubbed off as new ones grow; but at the tips of the fingers they are so firmly united that they form horny plates, or "nails," which are pushed forward as new cells form at the root.
In the skin of a bird where a new feather is to grow there is a little pit, and, at the bottom of this, an elevation or pyramid; extending up one side of this pyramid is a groove, or furrow, deepest at the base, and gradually growing shallower until it disappears near the top; from each side of this furrow a great many smaller grooves extend around to the other side of the pyramid, and these also decrease in depth, and at last disappear just as they are about to meet on the side opposite the large furrow. The whole pyramid is covered with skin, and the surface is made of the same scales, or flattened cells, that are found over the rest of the surface of the body; but, instead of falling off when they are pushed out by the new ones below them, they become united or welded to each other, so as to form a horny coat over the surface of the pyramid, with ridges on its lower or inner surface, corresponding to the grooves on the pyramid; and, as new cells grow at the base, this coat or cast of the surface is pushed upward till it breaks at its thinnest part, which is, of course, the smooth part without ridges opposite the large furrow; and then, as it is pushed outward and flattened, it assumes the form of a feather, the ridge formed in the main furrow being the shaft, while the casts of the side grooves form the separate barbs of the vane. When all of the vane has been formed and pushed forward, the pyramid loses its grooves and becomes smooth, and the wall now formed on its surface, being of the same thickness in all parts, does not break, but remains tubular and forms the quill, which is attached to what is left of the pyramid. A finger-nail or a hair is formed from the same kind of scales in the same way, the process differing only in those features which give to each organ its special character. Feathers, scales, hair, claws, and nails, all are made alike from the dead, flattened cells crowded to the surface by the process of growth.
If, passing from the feather to the wing, we study that in the same way, we shall find that it is made, part for part, on the same plan as the arm of a man, the fore-leg of a horse, the fore-foot of a turtle or frog, and the fin of a fish; and, when these organs are compared in their earlier stages of growth, the resemblance is very perfect; and it is only as one becomes fitted for swimming, another for flying, another for running, and another for handling and feeling, that the differences between them begin to appear. Studying now the whole body of the bird in the same way, and comparing it with a mammal, as the horse; a reptile, as the turtle; a batrachian, as the frog and a fish we find that all these animals are constructed on the same general plan, and here, also, the resemblance is stronger in the earlier life of the animals. We find, however, that they do not all resemble each other in the same degree, for the bird is more like the turtle than like any of the others, and, when full grown, it preserves some resemblance to reptiles; and there is an animal, found only in the fossil state, called the archiopteryx, which unites in itself many of the characteristics of birds, such as the possession of feathers, with other characteristics as unmistakably reptilian.
Such are the principal facts to be learned about the wing, and any explanation of its origin must account for them all; and the same or similar facts may be learned by studying almost any organ or animal.
To recapitulate: they are, first, the wonderful adaptation of all parts for their uses, rendered still more wonderful by the second fact, that the parts so adapted are modified forms of what are called homologous organs, that is, organs having the same plan, but adapted to quite different uses, and having very little superficial resemblance; third, the fact that, when the growth of these homologous parts is compared, it is found that in their earlier stages they are very much alike, and differ so far as and at the same time that they acquire those characteristics that fit them for their special uses; fourth, is the fact that there are or have been animals whose structure has been so little modified that they seem to connect animals of very different but homologous structure.
Now, what is the meaning of these relations between organs and between animals? For that they have a meaning must be clear to all, and it is fair to presume that it is one that can be discovered by investigation.
The fact that two or many different animals are constructed on the same plan seems to indicate some kind of connection between the animals themselves, and it is the work of the zoologist to find what it is that thus connects them.
Two theories have been proposed, each of which seems to meet most of the points to be explained, but each seems to fail in some respects. One of these is, that the connection between different groups of animals is to be found only in the mind of their Creator; the other is, that there is a direct genetic connection or relationship between them.
Each of these theories is conceivable and worthy of consideration, for we can find examples of the building up of systems some-what resembling the animal kingdom in each of these ways. The various kinds of steam-engines, for instance, are adapted each to its special work, with an accuracy rivaling that of Nature, yet all of them can be shown to be constructed on substantially the same plan.
If we trace the history of any form, such as the steep-grade locomotive, we find, as we go backward, that it loses, one by one, all of its special adaptations, until at last it is only a common locomotive at up-hill work. Tracing the history of the locomotive in the same way, we find that its special adaptations disappear, until it is nothing but a power-engine placed on wheels—not the improved power-engine of the present day, but an unimproved and rudimentary form. We might trace the history of other forms in the same way, until we had found the one source for them all.
In this case there can be no genetic connection; each engine is an independent thing, and the only connection is an ideal one in the minds of the inventors; that is, the idea of the steam-engine has gone through a process of evolution, expansion, and perfection, and most of the steps in the process have been embodied in real engines, so that together they form a manifestation or record of the changes that the idea has undergone.
According to the theory of which Agassiz is the most celebrated advocate, the phenomena of life are to be explained in a somewhat similar way. Recognizing all the facts which seem to indicate the evolution of the animal kingdom, and being himself the discoverer of very many of them, he says that the evolution is simply the evolution of an idea in the mind of the Creator, which idea has been embodied in material form in such a way that it can be traced by the study of the animals that form its expression.
The other theory may also be illustrated by an example: When we compare languages which philologists tell us have descended from one parent tongue, we are attracted by their differences only, and it needs careful study and comparison to understand the similarity of plan which underlies them all; but when their history is traced it is seen that they were originally the same, and have become different as the races using them have become more widely separated, and, coming under new and widely different physical conditions, have diverged in their habits, feelings, thoughts, and associations, and have required different forms of speech to supply their need. Here, unlike the case of the steam-engines, the language has been the same all the time, and, although men have been the means by which the change has been effected, they have not been the intelligent cause, but have been unconsciously acted upon by agencies around them.
According to the theory with which Darwin is identified, although he is not by any means the author, but has simply removed some of the most serious objections, all the different forms of life have been evolved from one source in substantially the same way that languages have originated; as animals become exposed to new conditions, new varieties adapted to these conditions arise, and, as animals thus grow different, the parent unimproved forms are unable to struggle with their more perfect descendants and become extinct, so that the animals which would connect dissimilar forms are no longer in existence.
The evidence necessary for the perfect establishment of either of these theories does not seem to have been obtained as yet, and we can only decide provisionally, according to probabilities; but the discussion of the evidence already collected, or even a bare outline of it, would lead us far beyond the limits of this article, which is simply designed to show how much food for study even a feather will supply, and what broad questions it will lead us into.