CHAPTER I.

Concerning Wings.

"Divinity within them breeding wings wherewith to scorn the earth."--_Milton._

What a wing is--The quill feathers and their function--The skeleton of the wing--The muscles of the wing--The great air-chambers of the body--The Bat's wing--The wing of flying Dragons--The wings of Dragon-flies and beetles.

The flight of birds has always aroused man's envy and stirred his imagination. David longed for the wings of a dove: the writer of the Book of Proverbs tells us that "the way of an eagle" surpasses his understanding. Icarus, spurred on by dire necessity, actually, we are told, contrived to fly--but his maiden effort ended in disaster! To-day we have, in a sense, succeeded where he failed. But only because we have given up the idea of flight by personal effort, and make our aerial journeys in a flying machine.

That we owe much of our success to a study of the flight of birds is common knowledge, but the machine which has evolved as a consequence of this study pursues its way through the air after a very different fashion from that of the birds, for its vast body is thrust, or drawn, through the air by means of a propeller, driven at incredible speed, its immobile wings sustaining the weight. The wings of the bird, on the other hand, not only lift the body from the earth, but they sustain it in the air by their marvellously complex movements. And this is true, in varying degrees of bird, and bat, and butterfly: of dragon-fly and beetle.

Even they who must perforce dwell in crowded cities see daily the miracle of flight performed. For even here sparrows and pigeons, at least, are everywhere, and it is just because this is so, just because they have become so "common-place," that their very presence escapes notice. Yet the wonder of their movements in the air might become a never-ending source of delight if only we went about our business with open eyes and minds alert.

Watch the wary sparrow spring from the ground and dart across the road, or up to the nearest house-top. How is it done with such incredible speed and accuracy?

To understand even the broad principles of flight, it is necessary to realize, at the very beginning, that the wing, in the case of the bird, or the bat, is a specially modified fore-leg. So also is the human arm and hand. But its transformation has not been so drastic as that of the bird, or the bat. Wherein the hand has been, as it were, completely re-modelled to fulfil the peculiar and complex functions demanded of it.

How should one describe the wing of a bird, as one sees it in flight?

The Dictionary, obscure and inaccurate as Dictionaries usually are, defines a wing as "the organ of a bird, or other animal, or insect, by which it flies--any side-piece." Might not the impression one gathers of a wing, during flight, be defined as of a lateral extension of the body, presenting a relatively large surface, but having no appreciable thickness? That surface, examined in a dead bird, is seen to be formed, for the most part, of a series of parallel, tapering, elastic rods, fringed with an innumerable series of smaller, similar, but much shorter rods, closely packed, and linked together by some invisible means to form an elastic web? These we call the "quill," or "flight-feathers." The rest of the wing, and the body itself, is clothed with precisely similar structures, differing only in their smaller size. We call them "feathers" commonly, without realizing that they are the "Hall-mark" of the bird, for no other creature has ever been similarly clothed.

These quill-feathers play such a tremendously important part in flight that their arrangement, and relation to the underlying skeleton must be carefully examined by all who would understand the flight of birds. To begin with, then, note that they are so arranged as to overlap one another, the free edges of the quills facing the outer edge of the wing. Only by this arrangement would flight be possible, for on the upstroke of the wing through the air the quills act like the shutters of the sails of a windmill, allowing the wind to pass between them and so relieving pressure on the uplifting wing-stroke. On the down-stroke, the opposite effect is produced. The full force of the stroke is conserved, because, owing to the overlap, the several feathers are now pressed closely together to form an impervious sheet.

How are they fixed to the skeleton? To see this all the smaller feathers and the muscles, or "flesh" of the wing must be removed. It will then be found that the flight-feathers are divisible into two series. One, widely spaced, runs along the upper surface of the fore-arm: the other, closely packed, along what answers to the back of the hand. In effect this is but a single rod of bone, but it is composed of three elements, answering to three of the digits of the human hand--the thumb and the first and second fingers. But they are scarcely recognizable as such, for the thumb is reduced to a mere stump, while the two fingers have become welded together. The third finger, indeed, has become reduced to the palm-bone, and a short stump answering to the first finger joint. To this frame-work, which can be folded up into the shape of a Z when the bird is at rest, the quills are fixed by their base by means of slender, but very strong elastic tendons. In birds which have a long upper arm bone, like the Albatross, Gull, or Heron, there is a third series of long, almost "quill-like" feathers running from the elbow to the body, thus closing up what would otherwise be a gap between the wing surface and the body, rendering flight impossible.

The most important muscles of the wing are those which have to provide the power for the down-stroke of the wing. And these are the "pectoral" or "breast-muscles"--which form such dainty meat in a roast fowl. Owing to their great bulk the breast-bone itself would be insufficient to afford them attachment. This is furnished by the development of a deep, median keel, so that the breast-bone of a bird, such as a pigeon, bears a fanciful resemblance, when seen in profile, to the hull of a ship--unusually shallow--with a very deep keel. The front end of the breast-bone supports two slender rods of bone, and these in their turn support the long, sword-like blade-bone, and the "merry-thought."

The general appearance of this frame-work for the support of the wing and its muscles can be seen in the adjoining illustrations. But it must be remembered that in their relative sizes and disposition these various parts present a very considerable range of differences. That these differences are correllated with different forms of flight goes without saying, but, be it noted, no one, as yet, has attempted to discover in what way they are related. Some of the readers of this book may, perhaps, be tempted to try and solve the problems which these differences present. To begin with, a collection of breast bones of different species of birds with their attached shoulder-girdles should be made, and these should be studied together with careful observations of the flight of the living bird. So far only a few comparisons of this kind have been made.

It must not be supposed that the whole secret of flight in birds is concentrated in the skeleton of the breast-bone and its shoulder-girdle, and the muscles attached thereto. But those who would investigate the modifications of the rest of the body which have taken place in harmony with the requirements of flight, must turn to more learned treatises. There is, however, one point which demands notice here. And this is the popular belief that birds have the power of materially reducing their weight when on the wing by drawing air into their lungs, and storing it in large air-chambers enclosed within the body. These chambers are indeed concerned with the needs of flight. But the precise part they play is yet to be discovered. They certainly have no effect of rendering the body lighter. So far as our knowledge goes it would seem that they act as regulators of the temperature and as reservoirs of breathing air, during the strenuous efforts of flight.

It is a mistake to suppose that it is unnecessary to consider other kinds of flight when studying that of birds. Even those who are not interested in the abstruse problems of the mechanism of bird's flight, will find that comparisons made between birds, bats, butterflies and beetles when on the wing, are immensely interesting, and help to bring out the peculiarities of each.

During the twilight hours of a still summer evening one may compare, with advantage, the rushing swoop of the screaming swift, borne with lightning speed upon long, ribbon-like pinions, with the curiously erratic flight of the woolly bat with beaded eyes, who has ventured abroad for his evening meal. One cannot but feel astonishment at the marvellous dexterity with which he twists and turns, now shooting up into the sky, now darting downward. What bird can beat him, or even match him, in the art of doubling back on his tracks? And one can put his skill at lightning turns to the test if one attempts to catch him in a butterfly net. Often indeed have I attempted this feat, but never yet with success.

In the glare of noon-day this aerial athlete may perhaps be found in a deep slumber, hanging head downwards behind the shutters of a cottage window, or in some crevice of a barn-roof. Gently seize him and as gently stretch out his wing. The moment one opens it one sees that it is constructed upon a totally different plan from that of a bird. In the first place a thin membrane, or fold of skin is seen to take the place of the series of quill-feathers found in the wing of the bird. In the second it will be found that this membrane is stretched between a series of long and very slender bony rods. These are excessively attenuated fingers. And if the hinder border of the wing-membrane be traced inwards it will be found to be attached to the hind limb. In some species it will be found that this membrane passes backwards beyond the leg to attach itself to the tail. Here, then, is a wing as efficient for its purpose as that of a bird, but constructed on a totally different plan.

Ages ago, before even the birds or the beasts had appeared on the earth, the winged dragons, which the Men of Science call Pterodactyles, held the proud position of being, not only the first, but the only creatures blessed with a backbone that could fly. Their wings resembled those of the bats, but differed in this, that instead of the wing-membrane being stretched between all the fingers, leaving only the thumb free, it was attached only to the fifth finger, leaving the remaining fingers free, and these were reduced to mere vestiges. As with the birds, the breast-bone was very broad and was furnished with a keel, while in the bats it takes the form of a jointed rod, down which no more than a slight keel is ever developed.

But millions of years before the Flying-dragons, birds, and bats came into being, the stupendous problem of flight had been solved. Far away in the distant Devonian Epoch, when the distribution of land and water over the earth's surface was totally different from that of to-day, dragon-flies and caddis-flies disported themselves in the summer sun, amid landscapes that would seem strange to our eyes. For there were no trees and flowering plants, such as we know.

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The dragon-flies of that remote epoch were very like those of to-day, whose dancing flights and graceful, swooping movements are such a delight to watch by reed-fringed pools, or river-banks, during the sweltering days of summer. This flight is very different from that of a bird, though it would be hard to say precisely in what it differs. But we have no such difficulty in regard to the broad outlines of the mechanism of such flight. To begin with there are two pairs of wings, and these appear to be fashioned out of some curiously gauze-like material, a sort of mesh-work tissue, often strikingly coloured. And they are obviously driven after a very different fashion from those of the bird. For in the bird they are moved by quivering muscles, attached to a bony, internal skeleton. In the dragon-fly--as with all insects--the hard skeleton, composed of a material known as "chitin," forms the outside of the body and encloses the muscles. Finally, for we may not dwell very long over this aspect of flight, it is clear that the wings cannot have been derived from modified fore-legs, like those of the bat, or the bird. Rather, it would seem, they have developed out of plate-like breathing organs.

The restful twilight hours of summer tempt not only bats from their hiding places, but a host of other winged creatures which are rarely to be seen, or heard, during the glare of noon. Among these is the lumbering dor-beetle, who, with lazy drone steers clear of solid objects only with difficulty. Many, indeed, are his failures. He and his kin are no match for bats and owls, who find them juicy morsels! On the next opportunity catch one and examine him. His wings are curiously interesting. There are the usual two pairs: but the fore-wings have been changed to serve as covers for the hind-wings. During flight they are spread outwards, and indirectly, no doubt, assist flight. But the hind-wings are the real propellers. And it will be noticed that when not in use they can be folded up in a perfectly wonderful manner, so as to lie completely underneath the fore-wings, or "elytra," so that when the creature is crawling it appears to be wingless.

Now compare these with the transparent wings of the bee, or the gorgeously scale-covered wings of the butterfly. It is well worth while. If this examination be done very carefully, and with the aid of a magnifying glass, it will be found that the fore and hind wings are yoked together in the wing of the bee, by a delicate mechanism of hooks. In the moths, but not in the butterflies, a bristle, or sometimes two or three bristles, serve the same purpose. Further, in the case of the bee it will be found that the fore-wing, when at rest, is folded longitudinally back upon itself.

Finally, turn to the flies. Herein it will be seen that there is but a single pair of wings, the hind pair having become reduced to mere stumps, known as "balancers."

Much, very much more, might have been said of these wings: but our conversation is of birds. We cannot, however, properly appreciate either the essential characters of their wings, or their flight, without some such standards of comparison as is afforded by the wings of other creatures.