Footnote 91. See picture-heading of this chapter.

Footnote 92. Ichthyornis, fish-bird.

Footnote 93. Hesperornis. companions. To explain the facts we must go much farther back than this. If any one were to ask us whether the Australian colonists came from the white Americans or the Americans from the Australians, we should answer, “neither the one nor the other, and yet they are related, for both have sprung from the English race.” In the same way, when we see how like the ancient birds and reptiles were to each other, so that it is very difficult to say which were bird-like reptiles and which were reptile-like birds, we can only conclude that they, too, once branched off from some older race which had that bone between the jaws, that single neck joint, and the other characters which birds and reptiles have in common.

But long ago they must have gone off each on their own road, the reptiles filling the world for a time, flying and walking and swimming, till they found at last that creeping was their most successful way of life; the birds on the other hand becoming more and more masters of the air and the water, so that, while keeping the same bones and parts as the reptiles, they have grown into quite different beings in their form and habits, giving up the long-jointed tail of the Archœopteryx, or ancient-winged bird, for the compact feathered fan which helps to balance them in their flight, and the teeth of the water-birds for the sharp and horny beak, which, together with their claws, is their chief weapon of attack and defence now that they have employed their front limbs as wings.

Nor shall we have far to look for the secret of their success in life. Just as the reptiles have an advantage over the naked frogs and newts by having strong scaly coverings in their skin, so the birds have an advantage over the reptiles in that beautiful feathery plumage which covers their body, and the powerful muscles which work their limbs. For it is by means of these that they have been able to move quickly and travel far, and to develop that bright nervous intelligence which has grown more and more active as they have been carried into fresh scenes and experiences, overcoming new difficulties and enjoying new pleasures.

Remember for a moment how weak the lizard’s limbs are, so that his body always drags upon the ground; and then look at the bird’s tight grasp of the bough and the rod-like legs which raise his body above it. Watch him as he beats the air with his wings, rising and sinking, turning and swerving at will, and you will see that he has earned freedom, strength, and active life, by means of the strong muscles which move these legs and wings, and the feathers which provide him with an instrument for beating the air. Feel a sparrow’s fat little breast, or see how much meat comes off the wing and breast of a pigeon, and then, if you consider that all this flesh is muscle used for moving his wings, you will not wonder at his easy flight. For the muscles of a bird’s breast often weigh more than all his other muscles put together, and while one enormous mass of muscle in front of the breast works to pull down the wing, another smaller one, ending in a cord or tendon, passing like a pulley over the top of a bone (c, Fig. 33, p. 126), pulls it up, so that by using these, one after the other, the bird flies.

But where have the feathers come from,—those wonderful beautiful appendages, without which he could not fly? They are growths of the bird’s skin, of the same nature as the scales of reptiles, or those on the bird’s own feet and legs; and on some low birds such as the penguins they are so stiff and scale-like that it is often difficult to say where the scales end and the feathers begin. All feathers, even the most delicate, are made of horny matter, though it splits up into so many shreds as it grows that they look like the finest hair, and Dr. Gadow has reckoned that there must be fifty-four million branches and threads upon one good-sized eagle’s feather.

When these feathers first begin to grow they are like little grooved pimples upon the flesh, then soon these pimples sink in till a kind of cup is formed all round them, and into this cup the soft layer just under the outer skin sends out fibres, which afterwards form the pith. Round these fibres rings of horny matter form, and then within these rings, in the grooves of the soft pimple, the true feather is fashioned. First the tips of the feathery barbs, then the shaft, and then the quill appear, as the feather grows from below, fed by an artery running up into the pimple; till at last, when the whole is full-grown, the quill is drawn in at the base, and rests in its socket, complete.

Some of these feathers are weak and soft, with slender shafts and loose threads growing all round them, and these are the downy feathers which lie close to the body and keep the bird warm. Others, which cover the outside and form the wings and tail are flat, with strong quills and shafts, and a double set of barbs growing upon each shaft; and if you look at these wing feathers under a strong microscope you will see that they have a special arrangement for helping them to resist the air. For not only have all the little featherlets or barbs rows of other featherlets or barbules growing upon them, but these again are covered with fine horny threads, often hooked at the tip, which cling to the next barb, so that the feather is woven together as it were, in a close web, and if you strike it against the air you will find that it resists it strongly.

Now in a bird’s wing the feathers are so arranged that they lap one under the other from the outside of the wing to the body, so that when the bird strikes downwards they are firmly pressed together, and the whole wing, which is hollow like the bowl of a spoon, encloses a wingful of air, and as this is forced out behind, where the tips of the feathers are yielding and elastic, he is driven upwards and forwards. When, however, he lifts his wing again, the feathers turn edgeways and are separated, so that the air passes through them, and he still rises while preparing for the next stroke. All this goes on so rapidly that even the heron makes 300 strokes in a minute, and the wild duck 500, while in most birds they are so rapid that it is impossible to count them; yet all the while the little creature can direct his flight where he will, can pause and direct his wings to the breeze so as to soar, can swoop or hover, wheel or strike, guiding himself by the outspread tail and a thousand delicate turns of the wing.

All this complicated machinery, however, would not have served the bird much if his body had been as heavy, and his blood as cold, as those of the lizard and the crocodile. But here he has made a great step forward. In the first place, he has a heart with four chambers, two on the right side and two on the left; and while one of those on the right side receives the worn-out blood from the body and pumps it to the lungs to be refreshed, one of those on the left side receives it from the lungs when it is refreshed, and the other pumps it into the arteries to feed the body. So here we see for the first time among our backboned animals a creature whose good and bad blood are never mixed in the heart (compare pp. 23 and 76), but it gets all the benefit possible from its breathing, and the blood is kept fresh and pure.

Moreover, a bird’s lungs are large, and are continued into several large air-sacs, which in their turn open out into tubes which carry air actually into the bones, many of which are hollow instead of containing marrow like those of other animals.[94]

And now we begin to see how wonderfully these little creatures are fitted for flying. With all this air within them, not only is their blood kept hot by constant purifying, but their bodies are much lighter than if their bones were solid, and they can present a much broader surface to float upon the air without increasing equally in weight. Meanwhile, their feathery covering prevents the cold air around from chilling them, so that they are not only warm-blooded animals, but actually warmer-blooded even than ourselves.

Thus, then, Life has spread her feathered favourites over the world. For them there are no limits except the extreme depths of the water below, and the

Footnote 94. Some chamæleons and geckos also have air-tubes passing from the lungs into the body, and the crocodile’s skull is full of air-cells; but the two phenomena are not connected as in birds, and other parts of the skeleton or of the skin-covering, being heavy, have a counteracting effect.

height beyond the atmosphere above. Wherever air-breathing creatures can go, there some bird may be found. On the dizzy ledges of inaccessible cliffs, on the wide bosom of the open ocean, on the sandy wastes of the desert, in the tops of the highest trees, on the cloud-capped peaks of the mountains, diving or swimming, flying or soaring, running, perching, darting, or sailing for miles and miles without one moment’s rest, they find their way everywhere, and there is no spot from the icebound countries of the Arctic zone to the warm bright forests of the tropics where they do not penetrate; while their sharp eyes, kept free from dust and harm by a third eyelid moving rapidly sideways,[95] see far into the distance, and thus as they soar into the sky they have a power, possessed by no other animals, of overlooking a wide domain. Nor have they been obliged, like the reptiles, to take up strangely different forms to suit their various habits, for so wonderfully does their body meet all their wants that very slight changes, such as a broad body and webbed feet for the swimmers, long bare legs for the waders, a long hind toe for grasping in the perchers, and sharp claws and beak for the birds of prey, fit each one for his work, and are some of the chief distinctions we can find between them.

Even the heavy running birds, the Ostriches of Africa, the Rheas of South America, and the Emus and Cassowaries of Australia, still remain truly bird-like, though their wings are unfit for flight. True, their breastbones are flat instead of keel-shaped, for they have no need of strong muscles to move their wings,

Footnote 95. This third eyelid is a fold on the inner side of the eye; some reptiles and amphibians have it, and so have the marsupials and many of the higher animals.

which now serve only as sails to catch the wind as they run, and in many other ways they are an older type than our flying birds; but their wing bones are formed as if they were used for flying, and their feathers, though loose and downy because they have no little booklets, are like those of other birds.

Fig. 35.

The Ostrich[96] at full speed.

Strong powerful creatures they are, even in confinement. Yet how little can we picture to ourselves, when we see the Ostrich trotting round his paddock in the Zoological Gardens, with his wings outspread, what he is when he courses over the free desert!—

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