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 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, 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. [Illustration: 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!--