The feet of Archæopteryx, it is important to remember, bear a very extraordinary likeness to the feet of a "perching" bird, say that of a crow. They are without any semblance of doubt, the feet of a bird which lived in trees. Archæopteryx, then, was an arboreal bird. And this being so, the most reasonable hypothesis of the origin of flight is that it developed out of "gliding" movements, made for the purpose of passing from the topmost branches of one tree to the lower branches of another, after the mode of the "flying-squirrels," and "flying-lemur" of to-day. The wing, at this primitive stage of its evolution, was even then, probably, a three-fingered limb, provided with a broad fringe of incipient feathers along its hinder border. At this stage the body would have been less bird-like than that of Archæopteryx, and have been still more like that of the ancestral reptilian stock from which the birds have sprung. That feathers are, so to speak, glorified reptilian scales cannot be certainly demonstrated, but men of Science are generally agreed that this was their origin. By the time that Archæopteryx had come into being, true flight had been arrived at, though probably it could not have been long sustained. As these primitive birds increased in numbers, and spread from the woodlands to the open country, life became more strenuous. New enemies had to be evaded, longer journeys had to be made for food. Only the very best performers on the wing could survive, and thus, in each generation, the failures would be speedily weeded out, while competition among the survivors would raise the standard. We see the result of this "struggle for existence" in the many and varied types of wings, and of flight, which are presented in this book. [Illustration: Archæopteryx. Pterodactyles.] CHAPTER III. The Sizes and Shapes of Wings and their relation to Flight. "... the fowls of heaven have wings, And blasts of heaven will aid their flight: * * * * * Chains tie us down by land and sea."--_Wordsworth._ The evasiveness of flight--The size of the wing in relation to that of the body--Noisy flight--"Muffled" flight--The swoop of the sparrow-hawk--The "flighting" of ducks--The autumn gatherings of starlings and swallows--"Soaring" flights of storks and vultures--The wonderful "sailing" feats of the albatross--The "soaring" of the skylark--The "plunging" flight of the gannet, tern, and kingfisher.