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.