We must enter into the matter of wing a little more closely--it is so important a feature of bird organisation. The wing, of course, although it performs so different a _rôle_, is the exact equivalent of the fore limb of mammals. We can easily recognise precisely the same bones, though they are diminished in number, and often of a different form. It will be noticed that in each case we can distinguish the three bones forming the arm, and which are known as the humerus, the radius, and ulna. The rest of the limb in the bird is not quite so obviously like the hand of the mammal; but a little attention will show that it is constructed upon a perfectly similar plan. The flexible wrist of the mammal is made up of many small bones; the hand itself is made up of a larger series still, of which those nearest to the wrist are technically termed the metacarpals, and those which follow, the phalanges. In many mammals there are five fingers; but there are many which have less, and the extreme is reached in the horse, which has to put up with a single finger and small rudiments of two others. Now the bird is better off in the way of fingers than the horse, as it has three fairly well-developed fingers, or rather two well developed and one less perfect. The shortest finger corresponds to the thumb of our hand. It is more freely movable than the others. The metacarpal bones of the second and third fingers are firmly welded together, and are long; each finger (as will be seen from a look at fig. 1, p. 4) has one or two phalanges, as the case may be. Now in mammals the end phalanx of each finger is tipped with a nail, or with a hoof. The powerful claws of the tiger, used for tearing, and the solid hoof of the ox or horse, upon which the creature walks, are one and the same thing. It might be supposed that the hand of the bird, which is not an organ of offence or meant to walk with, might be shorn of these appendages. But this is not the case: every bird has at least two nails (fig. 9), of a long and rather claw-like form when well developed, and sometimes three nails, that is, one to each of its fingers. It looks, therefore, very much as if the wing of the bird had been formed out of a limb that was once an organ for climbing or walking with. There is a curious bird, found in British Guiana, which is known as the Hoatzin (figs. 9, 11). In the very young nestlings of the hoatzin the claws of the fingers are so conspicuous that they are actually used by the callow chick to climb with, before the feathers of the wings have grown sufficiently to enable them to use their wings in the proper way in which a bird should; it has been said also, that other birds scramble about and use their claws when they are young. In the case of the hoatzin, it is stated that the thumb and the first finger can be brought together so as to lay hold definitely of an object. A very important thing to notice about the wing bones is that they are