pushed outward and flattened, it assumes the form of a feather, the ridge formed in the main furrow being the shaft, while the casts of the side grooves form the separate barbs of the vane. When all of the vane has been formed and pushed forward, the pyramid loses its grooves and becomes smooth, and the wall now formed on its surface, being of the same thickness in all parts, does not break, but remains tubular and forms the quill, which is attached to what is left of the pyramid. A finger-nail or a hair is formed from the same kind of scales in the same way, the process differing only in those features which give to each organ its special character. Feathers, scales, hair, claws, and nails, all are made alike from the dead, flattened cells crowded to the surface by the process of growth.
If, passing from the feather to the wing, we study that in the same way, we shall find that it is made, part for part, on the same plan as the arm of a man, the fore-leg of a horse, the fore-foot of a turtle or frog, and the fin of a fish; and, when these organs are compared in their earlier stages of growth, the resemblance is very perfect; and it is only as one becomes fitted for swimming, another for flying, another for running, and another for handling and feeling, that the differences between them begin to appear. Studying now the whole body of the bird in the same way, and comparing it with a mammal, as the horse; a reptile, as the turtle; a batrachian, as the frog and a fish we find that all these animals are constructed on the same general plan, and here, also, the resemblance is stronger in the earlier life of the animals. We find, however, that they do not all resemble each other in the same degree, for the bird is more like the turtle than like any of the others, and, when full grown, it preserves some resemblance to reptiles; and there is an animal, found only in the fossil state, called the archiopteryx, which unites in itself many of the characteristics of birds, such as the possession of feathers, with other characteristics as unmistakably reptilian.
Such are the principal facts to be learned about the wing, and any explanation of its origin must account for them all; and the same or similar facts may be learned by studying almost any organ or animal.
To recapitulate: they are, first, the wonderful adaptation of all parts for their uses, rendered still more wonderful by the second fact, that the parts so adapted are modified forms of what are called homologous organs, that is, organs having the same plan, but adapted to quite different uses, and having very little superficial resemblance; third, the fact that, when the growth of these homologous parts is compared, it is found that in their earlier stages they are very much alike, and differ so far as and at the same time that they acquire those characteristics that fit them for their special uses; fourth, is the fact that there are or have been animals whose structure has been so little modified that they seem to connect animals of very different but homologous structure.