A FEATHER. 691 



pushed outward and flattened, it assumes the form of a feather, the 

 ridge formed in the main furrow being the shaft, Avhile 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 sur- 

 face, 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, flat- 

 tened 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 ani- 

 mals. 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 rep- 

 tiles ; 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 ho- 

 mologous 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. 



