The case of the emu and the apteryx helps to throw light upon the problem thus disclosed. Where birds fly very little, their feathers never acquire or else soon lose the distinctive quill-like character; but where birds fly much, the quill-producing tendency becomes strong and pronounced. Primarily, this tendency ought to affect only those parts which are used in flight, namely, the wings and tail; and, as a matter of fact, we have seen that these are the parts which exhibit it in the highest degree. It would be almost impossible, however, that a change of such magnitude should be set up in some of the feathers, without to a lesser extent affecting all the rest. We might as well expect that the hair on a certain patch of some animal's skin would grow thick and spike-like, without any corresponding alteration in the rest of his body. True, natural selection does sometimes produce this result for some special purpose, when it is highly desirable that an acquired character should be confined to a small area. But, as a rule, when one part of the skin hardens, like that of a turtle or crocodile, the tendency to bony development shows itself in every part; and when certain hairs become converted into thick spines, like those of the hedgehog, the echidna, and the porcupine, a general bristly tone pervades almost all the coat. The scaly plates of the armadillo and the pangolin in like manner communicate a universal scaliness to the whole external surface of the animal. We may say in simple language that the body has got into the habit of producing certain structures, and that the habit extends to analogous parts in which it is not strictly necessary.
This is the case with the flying birds. Some of their feathers—modified scales or hairs—having become specially adapted for flying, all the rest follow suit to a greater or less extent. Indeed, we can hardly imagine how quills could come into existence at all, unless we allow that there must first have been an adventitious tendency toward the production of light-barbed shafts over the whole body. Those birds which exhibited this adventitious habit in the highest degree would become the ancestors of the aërial species, in whom it is still further developed by natural selection; while those birds which exhibited it in the least degree would become the ancestors of the diving, running, and scraping tribes, in whom natural selection favors rather such special adaptations as web-feet, fin-like wings, long and powerful legs, and ornamental plumage.[1]
The æsthetic philosopher, however (if the reader will permit me to
- ↑ Of course no effect, in nature is really accidental, that is to say, uncaused; but, in organic nature, effects which arise from special collocations of causes, unconnected with the previous habits of a plant or animal, may fairly be called adventitious. If they result in some alteration beneficial to the species, the alteration will be further strengthened by natural selection, and its final outcome will be a purposive structure—that is to say, a structure specially adapted to its peculiar function. But it must be remembered that almost all purposive structures were in their origin adventitious. I say "almost all" and not "all," because an exception must be made in favor of what Mr. Herbert Spencer calls "functionally-produced structures."
