"PLEASED WITH A FEATHER:' 37 i 



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 pro- 

 nounced. 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 com- 

 municate a universal scaliness to the whole external surface of the ani- 

 mal. 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 aerial 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 spe- 

 cial adaptations as web-feet, fin-like wings, long and powerful legs, 

 and ornamental plumage.* 



The aesthetic 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." 



