Characters as Adaptive and Specific. 219 



directly affecting the colours of plumage when eaten 

 by birds. Therefore the presence of such materials 

 in the food-stuffs of birds occupying difTerent areas 

 may very well in many cases determine differences 

 of colouration, which are constant or stable so lonp- 

 as the conditions of their production are maintained. 



III. Sexual Selection. 



Passing on now to causes of specific change which 

 are internal, or comprised within the organisms 

 themselves, we may first consider the case of Sexual 

 Selection. 



Mr. Wallace rejects the theory of sexual selection 

 in toto, and therefore nothing that can be said under 

 this head would be held by him to be relevant. 

 Many naturalists, however, believe that Darwin was 

 right in the large generalization which he published 

 under this title ; and in so far as any one holds that 

 sexual selection is a true cause of specific modification, 

 he is obh'ged to believe that innumerable specific 

 characters — especially in birds and mammals — have 

 been produced without reference to utility (other, 

 of course, than utility for sexual purposes), and 

 therefore without reference to natural selection. This 

 is so obvious that I need not pause to dilate upon it. 

 One remark, however, may be useful. Mr. Wallace 

 is able to make a much more effective use of his 

 argument from " necessary instability " when he 

 brings it against the Darwinian doctrine of sexual 

 selection, than he does when he brings it against the 

 equally Darwinian doctrine of specific characters in 

 general not being all necessarily due to natural 



