EVOLUTION OJ? THE COLORS OF BIRDS. 119 



natural selection is based upon the assumption that cer- 

 tain characters are useful, but we are not justified in 

 arguing from this that all characters have been devel- 

 oped by natural selection and are therefore useful. 

 Darwin himself freely conceded the inutility of many 

 characters, and, as Mr. Eomanessays, " there is positive 

 evidence to show that the slight changes of form and 

 colour which chiefly serve to distinguish allied species 

 are often due to what Mr. Darwin calls ' the direct action 

 of external conditions,' such as changes of food, 

 climate, etc., as well as to mere independent variation 

 on isolated areas, and in some of our domesticated pro- 

 ductions, etc.; and in none of these cases do the specific 

 changes which result present a meaning of any kind." 

 In refuting Mr. Wallace's argument on the utility of 

 color Mr Romanes, in a footnote, quotes Darwin to the 

 effect that "each of the endless variations which we see 

 in the plumage of our fowls must have had some 

 efficient cause; and if the same causes were to act 

 uniformly during the long series of generations on many 

 individuals, all probably would be modified in the same 

 manner," and he adds: "The obvious truth of this 

 remark serves to dispose of Mr. Wallace's argument in 

 the Fortnightly, that ' the general constancy of colouration 

 we observe in each wild species,' of itself furnishes 

 sufficient proof that the colouration must be ' a useful 

 character.' Moreover, when using this argument Mr. 

 Wallace forgets that uniformity of colouration (whether 

 useful or unuseful) is preserved in wild species by free 

 intercrossing. Where this is prevented — as by isola- 

 tion or migration — variations of colour very frequently 

 do take place, just as in the thenanalgous case of our do- 

 mesticated strains." 



Concerning the swamping effects of intercrossing, Mr. 

 Romanes shows that the variations cited from Mr. 



