REPLY TO CRITICISM. 237 



"The advantage of divergence," the principle on which he relied to 

 account for divergent habits, producing divergent natural selection, 

 he never attempted to apply here ; and, above all, when he believed 

 the newer instincts to be either non-advantageous or disadvantageous 

 as contrasted with the older instincts, he certainly could not have 

 attributed advantage to the resulting divergence. As I have pointed 

 out on previous occasions, Darwin assumed a psychological divergence 

 in the sexual instincts of a species in order to account for the diver- 

 gence in their secondary sexual pharacters relating to form, color, etc. ; 

 and as there is no reason given why the psychological divergence 

 should take place, or why it should precede the change in form and 

 color, the theory of sexual selection, as presented by Darwin, is in- 

 complete, especially in its relations to divergent evolution. If he 

 had thrown light on the causes of divergence in sexual instincts, he 

 would have found the same or similar principles applicable to the 

 explanation of divergence of all kinds. But my object in referring 

 to his opinion here is to point out that he was free to admit that per- 

 manent divergence in sexual instincts may be non-advantageous, or 

 even somewhat disadvantageous ; and if this is true of sexual instincts, 

 I do not see why it may not be equally true of industrial instincts. I 

 think there is ample evidence that, when segregation has been estab- 

 lished, divergence which is neither advantageous nor disadvantageous 

 often arises in industrial as well as other instincts, and that these in- 

 stincts may introduce new forms of environal, sexual, or social selec- 

 tion. The relations which exist between habits and their objects are 

 in many species constantly varying in such a way as to constitute a 

 series of experiments; and when independent generation exists be- 

 tween different sections of a species, there is nothing to prevent diver- 

 gence in the results of those experiments in the different sections, even 

 when exposed to the same environment. 



In Darwin's "Posthumous Essay on Instinct," published as an 

 appendix to Romanes's "Mental Evolution in Animals," on pages 378 

 to 384, mention is made of certain "imperfections and mistakes of 

 instinct," and of certain instincts "that are carried to an injurious 

 excess," and of others that are "small and trifling." Of the last- 

 named he says : 



I have not rarely felt that small and trifling instincts were a greater difficulty in 

 our theory than those which have so justly excited the wonder of mankind ; for an 

 instinct, if really of no considerable importance in the struggle for life, could not 

 be modified or formed through natural selection. 



After mentioning several which might perhaps be considered tri- 

 fling but are really of great importance to the species, he alludes to a 



