INTENSIVE SEGBEttA-TION. 375 



sexual, or social selection. The relations whicli 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 between different sections of a 

 species, there is nothing to prevent divergence 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 Eomanes's ' Mental Evolution in Animals,' on 

 pages 378-384 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 difiiculty 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 selec- 

 tion." After mentioning several which might perhaps be con- 

 sidered trifling but are really of great importance to the species, 

 he alludes to a few that seem to be " mere tricks " or " habits 

 without use to the animals." Mr. Eomanes, referring to these 

 cases, offers the following explanation on p. 275 of the same work 

 (I quote from the New York edition, Appleton & Co., 1884) : — 

 " We have seen abundant evidence that non-adaptive habits 

 occur in individuals, and may be inherited in the race. There- 

 fore, if from play, affection, curiosity, or even mere caprice, the 

 animal should perform any useless kind of action habitually . . . . , 

 and if this habit were to become hereditary in the similarly con- 

 stituted progeny, we should have a trivial or useless instinct." As 

 an example of a strongly inherited non-adaptive instinct in a 

 wild creature may be mentioned the cackling of the wild hen of 

 India after having laid an egg. This habit is referred to by 

 Darwin as one that may be slightly detrimental ; but all that is 

 necessary to put it beyond the developing influence of natural 

 selection is that it should fail of bringing advantage to the 

 species ; and that it is of no advantage will, I think, be generally 

 admitted. If, then, species differ in regard to instincts that are 

 non-advantageous, they are liable to present non-advantageous 

 differences in form and colour, resulting either from the same 

 causes that have produced the divergent instincts, or from diver- 

 gent forms of natural, sexual, and social selection produced by 



LINN. JOTTEN. ZOOLOUT, VOL. XXIII. 25 



