IXSTIXCTS APPARENTLY DETRIMENTAL TO THE SPECIES. 277 



But of course the case is entirely altered where we appear to 

 meet with an instinct the operation of which is detrimental 

 to the individual, without being attended with any com- 

 pensating benefit to the species ; for in such a case the 

 detriment to the individual would also become a detriment 

 to the species. Such apparent cases, in fact, are precisely 

 analogous to those in which certain structures appear to be 

 detrimental to their possessors, without seeming to confer 

 any compensating benefit upon their species;* and, as Mr. 

 Darwin observes, such an apparent case, if it could be shown 

 to be a real one, would be incompatible with the theory of 

 natural selection, inasmuch as '' natural selection acts solely 

 by and for the good of each." further, as Mr. Darwin 

 adds, " if it could be proved that any part of the structure of 

 any one species had been formed for the exclusive good of 

 another, it would annihilate my theory ; " and it is obvious 

 that the same remark would equally apply to the case of 

 instincts. 



It is therefore of the utmost importance to take a survey 

 of all known instincts, in order to see whether there is any 

 one case, either of an instinct which is detrimental to the 

 species exhibiting it, or of one which has exclusive reference 

 to the benefit of other species. For, on the one hand, if 

 there is any one such case of an indisputable kind, we should 

 clearly have to modify our wdiole theory in order to meet it ; 

 while, on the other hand, if there is no such case, the fact of 

 all the innumerable multitude of animal instincts being of 

 obvious use to the species which manifest them, and never 

 of exclusive use to other species, must be taken as the 

 strongest possible evidence of the theory that ascribes all 

 instincts to the causes which we have assigned. 



I may as well say at once that there is only one apparent 

 case of an instinct in one species having exclusive reference 

 to the benefit of another, although there are cases of instincts 

 beneficial to the species presenting them being also beneficial 

 to other species. With the latter cases we are not, of course, 

 concerned. The former is the case of aphides yielding up 

 their secretion to ants, and has already been considered by 

 Mr. Darwin. His explanation is that, " as the excretion is 

 extremely viscid, it is no doubt a convenience to the aphides 



* See Origin of Species, 162-4, wliere the case of tlie rattle of the rattle- 

 snake, &c., is considered. 



