Chap. VII. INSTINCT. 203 



that instincts do vary ever so little, then I can see no dilTiculty 

 in natural selection preserving^ and continually accumulating 

 variations of instinct to any extent that was profitable. It is 

 thus, as I believe, that all the most complex and wonderful in- 

 stiijcts have originated. As modifications of corporeal struct- 

 ure arise from, and arc increased b}'", use or habit, and are 

 diminished or lost by disuse, so I do not doubt it has been 

 with instincts. But I believe that the effects of habit arc of 

 quite subordinate importance to the effects of the natural selec- 

 tion of what may be called spontaneous variations of instincts; 

 tliat is, of variations produced by tlie same unknown causes 

 which produce slight deviations of bodily structure. 



No comj^lex instinct can possibly be produced througli nat- 

 ural selection, except by the slow and gradual accumulation 

 of numerous, slight, yet profitable, variations. Hence, as in 

 the case of corporeal structures, we ought to find in nature, not 

 the actual transitional gradations by which each complex in- 

 stinct has been acquired — for these could be found only in the 

 lineal ancestors of each species — but we ought to find in the 

 collateral lines of descent some evidence of such gradations ; 

 or we ought at least to be able to shoAV that gradations of 

 some kind are possible ; and this we certainly can do. I have 

 been surprised to find, making allowance for the instincts of 

 animals having been but little observed except in Europe and 

 North America, and for no instinct being- kno^vn among extinct 

 species, how very generall^'^ gradations, leading to the most 

 complex instincts, can be discovered. Changes of instinct may 

 sometimes be facilitated by the same species having different 

 instincts at different periods of life, or at different seasons of the 

 year, or when placed imder difi'erent circumstances, etc. ; in 

 which case cither the one or the other instinct might be pre- 

 served by natural selection. And such instances of diversity 

 of instinct in the same species can be shown to occur in nature. 



Again, as in the case of corporeal structure, and conform- 

 ably to my theorj-, the instinct of each species is good for it- 

 self, but has never, as far as we can judge, been produced for 

 the exclusive good of others. One of the strongest instances 

 of an animal apparently performing an action for the sole good 

 of another, witli which I am acquainted, is that of ajihides vol- 

 untarily yielding, as was first observed l)y Hu})er, their sweet 

 excretion to ants ; that they do so voluntarily, the following 

 facts show : I removed all the ants from a group of about a 

 dozen aphides on a dock-plant, and prevented their attendance 



