INSTINCT. " 229 



that was profitable. It is thus, as I believe, that all the 

 most complex and wonderful instincts have originated. As 

 modifications of corporeal structure arise from, and are in- 

 creased by, use or habit, and are diminished or lost by dis- 

 use, so I do not doubt it has been with instincts. But I 

 believe that the effects of habit are in many cases of sub- 

 ordinate importance to the effects of the natural selection 

 of what may be called spontaneous variations of instincts — 

 that is of variations produced by the same unknown causes 

 which produce slight deviations of bodily structure. 



No complex instinct can possibly be produced through 

 natural selection, except by the slow and gradual accumu- 

 lation 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 instinct 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 show 

 that gradations of some kind are possible ; and this we cer- 

 tainly can do. I have been surprised to find, making allow- 

 ance for the instincts of animals having been but little 

 observed, except in Europe and North America, and for no in- 

 stinct being known among extinct species, how very generally 

 gradations, leading to the most complex instincts, can be dis- 

 covered. 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 under different circumstances, etc. ; in which case 

 either the one or the other instinct might be preserved by 

 natural selection. And such instances of diversity of in- 

 stinct in the same species can be shown to occur in nature. 



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

 ably to my theory, the instinct of each species is good for 

 itself ; but has never, as far as we can judge, been produced 

 for the exclusive good of others. One of the strongest in- 

 stances of an animal apparently performing an action for 

 the sole good of another, with which I am acquainted, is 

 that of aphides voluntarily yielding, as was first observed by 

 Huber, their sweet excretion to ants ; that they do so volun- 

 tarily, the following facts show : I removed all the ants from 

 a group of about a dozen aphides on a dock-plant, and pre- 

 vented their attendance during several hours. After this 

 interval, I felt sure that the aphides would want to excrete* 



