210 THE ORIGIN OF SPECIES 



doubt it has been with instincts. But I believe that the effects of 

 habit are in many cases of subordinate importance to the effects 

 of the natural selection of what may be called spontaneous varia- 

 tions of instincts — that is of variations produced by the same 

 unknown causes which produce slight deviations of bodily struc- 

 ture. 



No complex instinct can possibly be produced through natural 

 selection, except by the slow and gradual accumulation of numer- 

 ous slight, yet profitable, variations. Hence, as in the case of cor- 

 poreal structures, we ought to find in nature, not the actual transi- 

 tional 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 allowance for 

 the instincts of animals having been but little observed, except in 

 Europe and North America, and for no instinct being known 

 among extinct species, how very generally 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 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 instinct in 

 the same species can be shown to occur in nature. 



Again, as in the case of corporeal structure, and conformably 

 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 instances of an animal ap- 

 parently 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 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 during several hours. After this 

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

 watched them for some time through a lens, but not one excreted; 

 I then tickled and stroked them with a hair in the same manner, 

 as well as I could, as the ants do with their antennae; but not one 

 excreted. Afterward, I allowed an ant to visit them, and it im- 

 mediately seemed, by its eager way of running about to be well 

 aware what a rich flock it had discovered; it then begun to play 



