CHAP. VIII.] INSTINCT. 193 



modifications of corporeal structure arise from, and are increased 

 by, 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 are in many cases of subordinate 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 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 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 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 known amongst 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, &c. ; 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 

 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 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. Afterwards I allowed an ant to visit them, and it 

 immediately seemed, by its eager way of running about, to be 

 well aware what a rich flock it had discovered ; it then began to 

 play with its antennae on the abdomen first of one aphis and then 



