264 ORIGIN OF SPECIES 



It will be universally admitted that instincts are as im- 

 portant as corporeal structures for the welfare of each spe- 

 cies, under its present conditions of life. Under changed con- 

 ditions of life, it is at least possible that sHght modifications 

 of instinct might be profitable to a species; and if it can be 

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

 difficulty in natural selection preserving and continually accu- 

 mulating variations of instinct to any extent that was profit- 

 able. 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 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 spon- 

 taneous variations of instincts; — that is of variations pro- 

 duced 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 accumula- 

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

 in the cases 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 be able to show that grada- 

 tions of some kind are possible ; and this we certainly can do. 

 I have been surprised to find, making allowance for the in- 

 stincts 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, lead- 

 ing 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, &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. 



