344 INSTINCT. 



portant as corporeal structures for the welfare of each 

 species, under its present conditions of life. Under 

 changed conditions of life, it is at least possible that slight 

 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 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 

 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 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 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 among extinct species, how 

 very generally gradations, leading to the most complex in- 

 stincts, can be discovered. Changes of instinct may some- 

 times be facilitated by the same species having.different in- 

 stincts 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 in- 

 stances of diversity of instinct in the same species can be 

 shown to occur in nature. 



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

 formably to my theory, the instinct of each species is good 



