- 322 INSTINCT. [Chap. VHL 



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 accu- 

 mulation 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 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. 



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

 formably 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 perform- 



