INSTINCT 349 



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

 effects of habit are in many cases of subordinate impor- 

 tance 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 

 among extinct species, how very generally gradations, 

 leading to the most complex instincts, can be discov- 

 ered. 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 pre- 

 served by natural selection. And such instances of di- 

 versity 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 



it 



