322 INSTINCT. [Chap. VIIL 



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, varia- 

 tions. Hence, as in the case of corporeal structures, we 

 ought to find in nature, not the actual transitional gra- 

 dations by which each complex instinct has been ac- 

 quired — for these could be found only in the lineal an- 

 cestors of each species — ^but we ought to find in the col- 

 lateral 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 ex- 

 cept in Europe and North America, and for no instinct 

 being known amongst extinct species, how very gener- 

 ally gradations, leading to the most complex instincts, 

 can be discovered. Changes of instinct may sometimes 

 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 circum- 

 stances, &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 spe- 

 cies 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 



