Page - 264 - in The Origin of Species

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.