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 of quite
subordinate importance to the effects of the natural selection of what
may be called accidental 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, 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.
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