, and thus tried to complete the already
finished work. {209}

If we suppose any habitual action to become inherited--and I think it can
be shown that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at
all, he might truly be said to have done so instinctively. But it would be
the most serious error to suppose that the greater number of instincts have
been acquired by habit in one generation, and then transmitted by
inheritance to succeeding generations. It can be clearly shown that the
most wonderful instincts with which we are acquainted, namely, those of the
hive-bee and of many ants, could not possibly have been thus acquired.

It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight 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
accumulating variations of instinct to any extent that may be profitable.
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 {210} 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 tha