In the third place, we have to allow for the direct
and definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud-variations, such as the appearance of
a moss-rose on a common rose, or of a nectarine on a peach-tree, offer good
instances of spontaneous variations; but even in these cases, if we bear in
mind the power of a minute drop of poison in producing complex galls, we
ought not to feel too sure that the above variations are not the effect of
some local change in the nature of the sap, due to some change in the
conditions. There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently, it
is almost certain that all the individuals of the species would be
similarly modified.
In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to spontaneous
variability. But it is impossible to attribute to this cause the
innumerable structures which are so well adapted to the habits of life of
each species. I can no more believe in this than that the well-adapted
form of a race-horse or greyhound, which before the principle of selection
by man was well understood, excited so much surprise in the minds of the
older naturalists, can thus be explained.
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