simile occurs in _Origin_, Ed. i. p. 67, not in the
later editions.

{231} <Note in the original.> In case like mistletoe, it may be
asked why not more species, no other species interferes; answer
almost sufficient, same causes which check the multiplication of
individuals.

But let the external conditions of a country change; if in a small
degree, the relative proportions of the inhabitants will in most cases
simply be slightly changed; but let the number of inhabitants be small,
as in an island{232}, and free access to it from other countries be
circumscribed; and let the change of condition continue progressing
(forming new stations); in such case the original inhabitants must cease
to be so perfectly adapted to the changed conditions as they originally
were. It has been shown that probably such changes of external
conditions would, from acting on the reproductive system, cause the
organization of the beings most affected to become, as under
domestication, plastic. Now can it be doubted from the struggle each
individual (or its parents) has to obtain subsistence that any minute
variation in structure, habits, or instincts, adapting that individual
better to the new conditions, would tell upon its vigour and health? In
the struggle it would have a better _chance_ of surviving, and those of
its offspring which inherited the variation, let it be ever so slight,
would have a better _chance_ to survive. Yearly more are bred than can
survive; the smallest grain in the balance, in the long run, must tell
on which death shall fall, and which shall survive{233}. Let this work
of selection, on the one hand, and death on the other, go on for a
thousand generations; who would pretend to affirm that it would produce
no effect, when we remember what in a few years Bakewell effected in
cattle and Western in sheep, by this identical principle of selection.

{232} See _Origin_, Ed. i. pp. 104, 292, vi. pp. 127, 429.

{233} Recognition of the importance of minute differences in the
struggle occurs in the Essay of 1842, p. 8 note 3.{Note 59}

To give an imaginary example, from changes in progress on an island, let
the organization{234} of a canine animal become slightly plastic, which
animal preyed chiefly on rabbits, but sometimes on hares; let these same
changes cause the number of rabbits very slowly to decrease and the
number of hares to increase; the effect of this would be that the fox