ysical change; for
instance, of climate. The proportional numbers of its inhabitants will
almost immediately undergo a change, and some species will probably
become extinct. We may conclude, from what we have seen of the intimate
and complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of the
inhabitants, independently of the change of climate itself, would
seriously affect the others. If the country were open on its borders,
new forms would certainly immigrate, and this would likewise seriously
disturb the relations of some of the former inhabitants. Let it be
remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up if some of the
original inhabitants were in some manner modified; for had the area been
open to immigration, these same places would have been seized on by
intruders. In such cases, slight modifications which in any way favored
the individuals of any species by better adapting them to their altered
conditions, would tend to be preserved; and Natural Selection would have
free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter, that
changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions have changed, and
this would manifestly be favorable to Natural Selection by affording a
better chance of the occurrence of profitable variations. Unless such
occur, Natural Selection can do nothing. Under the term of "variations,"
it must never be forgotten that mere individual differences are
included. As man can produce a great result with his domestic animals
and plants by adding up in any given direction individual differences,
so could Natural Selection, but far more easily from having incomparably
longer time for action. Nor do I believe that any great physical change,
as of climate, or any unusual degree of isolation to check immigration,
is necessary in order that new and unoccupied places should be left, for
Natural Selection to fill up by improving some of the varying
inhabitants. For as all the inhabitants of each country are struggling
together with nicely balanced for