ld
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 case, every slight modification, which in the course
of ages chanced to arise, and which in any way favoured the individuals
of any of the species, by better adapting them to their altered
conditions, would tend to be preserved; and natural selection would thus
have free scope for the work of improvement.

We have reason to believe, as stated in the first chapter, that a change
in the conditions of life, by specially acting on the reproductive
system, causes or increases variability; and in the foregoing case the
conditions of life are supposed to have undergone a change, and this
would manifestly be favourable to natural selection, by giving a
better chance of profitable variations occurring; and unless profitable
variations do occur, natural selection can do nothing. Not that, as
I believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction
mere individual differences, so could Nature, but far more easily, from
having incomparably longer time at her disposal. Nor do I believe that
any great physical change, as of climate, or any unusual degree of
isolation to check immigration, is actually necessary to produce new
and unoccupied places for natural selection to fill up by modifying and
improving some of the varying inhabitants. For as all the inhabitants
of each country are struggling together with nicely balanced forces,
extremely slight modifications in the structure or habits of one
inhabitant would often give it an advantage over others; and still
further modifications of the same kind would often still further
increase the advantage. No country can be named in which all the native
inhabitants are now so perfectly adapted to each other and to the
physical conditions under which they live, that none of them could
anyhow be improved; for in all countries, the nati