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 natives
have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being.
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