hanced 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 {83} 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. She
can act on every internal organ,