ess 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 forces, extremely slight modifications in
the structure or habits of one species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. 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
be still better adapted or improved; for in all countries, the natives
have been so far conquered by naturalised productions that they have
allowed some foreigners to take firm possession of the land. And as
foreigners have thus in every country beaten some of the natives, we
may safely conclude that the natives might have been modified with
advantage, so as to have better resisted the intruders.

As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far
as they are useful to any being. She can act on every internal organ,
on every shade of constitutional difference, on the whole machinery of
life. Man selects only for his own good; Nature only for that of the
being which she tends. Every selected character is fully exercised by
her, as is implied by the fact of their selection. Man keeps the natives
of many climates in the same country. He seldom exercises each selected
character in