n in
however slight a degree to the surrounding physical conditions, will, in
the long run, turn the balance.

With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with
their conditions of life, will generally leave most progeny. But success
will often depend on the males having special weapons or means of
defence or charms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication.
And if there has been any variability under nature, it would be an
unaccountable fact if natural selection had not come into play. It has
often been asserted, but the assertion is incapable of proof, that the
amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously,
can produce within a short period a great result by adding up mere
individual differences in his domestic productions; and every one
admits that species present individual differences. But, besides such
differences, all naturalists admit that natural varieties exist,
which are considered sufficiently distinct to be worthy of record
in systematic works. No one has drawn any clear distinction between
individual differences and slight varieties; or between more plainly
marked varieties and subspecies and species. On separate continents, and
on different parts of the same continent, when divided by barriers of
any kind, and on outlying islands, what a multitude of forms exist,
which some experienced naturalists rank as varieties, others as
geographical races or sub species, and others as distinct, though
closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or
slowly, why should not variations or individual differences, which are
in any way beneficial, be preserved and accumulated through natural
selection, or the survival of the fittest? If man can by patience select
variations useful to him, why, under changing and complex conditions
of life, should not variations useful to nature's living products often
arise, and be preserved or selected? What limit can be put to this
power, acting during long ages and rigidly scrutini