On the other hand the
struggle will often be severe between beings remote in the scale of nature.
The slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation 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.
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