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.