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.