We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the conditions the changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations.

Unless 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.