"How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? Can the principle of selection, which we have see is so potent in the hands of man, apply in nature? I think we shall see that it can act most effectually. . . .

"Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations?

"If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.

"This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection."  (130-31)

For an example of Natural Selection observed in 19th Century England, see The Case of the Peppered Moths. For an overview of natural selection, see Principles of Natural Selection.