Darwin made extensive studies of the effect of artificial selection by plant and animal breeders, and himself became an avid "pigeon fancier." The experience showed him that it was possible to breed for a wide variety of body forms artificially, by intentionally selecting as breeding stock from among the small differences that arose in each generation. The analogy between Artificial and Natural Selection, in which the effect of the latter is differential survival and reproduction in the struggle for existence, forms a key part of his theory.

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