theory of the evolutionary (evolution) mechanism propounded by Charles Darwin as an explanation of organic change. It denotes Darwin's specific view that evolution is driven mainly by natural selection.

Beginning in 1837, Darwin proceeded to work on the now well-understood concept that evolution is essentially brought about by the interplay of three principles: (1) variation—a liberalizing factor, which Darwin did not attempt to explain, present in all forms of life; (2) heredity—the conservative force that transmits similar organic form from one generation to another; and (3) the struggle for existence—which determines the variations that will confer advantages in a given environment, thus altering species through a selective reproductive rate.

On the basis of newer knowledge, neo-Darwinism has superseded the earlier concept and purged it of Darwin's lingering attachment to the Lamarckian theory of inheritance of acquired characters. Present knowledge of the mechanisms of inheritance are such that modern scientists can distinguish more satisfactorily than Darwin between non-inheritable bodily variation and variation of a genuinely inheritable kind.