Origin’s Chapter IV: The Newton of the Blade of Grass

Charles Darwin, the leading evolutionist, introduces and discusses his key mechanism, natural selection, in Chapter IV of his On the Origin of Species (1859). He shows how the mechanism follows from the struggle for existence, together with random variation, and he argues that it not only explains change, but change in the direction of features of adaptive worth. He introduces the secondary mechanism of sexual selection and then, through examples, shows how selection might be expected to work in nature. He argues, based on the economic notion of the division of labor, that this shows how selection can lead to there being different species. He ends the chapter by showing how the overall result is a tree of nature, with the primitive at the bottom and the complex at the top.