Darwin’s Origin of Species(1859) argues for two big ideas, both metaphorically expressed: the tree of life and natural selection. New species descend from earlier, ancestral species; and these lines of descent with divergent modifications branch and rebranch, like the branches on a tree. So, if every line traces to one first species, all life forms one tree. Natural selection has been the main cause of these changes. By selective breeding, humans make, in a domesticated species, varieties fitted for different ends: heavy horses for plowing, fast ones for racing. In the wild, over eons, natural selective breeding due to the struggle for existence works unlimited changes in branching lines of adaptive, divergent descents, from fish ancestors fitted for swimming to bird descendants fitted for flying and mammals for running.

Darwin first had these ideas more than twenty years before publishing them in the Origin. In October 1836, the Beagle voyage ended. In July 1837, he opened his private Notebook B with a comprehensive account of the course and causes of life’s changes, including a first version of his tree of life. He has the idea of natural selection late in 1838, in Notebook E. The ideas may look like instant insights; but the story is not so simple. Any short telling of the origins of the Origin commits misleading omissions and condensations. However, even this very short one can counter two contrasting demands: from rationalists hoping for an edifying tale of universal methodological principles consistently yielding successful solutions to certain given problems specifiable in advance; and from romantics yearning for an epic saga of individual genius bringing imagination and intuition to transcendent reconfigurations of experience, man, nature, and so the whole world. (For documentation of what is said here about Darwin’s early theorizing, and for references to the secondary literature, see M. J. S. Hodge 2009b; for the texts of the notebooks, see Barrett et al. 1987. Becquemont 2009 is an important recent study.)