Charles Darwin wrote _On the Origin of Species_ in a hurry. He had, it was true, been formulating his ideas and arguments for several decades—since his round-the-world _Beagle_ voyage of 1831-1836. These ideas and arguments had been slow to take definitive shape; Darwin had nurtured and reworked them, amassing evidence for what he projected to be a weighty magnum opus. Although he had shared his developing evolutionary speculations with his closest professional colleagues, Darwin was reluctant to publish them on several grounds. He was aware that his theory of evolution by natural selection (or descent with modification) was complex, that it rested on vast but not incontrovertible evidence, and that the chain of his reasoning was not uniformly strong. Further, his conclusions challenged not only the scientific assumptions of many fellow specialists but also the theological convictions of a much wider circle of fellow citizens.
In 1859, Darwin did not feel quite ready to expose his cherished theory to the harsh light of public scrutiny. In the introduction to the _Origin_ he confessed that although his work on evolution by natural selection was “nearly finished,” he would need “two or three more years to complete it.” The _ Origin_ was, he suggested, merely a stopgap, a schematic “abstract” of a much longer and more fully supported treatise yet to come. He had been moved to preview his labors in this way, he explained, because his health was “far from strong” and, perhaps more importantly, because Alfred Russel Wallace, a younger naturalist working in isolation in southeast Asia, had sent a paper to the Linnean Society of London in which he “arrived at almost exactly the same general conclusions that I have on the origin of species.” If Darwin had not gone public with his theory at this point, he would have risked losing credit for the work of many years.
As its reception showed immediately and has continued to show, the _Origin_ benefited from the succinctness imposed by circumstances. Darwin himself may have appreciated this point; at any rate, he never produced the massive treatise, although he repeatedly issued revised editions of the _ Origin._ But he did not abandon his intention to buttress his initial schematic presentation with additional evidence. In the course of the next two decades he published several full-length elaborations of topics summarily discussed in the _Origin: The Variation of Animals and Plants under Domestication; The Descent of Man, and Selection in Relation to Sex;_ and _The Expression of the Emotions in Man and Animals._ In addition to fleshing out the _Origin,_ these subsequent studies bolstered its arguments and responded to questions raised by critical readers, especially pragmatic questions about the way that descent with modification actually operated.