Several implications for the understanding of the Darwinian revolution follow from an analysis of the role of colloquial language and prudential reason in Charles Darwin's quest for a theory of evolution. First, the term "natural selection" is not merely or even primarily a technical term and thus cannot be understood accurately apart from the circumstance for which it was designed. Second, an analysis of Darwin's inventional logic invites the expansion of the concept of scientific reason to include timing and a sense of the appropriate. Third, Darwin's rhetoric challenges the received notion that clarity or univocacy is the norm or goal of scientific language. Fourth, any attempt to distinguish Darwin's process of "discovery" from his process of "justification" on the ground that the former is private, subjective, and imaginative while the latter is public, objective, and falsifiable is itself falsifiable--for Darwin's reasoning cannot be accounted for by setting aside his language. Both before and after the "Origin," Darwin originated not so much a theory as a family of theories, each with distinct species and subspecies, and each in response to a particular occasion. (Fifty-five notes are included.) (RS)