However strongly we may see scientific ideas as socially and culturally contingent in their origin and expression, we must acknowledge that they are also the products of individuals. Hence, even if we consider all scientific activity to be but a reworking of prior scientific activity, the dynamics by which individual scientists develop their theories is a subject integral to the history of science. If we accept the proposition that knowledge grows by public and critical dialogue, we should not ignore the fact that important phases of the dialogue may occur within an individual. Such is the case with Charles Darwin, who over the decades prior to the publication of Origin of Species, engaged in an extended reworking not only of natural history, but also of his own emerging ideas. For a scientist such as Darwin, the internal personal debate is as fierce and as fertile as many a public debate.