Scholars have often considered evolutionary social theories a product of Positivist scientism and the naturalization of ethics. Yet the theistic foundations of many evolutionary theories proposed between the nineteenth and the twentieth centuries bolstered the belief that following natural laws was morally desirable, if not vital, to guaranteeing social and moral progress. In the early twentieth century, American paleontologist and leading evolutionist Henry Fairfield Osborn represented one of the most authoritative advocates of this interpretation of natural normativity. Particularly during the (...) last years of his career, Osborn used theological arguments to reinforce his advocacy of evolutionary ethics and social control policies, which led him to challenge his “old master” Thomas Huxley regarding the separation between evolution and moral conduct. This article examines the development of Osborn’s evolutionary ethics, with particular regard to its bearing on the American debate on euthenics. I argue that theistic topics played a major rhetorical role in the attempt to justify normative conclusions drawn from ostensible laws of evolution. (shrink)