This chapter reads the late nineteenth-century genres of American naturalism and regionalism through the prism of climate, and finds that their authors depict characters whose characteristics are shaped by their responses to their ambient environments, including climate, and by the inherited effects of their ancestors’ adaptations to theirs. It argues that their thinking about climate was informed by the popular Lamarckian science of their post-Darwinian evolutionary era, by the climate theory of the historian Hippolyte Taine, and by turn-of-the-century geography. In the decades during which Frank Norris, Charles Chesnutt, Edith Wharton, and Hamlin Garland were at work on questions of determinism and/or a “determined indeterminacy,” climatic, genetic, medico-psychiatric, and sociological models of identity vied for authority. The writers drew their representations of the making of Americans from these competing claims.