In the final decades of the nineteenth century, Darwin’s theory of evolution reverberated across nearly every part of culture, giving rise to new movements in philosophy, psychology, the social sciences, literature, and the visual arts, which collectively took up “naturalism” as their rallying cry. Rather than embodying a single idea, however, naturalism formed a contested terrain, wherein a mixture of old and new ideas about nature and human agency vied for cultural authority. For British and American photographers at the turn of the century, naturalism took root as a key organizing term, representing both an aesthetic ideal and a deeper set of philosophical commitments that engendered a new understanding of the medium. These photographers visualized the landscape, its human inhabitants, and wildlife subjects in ways that registered their novel beliefs about nature, and they simultaneously drew from those beliefs as they laid claim to photography as a mode of individual expression. The narrative follows the transatlantic flow of naturalism within photography, starting with a reappraisal of British photographer P. H. Emerson’s famed decision in late 1890 to withdraw his controversial 1889 treatise, Naturalistic Photography for Students of the Art. The opening chapter details Emerson’s formative engagement with Herbert Spencer’s evolutionary philosophy, revealing how an encounter with William James’s writings on psychology led him to reconsider his core convictions about photography and art. The second chapter traces the influence of evolutionary thought on American pictorialism through a comparative analysis of two competing claimants to Emerson’s legacy, Alfred Stieglitz and Rudolf Eickemeyer, Jr. The third chapter furnishes a culturally situated account of the emergence of nature photography as a distinct genre and a popular activity starting in the mid-1890s, focusing on the peripatetic life and career of British-American photographer Arthur Radclyffe Dugmore. His work is read through his peripheral engagement with in the New York amateur photography scene and his involvement with the American nature study movement. Lastly, a chapter-length appendix recounts Emerson’s attempt starting in the 1920s to secure his legacy by writing a history of artistic photography, which he completed but never published. Parsing the details of Emerson’s unrealized history through his private correspondence with Stieglitz and Eickemeyer casts light on the fate of naturalism and the efforts of each photographer to shape the history of the medium in their own image.