This article examines atavism as a theory of racial science in the nineteenth-century United States that illuminates how the developing medical profession reinforced racial, class, and gender hierarchies to gain cultural authority. I use John S. Partridge's "The Pineal Eye," a little-known short story published in San Francisco's The Wave in 1894, as a case study that reveals how atavism was conceived as pathology within the purview of medical study. Partridge intertwines established atavistic discourse that asserted the Anglo-Saxon female body as paradoxically modern in terms of racial identity and primitive in terms of sex with scientific experimentation and male medical authority, resulting in evolutionary regression. Partridge portrays atavism as a lens with which to challenge medicine that relied on experimentation and scientific discovery rather than recuperative treatment. I argue that these connections between atavistic and medical discourse blur the boundaries between science and fiction during the period.