The interim between Darwin's first publication of the theory of natural selection (The Origin of Species, 1859) and his extended application of it to human development (The Descent of Man, 1871) corresponds with the reign of sensation fiction, a genre built upon older gothic tropes but imbricated with cultural and scientific modernization. Joseph Sheridan Le Fanu's Uncle Silas, a novel that straddles generic and national literature categories, typifies the sensation era's uneasy awareness of not only similarity but interdependence across boundaries. While eschewing typical sensation engagement with technology such as trains and telegrams, Uncle Silas reflects theories of species transmutation and particularly Darwin's Origin, which posited a malleability of seemingly distinct categories through species interaction and environmental influence. The plot of Uncle Silas centers on dislocation and efforts to fortify self‐identity and reestablish security. Le Fanu's narrator‐protagonist, the orphaned English or Anglo‐Irish heiress Maud Ruthyn, practices bodily self‐regulation and careful discernment to shore up boundaries of nationality and class that sustain her hereditary privilege. She stabilizes her class, national, and gender identity through rational revulsion from her French governess and implicitly Irish cousin. Narrator‐protagonist Maud resolutely positions Englishness against foreignness and humanity against animality, but these categorical divisions, which she enforces as a character in the novel's imagined world, are exposed as false by her narration. Uncle Silas presents unregulated animal appetites as threatening to rational community governance, but it also implies that our bestial shared capacity for pain, which crosses boundaries of nationality, class, and even species, enables ethical response.