The present thesis analyses the work of four Lusophone Naturalist writers, two from Portugal (Abel Botelho and Eça de Queirós) and two from Brazil (Aluísio Azevedo and Adolfo Caminha) to argue that the pseudoscientific discourses of Naturalism, positivism and degeneration theory were adapted on the periphery of the Western world to critique the socio-economic order that produced that periphery. A central claim is that the authors in question disrupt the structure of the patriarchal family — characterised by exogamy and normative heterosexuality — to foster alternative notions of kinship that problematise the hegemonic mode of transmitting name, capital, bloodline and authority from father to son. It was this rapidly globalising form of patriarchal capitalism that saw Portugal and Brazil slip into positions of economic disadvantage and dependency, events that were then naturalised in centres of dominance as incidences of national, racial and sexual “degeneracy”. The thesis thus draws links between contemporaneous disquiet about the nation’s race and bloodline; the various “homosexual scandals” that rocked the period; the considerable prevalence of incest and non-normative desire in the literature concerned, and the supposed “inconsistencies” in the style of Lusophone Naturalism that have often been regarded as imperfections in the face of Zola’s model. I propose instead that such adaptations to the Naturalist model can be read as attempts to reassess its potentially marginalising discourse from the margins themselves, exposing something “queer” at the textual, discursive level. This is the process that I call writing “against nature”, relating non-normative kinship to the disruption of the Naturalist aesthetic more generally. Drawing on postcolonial theory, psychoanalysis and queer theory, I argue that the Lusophone Naturalist perspective presents the divided world of the period as anything but a “natural” state of affairs. In this sense, a second line of reasoning is developed: that its authors formed a tentative transatlantic movement that criticised Naturalism as conceived in centres of dominance, calling for a revision of the role that the “scientists” played in shaping and understanding the fin-de-siècle world.