Degeneration theory was elaborated in the nineteenth century based on the very old belief in human trans-generational decline. After incorporation into social and biological theories in the eighteenth century, it flourished in the fields of medicine and public health in the early nineteenth century and was emphasized, above all, in some fields of psychiatry. Degenerationist beliefs were influential in the professional middle classes of France, England and Spain. The discourse was incorporated by Émile Zola into the aesthetics of his Naturalist novels in the 1870s and 1880s and directly influenced Benito Pérez Galdós in Spain in the 1880s. An analysis of a quartet of Galdós's Naturalist novels shows evidence of degenerationist thinking, under the influence of Spanish and French medicine.