As he wrote his Naturalist novels, Galdós was much influenced by Émile Zola and the French author's vision of determinism and degeneration that he developed as a result of his study of Darwin, Bénédict Morel, Moreau de Tours and Prosper Lucas, also owing much to Comte's positivism and to Taine's social determinism. In addition, degeneration theory entered Spain from France and England in medical discourse, though there are examples of degenerationist thinking in Spain preceding the major influx from France in the 1870s. Galdós interpreted the Naturalist vision with humour and irony using little of the detached "scientific" narrative of the French model. A vein of degenerationist determinism can be perceived in his writing, especially as he portrays the great "degenerative social diseases" of alcoholism, syphilis, tuberculosis and madness. Such an interpretation of these infirmities was common among Spanish physicians as they confronted these conditions of epidemic magnitude that appeared to threaten public order and, even, the stability of society itself. This book studies how Galdós used the accepted medical wisdom of his time as he sought to raise public awareness of these ills.