Morel, influenced by various pre-Darwinian theories of evolution, particularly those that attributed a powerful role to acclimation, saw mental deficiency as the end stage of a process of mental deterioration. In the 1850s, he developed a theory of "degeneration" in regards to mental problems that take place from early life to adulthood. In 1857 he published "Traité des dégénérescences physiques, intellectuelles et morales de l'espèce humaine et des causes qui produisent ces variétés maladives", a treatise in which he explains the nature, causes, and indications of human degeneration. Morel looked for answers to mental illness in heredity, although later on he believed that alcohol and drug usage could also be important factors in the course of mental decline.