ABSTRACT Emile Zola is considered one of the fathers of 19th century French Naturalist literature. He is famous for his eloquence, sarcasm and is well known for being a provocateur. He wants to follow the principles of science: observation of his characters in their living environment (or milieu). He holds that individuals inherit physical and personality traits from their ancestors, such as atavism, which can be passed from grandfather to father and father to son. This assumption leads to Social Darwinism and impacted Zola like many other European intellectuals who believed in the new social sciences. Religion was going extinct on the old continent and the trend was to apply these theories to literature and humanities. The author also captures the political and social unrest of a struggling working class in his novel Germinal, where starving miners rebel against the bourgeois class that exploits them. Baldomero Lillo is a Chilean naturalist follower of Emile Zola who found inspiration in Germinal to write Sub Terra-short stories depicting the grim life of the coal miners. The author knows them well since he shared his existence with the miners in Lota, in the southern region of Santiago. Unlike Zola, Lillo, who was less educated and less inclined to trust science, opts for a compassionate Naturalism which relates more to his culture and personal inclinations. Le milieu or el medio ambiente in the Sub Terra stories is dreadful and the author seeks to expose the master/slave relationship in a society that still resembles the European Middle Ages. Le milieu, that is to say the external forces that surround the miners (their geographical, social and political environment), eventually engulfs and condemns them to a life of servitude and misery. Determinism on both continents decides the fate of each member of the society.