Literary naturalism derives mainly from a biological model. Its origin owes much to Charles Darwin and his theory of evolution, based in turn on his theory of natural selection. Darwin created a context that made naturalism - with its emphasis upon theories of heredity and environment - a convincing way to explain the nature of reality for the late nineteenth century. But before Darwin's ideas were available in literary form, they had to be transformed by Emile Zola in his Roman expérimental (1880). Zola, in turn, based his theories of heredity and environment on Prosper Lucas's Traité. . . de l'héredité naturelle (1850) and especially Claude Bernard's Introduction l'étude de la médecine expérimentale (1865). Zola believed that the literary imagination could make use of the ideas in these books so long as the novelist functioned like a scientist, observing nature and social data, rejecting supernatural and transhistorical explanations of the physical world, rejecting absolute standards of morality and free will, and depicting nature and human experience as a deterministic and mechanistic process. All reality could be explained by a biological understanding of matter, subject to natural laws, available in scientific terms.