What did naturalists write about in their novels?

Naturalistic writers thus used a version of the scientific method to write their novels; they studied human beings governed by their instincts and passions as well as the ways in which the characters' lives were governed by forces of heredity and environment.

Instead of using supernatural or spiritual explanations, naturalism focuses on explanations that come from the laws of nature. Beyond the belief that everything can be explained using nature, naturalism is also a term for a particular style of art and literature from the 19th century. So I think naturalist focused on laws of nature instead when they were writing novels, instead of supernatural.