Naturalism developed as an extension of Realism in the late 19th century, influenced by scientific theories of the time. Naturalist writers portrayed humans as products of their environment and heredity, subject to forces outside their control. Major themes included humanity's smallness in an indifferent universe and the influence of factors like genetics and socioeconomic conditions on people's lives and ability to make choices. Works often portrayed the harsh realities of life for the working class and sought to expose cruelty in economic and social systems.