2.1 Definition
Naturalism has sprung up and flourished in the European countries. But, the definition of naturalism has sparked a heated debate. When it comes to it, critics’ notions vary from person to person. Naturalism derives from a biological model. Its origin owes much to Charles Darwin and his theory of evolution. Naturalism is sometimes claimed to give an even more accurate depiction of inner life than realism. However, naturalism is not the same as realism. “The term naturalism refers to a literary movement in Europe and America in the late nineteenth-century and early twentieth-century. It uses the techniques and theories of science to convey a faithful picture of inner life and people. The characteristics of naturalism include a carefully detailed presentation of modern society, often featuring lower-class characters in an urban setting or a panoramic view of contemporary life; a deterministic philosophy that emphasizes the effects of heredity and environment; characters who act from passion rather than reason and show little insight into their behavior.” (Logan, 2011:566) “Terrible things must happen to the characters of the naturalistic tale. They must be twisted from the ordinary.” (Matthews, 2009:160) Thus, naturalistic writers apply to a scientific method to create their works by studying human beings governed by their instincts and passions. Some critics identified naturalism as “pessimistic materialistic determinism”, but its elements are more complex than that phrase would suggest. “Naturalism, in letters, is equally a return to nature and to man; it is direct observation, exact anatomy, the acceptance and depiction of what is.” (Logan, 2011:566) Naturalism also emphasizes on the philosophy that life is unpredictable and contains misadventures.
It seems hard to give an authoritative definition to naturalism that all of the people can accept. In spite of the substantial pergences of different scholars, they still have something in common. Generally speaking, naturalism meant turn-of-the-century literature that dealt with lower-class urban characters in whom traits roiled beneath a veneer of civilization. Driven by forces of heredity and environment that one can not control or understand, these characters were pushed to the verge of panic and limits of endurance. They responded to the sordid, frightening, and certain compelling circumstances in which they found themselves by striking out in desperate ways unacceptable to middle-class: through drinking or taking drugs, resorting to prostitution or theft, or dealing with things in violent ways, even committing murder on certain occasions.

2.2 Darwinism and His Evolutionary Theory
Charles Robert Darwin was an English naturalist. He proposed the scientific theory that his branching pattern of evolution resulted from a process that he called natural selection. Darwin published his theory of evolution with compelling evidence in his famous book On the Origin of Species in 1859. Darwin pointed out that creatures possessing advantageous mutations would be favored, eventually evolving into new species. Darwinism and his evolutionary theory indicate that the origin of species is derived by descent through the natural selection of those best adapted to survive in the struggle for existence. Natural selection refers to a process in nature resulting in the survival and perpetuation of only these forms of plant and animal life best fitted for existing conditions, while related but less fit forms become extinct. “Darwinism was both a continuation of and a challenge to Enlightenment assumptions. As a theory of evolution, it revealed the physical process of the universe, matter unfolding in time. But as a natural selection-that species change through a process of adaptation to their immediate environment–Darwinism emphasized the accidental rather than a necessary unfolding, seriously challenging the notion of design. Darwin’s theory of evolution contained the idea of devolution and degeneration. Natural selection argues that the best in the species are attracted to and mate with each other. This leaves the worst in the species to mate and generate their own offspring.” (Pizer, 2000:56) 自然主义在《珍妮姑娘》中的体现(4):http://www.751com.cn/yingyu/lunwen_2218.html