Why are naturalists inevitably pessimistic in their view?

Why are naturalists inevitably pessimistic in their view?
【正确答案】：A.Naturalism was greatly influenced by Darwin's evolutionary theory and French literature.
B.Naturalists accepted the more negative implications of Darwin's theory and used it to count for the behavior of those characters in literary works who were conceived as more or less complex combinations of inherited attributes, their habits conditioned by social and economic forces.
C.Naturalism is evolved from realism when the author's tone in writing becomes less serious and less sympathetic but more ironic and more pessimistic.