How does the theory of "survival of the fittest" fit in with naturalism?

Of course, Darwin's Theory of Evolution has many points
that correspond with Naturalism, including the survival of the fittest. Let us just
remind ourselves of what Naturalism is. Naturalism is a movement that originated in the
nineteenth century and focussed on the ideas of thinkers such as Darwin. They concluded
that human behaviour is determined by heredity and environment. Key themes of Naturalism
involve presenting human beings as mere players in a world where they are controlled by
natural forces of which they have little understanding or comprehension. Key writers who
can be described as Naturalist are Jack London and Stephen
Crane.

Of course, we can see the relation to the survival
of the fittest, as Darwin argued that creatures survive on the basis of their genes
alone, rather than their intelligence or wit.