In addition to the above, Jack London is considered a naturalist, which is kind of a kicked up version of realism.
London tried to do everything realists did but represented the real world in specific ways. He emphasized humans as victims of natural forces and social environments. London's real world is a nasty place. Humans are creatures like any other creatures in London's works, and in fact are inferior to other creatures in many ways.
For instance, in London's classic short story, "To Build a Fire," the human protagonist cannot overcome the force of nature, while the dog traveling with him survives with no trouble.