In the story "To Build a Fire," London contrasts the reactions of both a man and a dog to the extreme cold of the Arctic.
The dog mostly relies on his instinct, and this indicates to him what the best thing to do is when faced with an extreme situation of this kind. On the other hand, the man relies on help from civilization as well as his own judgement. While the dog is able to survive, the man dies due to his unpreparedness and his arrogance.
The point that London is trying to make is that humans are extremely detached from nature, and that this makes them unable to survive without the help of society and technology. This is compatible with the characteristics of Naturalism, as the situation is extremely realistic, and it is the environment that has power over the man's character and destiny.