Jack London's naturalistic story depicts the conflict of Nature vs. Man; that is, civilized man, who has lost some of his atavistic instincts. Without these instincts and development according to natural selection, modern man is ill-equipped at times to deal with an indifferent universe that is at times harsh.
In the exposition of London's story, the main character, who is called only "the man," does notice that there is no sun as he sets out, but "[T]his fact did not worry the man." Moreover, other harsh conditions, such as the tremendous cold and the "far-reaching hair-line trail," make no impression upon him. Clearly, he has no real experience of his environment, one that heredity would provide him.
Fifty degrees below zero was to him just precisely fifty degrees below zero.
London expresses this condition as the man's lacking "imagination," an imagination that develops through natural selection and becomes part of man's heredity. On the other hand, the dog, through this same process of natural selection, has developed instincts that become a part of his genetic code. These instincts tell him that without the man who provides fire, it is too cold for him to be away from the camp:
But the dog knew; all its ancestry knew, and it had inherited the knowledge...that it was not good to walk abroad in such fearful cold....It was not concerned in the welfare of the man; it was for its own sake that it yearned back toward the fire.
The dog, too, senses the ice beneath the snow as they travel, and he does not tread upon a spot where the man's foot breaks through. Having done so, the man knows that he must build a fire in order to dry his foot and ward off frostbite. But, because he "lacks imagination" and does not have the instincts coded into the dog, the man builds his fire under a snow-filled tree; the heat melts this snow, and it falls upon the fire, putting it out:
The man was shocked. It was as though he had just heard his own sentence of death.
Only after he fails, does the man realize the formidable forces of an indifferent nature that are at work. So, after "the fire provider had failed," the dog indifferently returns to the camp, especially after it "caught the scent of death." The dog follows his instincts and survives; the man, who has none, dies.