Any answer to this question needs to focus on the bleak and dangerous setting and the way in which it highlights man's fragile place in the world. One of the central themes of this story is that of naturalism, which seeks to place man in his proper relationship with nature. Man is not the conquering force in every sphere of his life, and, in the face of the awesome powers of nature, is shown to be ultimately incredibly weak and frail. Consider the following quote and how this highlights this theme:
The Yukon lay a mile wide and hidden under three feet of ice. On top of this ice were as many feet of snow. It was all pure white, rolling in gentle undulations where the ice jams of the freeze-up had formed. North and south, as far as his eye could see, it was unbroken white, save for a dark hairline that curved and twisted from around the spruce-covered island to the south, and that curved and twisted away into the north, where it disappeared behind another spruce-covered island.
The power of nature is shown in the beauty and purity yet rather disturbing way in which the snow and ice have left no trace of any other humans or civilisation at all. Man is literally exposed and tremendously vulnerable in such a setting, and this is emphasised by the sparse nature of the few trees that dot the landscape.