The mood of the novel is bittersweet, often mixing beauty with violence or deep melancholy. For example, in Chapter 5, a beautiful spring takes over the land while the dogs starve under the mismanagement of Hal, Mercedes, and Charles:
The whole long day was a blaze of sunshine. The ghostly winter silence had given way to the great spring murmur of awakening life. This murmur arose from all the land, fraught with the joy of living.
The phrase "fraught with the joy of living" encapsulates the bittersweet mood of this moment and the novel in general. Living itself is inherently joyful, the novel seems to communicate. At the same time, though, living is also inherently "fraught." Being alive sometimes means enduring incredibly grueling and cruel circumstances. Living is "fraught" for Buck and the other dogs right now because they are being forced to work themselves to death while starving. Their bodies remain stubbornly alive for longer than might be expected under these circumstances. The "joy of living," in other words, prolongs their suffering. Meanwhile, the phrase "spring murmur of awakening life" fails to acknowledge all this suffering.
London invites the reader to imagine the happy experience of hearing new wildlife in the spring just as he is asking the reader to sympathize with the near-dead dogs, whom he compares to "skeletons." Readers and dogs alike may want to abandon the hope that the dogs will make it through this experience and move on to a better life, but the spring forces a painful optimism. This painful optimism is the mood London seems to want readers to take toward social issues, particularly labor rights. Workers were treated terribly at the time London wrote this novel, and he wanted readers to see the way this poor treatment was chipping away at people's souls. Still, he wanted to encourage readers to hope for a better world rather than giving up altogether.