The moral of the novel concerns the human connection to the primal aspects of nature and ways in which our connection to civilization has interfered with that important connection. Within that connection lies something almost spiritual, where a man can understand his strength and essence. Many critics point out that the story presents a view of life that is very masculine, celebrating manhood through a rugged individuality over the human relationships often associated with a “feminine” world view. Enotes has an excellent discussion of the novel's themes, which you can find at the link below.
The Moral to this story is Survival of the fittest. If you cannot survive, people, or dogs for that matter, will simply forget you. civilization vs. primitive nature is also another aspect of this story that is significant here. when buck was back at Judge Miller's place, he was civilized and now that he is in the Klondike, it is a dog eat dog world out there. let the best dog win, or strongest, for that matter.