John Thornton pays off his debts with money he earns from the bet, and he sets off to the east to find a fabled lost mine that is supposed to make a man rich. Together with his other dogs and his friends Pete and Hans, he and Buck wander in the wilderness, hunting and fishing and living off the land, until they reach a shallow place in a valley full of gold. The men earn thousands of dollars a day panning for gold, and the dogs have nothing to do. Buck begins to feel wild yearnings. One night, he springs up from sleep with a start, hearing a call from the forest. He dashes through the woods and finds a timber wolf, one-third his size. Buck begins to circle the wolf and make friendly advances, but the wolf is afraid.

Finally, the two show their friendship by sniffing noses, and the wolf leads Buck away through the forest. They stop to drink, and Buck remembers John Thornton. The wolf encourages him to keep following, but Buck starts back toward the camp. When he arrives, Thornton is eating dinner, and Buck showers him with affection. For two days, he never allows Thornton out of his sight. Then, he hears the call more loudly than ever and is haunted by recollections of his wild friend. He begins to stay away from the camp for days at a time, hunting his own food.

Buck has two identities at this point: one as sled dog in Thornton’s camp, another as wild hunter in the forest. He kills a bear and fishes for salmon the river; when the moose come in the fall, Buck hunts them eagerly. He cuts a bull away from the pack to kill him and finally brings him down after four days. Then he heads back to the camp. On the way, he feels a strange stirring in the wilderness, of something new abroad, and he feels a premonition of calamity. His feeling is proven correct when he finds Thornton’s dog Nig and one of the dogs bought in Dawson, both dying on the trail. As he approaches the camp, he sees Hans lying facedown, arrows covering him. He peers out to where the lodge had been and sees Yeehat Indians dancing in the wreckage. Buck charges, cutting their throats with his fangs and killing several of them. The Indians scatter, and Buck finds the rest of his camp, including Thornton, dead.

Buck mourns his dead master but feels pride at having killed the Yeehats. Henceforth, he will not fear men unless they carry weapons. He hears the call of the wolf again. His ties to Thornton broken by death, he heads off to follow it. He finds the pack, and one wolf lunges for his throat, but he breaks its neck easily. Three others try but pull back. After half an hour they all draw back, and one of them approaches Buck in a friendly manner. Buck recognizes him to be the wolf he encountered in the woods. Buck joins the wolf pack, and the Yeehats notice a difference in the local breed of timber wolves as years pass. They also tell of a Ghost Dog that runs at the front of the pack, singing songs and leaping above his fellows. They tell of a haunted valley—where Thornton lies dead—where an evil spirit dwells, and where, every year, Buck comes and mourns for a time beside the stream before loping away to rejoin the pack.

Early in this chapter, Buck’s vision of primitive man recurs, and this time, he sees himself running alongside the “hairy man,” hunting with him in the forest, and guarding him while he sleeps. In these images, London once again emphasizes the primitive nature of the man-dog relationship and the strength of the bond that ties Buck to John Thornton. But the bond is constantly tested by the equally strong call that draws Buck away from human life and deeper into the wilderness—a call that fills Buck with “a great unrest and strange desires.” As Thornton and his friends sift for gold in the wild, Buck’s soul is in a state of extreme tension, torn between his loyalty to his master and his destiny as a wild animal.

His encounter with the timber wolf, whose smallness reminds us of Buck’s remarkable size and power, is an important step in his development as a wild creature, since it offers the promise of a community of wild creatures. Buck need not be alone in the wild; he can find companionship not only from humans and dogs but also in the tight-knit world of the pack. Meanwhile, Buck’s long hunt of the moose enables London to emphasize the importance of what he terms “blood-longing” in Buck; once again, the novel emphasizes the kill-or-be-killed nature of life in the wild and shows us how Buck, the “dominant primordial beast,” is the ultimate killer. “He was a killer,” the novel insists, “a thing that preyed, living on the things that lived, unaided, alone, by virtue of his own strength and prowess, surviving triumphantly in a hostile environment where only the strong survive.” These sentiments are the language of Darwin and Nietzsche, portraying life as an unceasing struggle for survival in which only the strong—only the Bucks of the world—can last for long.

Still, it takes Thornton’s death to enable Buck to enter this wild world fully. For the first time in the novel, he has no master: he has been passed from Judge Miller to the dog traders to Francois to Hal and finally to Thornton. But Thornton’s death ends the succession of masters and leaves him the master of his own fate. The only humans that remain in his world are the Yeehats, and Buck scatters them, triumphantly demonstrating that he is the master, not they. His attack on them marks the final step in his escape from the world of men. Earlier, he learns that humans can be violent, like the man who beats him with the club, and foolish, like Hal, Charles, and Mercedes. After all that has happened in the North, he learns he can kill men at will. The last traces of the old, civilized morality vanish, and Judge Miller’s Buck, who would die for a principle, is transformed into a beast who kills with impunity and without remorse.

London treats this transformation as triumph, not tragedy. Morality functions well in the civilized world, the novel suggests, but Buck’s authentic, animal nature is amoral—it obeys the law of the wild, in which brute strength is the only arbiter of justice. This strength wins the respect of the wolves, who first fight Buck and then obey him; and this strength makes him a legend among the Yeehats. When the novel opens, Buck is a king, but a soft monarch ruling a gentle land obtained only by his birthright. As the novel closes, he is a king again, but his kingdom is a very different place from Judge Miller’s warm Santa Clara spread. More important, he has won his kingdom by his own efforts and nothing else. He is a self-made monarch, having faced a cruel, uncaring world—and mastered it.