The money Buck wins for him enables Thornton to pay off some debts and journey with his partners in search of a fabled lost gold mine that no living man has ever found. They travel east on an unknown trail. Buck enjoys himself like never before. They journey many months but do not find the lost mine. But they do find gold, in a shallow area in a valley, and they collect as much gold dust and nuggets as they can.
Buck muses by the fire and has more visions of cave men, and he sees himself wandering with the man. He feels the call of the wild, and sometimes pursues it, spending hours in the forest. "irresistible impulses" seize him. He hears a wolf howl, and feels an instant kinship with it. When he sees the wolf, the wolf runs, and Buck follows. When Buck corners it, he does not attack, but makes friendly advances. They continue the chase, and finally sniff noses and become friendly. For hours they run together, and Buck knows he is at last answering the call of the wild. But then he remembers Thornton and returns to the camp. For two days he does not leave, but then he hears once more the call of the forest. He starts to wander in the woods, and stays away from the camp for days at a time, hunting and fishing for his food. He exhibits a wild, wolf-like cunning. As a fearsome hunter, he kills a moose by separating it from the herd and remorselessly pursuing and attacking it. He shows great patience and ferocity. It takes him four days to pull the moose down.
When he returns to Thornton's camp, he finds it has been overrun by Yeehat Indians. The men and dogs are dead, and the Indians are dancing around the wreckage of the lodge. Buck attacks them and kills several of them. The remainder flee in panic, thinking that an evil spirit has come. Buck discovers that Thornton's body is submerged in a pool.
At night, Buck hears the yelping of the wolves in the forest, and he finds their call more compelling than ever before. With the death of Thornton, the last of his ties to the human world is gone. When Buck encounters the wolf pack, he kills one of them that attacks him. The other wolves attack him, but Buck positions himself well and fights them off. After half an hour, the wolves back off, and one wolf approaches him in a cautious but friendly manner. It is the wolf Buck has met before. They touch noses. One wolf begins to howl, and the others join in, as does Buck. The pack then runs off, and Buck runs with them.
For years Buck runs in the wild as a wolf. The Yeehats tell of a Ghost Dog that runs at the head of the pack, and they are afraid of him. They know he continues on occasion to kill human hunters. They will not enter the valley where he lives.
Buck continues to hear the call of the wild, and hears it more strongly than ever. From the moment he meets the wolf, and makes friends with him, he exists in two worlds and two communities: the human one, and the wolf community that he is discovering in his "wild brother."
Perhaps because London wanted to emphasize the depth of Buck's attachment to Thornton, and distinguish it from the former relationship between Buck and Judge Miller, he gives to Buck a second vision of the cave-man, but this time the cave-man is accompanied by Buck. The tie between man and dog, it is implied, goes back to the dawn of the human race, when man, at the mercy of wild beasts, needed dogs for protection. Thus the relationship between Thornton and Buck is as much a product of primitive roots as civilized ones.
The death of Thornton severs the last ties Buck has to the human world. He now becomes completely wild, running with the pack and establishing his mastery through strength and cunning. He is the living example of the Darwinian law of the survival of the fittest. But London does not present this as a regression to be regretted. Buck has in fact triumphed. He has discovered for himself the ancient springs of his own nature, and is more fully himself than when he was cozy and enjoying himself on the ranch at Judge Miller's, or lazing around the fire at John Thornton's. He is fulfilling the deepest impulses of his own being, and in doing so he acquires an almost mythic status as the Ghost Dog or Evil Spirit of Indian legend.