How did Buck meet John Thornton ?

Charles, Hal, and Mercedes are abusive to their dogs. They put much too much weight on the sled and work their dogs past exaustion. They all stagger into John Thornton's camp for a rest. John warns the travelers that the ice is thin, and the trail is likely to drop out at any moment. Hal refuses to listen, and calls the team to order. Only the merciless lashing of his whip can make any of them rise. But Buck remains on the ground. He refuses to move. Hal takes up the club, but Buck will not move. He is too numb.Suddenly John Thornton springs upon Hal and drives him back, telling him that he will kill him if he hits the dog one more time.