John Thornton was "the ideal master", and because of this he evoked in Buck a love that was "feverish and burning, that was adoration, that was madness".  Buck was completely devoted to Thornton; "nothing was too great for Buck to do, when Thornton commanded".  One day, on a whim, Thornton thought to test Buck's loyalty to him.  He was sitting with some others "on the crest of a cliff which fell away, straight down, to naked bed-rock three hundred feet below".  Thornton commanded Buck to jump, and the faithful dog attempted to do so without hesitation; it was only because Thornton caught him and dragged him back to safety that Buck did not leap to his death.  The fierceness of Buck's devotion was "splendid...and terrible", and it made Thornton afraid because of its utter totality.  The others noted that they would not want "to be the man that lays hands on (Thornton) while (Buck) (was) around", an apprehension that was realized before the year was over.  In that situation, Thornton "good-naturedly" tried to stop a fight at a bar, and was roughed-up a little by one of the combatants.  Buck was upon the attacker in a flash, with the intent to kill.  Fortunately, the crowd managed to pull the dog off the man, but not before Buck had torn open his throat (Chapter VI).