How did Buck show his love for Thornton?

Buck gets a chance to prove his love when John Thornton interferes in a bar fight between "Black" Burton and a tenderfoot. Buck nearly kills the instigator, and suddenly he has a reputation in every camp in Alaska. When John falls into the rapids of a river, Buck aids Hans and Pete in rescuing him and breaks three ribs. Later that winter in Dawson, Buck wins a $1600 bet for his master by pulling a sled loaded with a thousand pounds of flour. After that Buck becomes the most famous dog in Alaska. All who see it are astounded, and a man offers to buy him from John Thornton for 1200 dollars, but Thornton doesn't blink before he refuses.