Quote 1
John Thornton asked little of man or nature. He was unafraid of the wild. With a handful of salt and a rifle he could plunge into the wilderness and fare wherever he pleased and as long as he pleased. Being in no haste, Indian fashion, he hunted his dinner in the course of the day's travel; and if he failed to find it, like the Indian, he kept on traveling, secure in the knowledge that sooner or later he would come to it. So, on this great journey into the East, straight meat was the bill of fare, ammunition and tools principally made up the load on the sled, and the time-card was drawn upon the limitless future. (7.3)
Like Buck, Thornton learns to work with nature to survive in it.
"You poor devil," said John Thornton, and Buck licked his hand. (5.66)
Buck’s feelings for Thornton are immediate.
He had a way of taking Buck's head roughly between his hands, and resting his own head upon Buck's, of shaking him back and forth, the while calling him ill names that to Buck were love names. Buck knew no greater joy than that rough embrace and the sound of murmured oaths, and at each jerk back and forth it seemed that his heart would be shaken out of his body so great was its ecstasy. And when, released, he sprang to his feet, his mouth laughing, his eyes eloquent, his throat vibrant with unuttered sound, and in that fashion remained without movement, John Thornton would reverently exclaim, "God! you can all but speak!" (6.4)
London uses the word "ecstasy" both to describe Buck’s emotion at killing a rabbit and his feelings with Thornton. This suggests a connection between the extremes of love and death.