Quote 7
The dominant primordial beast was strong in Buck, and under the fierce conditions of trail life it grew and grew. (3.1)
As Buck adapts to his new life, his primitive side becomes more prominent.
Quote 8
And strange Buck was to him, for of the many Southland dogs he had known, not one had shown up worthily in camp and on trail. They were all too soft, dying under the toil, the frost, and starvation. Buck was the exception. He alone endured and prospered, matching the husky in strength, savagery, and cunning. Then he was a masterful dog, and what made him dangerous was the fact that the club of the man in the red sweater had knocked all blind pluck and rashness out of his desire for mastery. He was preeminently cunning, and could bide his time with a patience that was nothing less than primitive. (3.22)
It is Buck’s primitive nature that allows him to survive in the frozen North where others of his kind have failed.
Quote 9
In the main they were the wild wolf husky breed. Every night, regularly, at nine, at twelve, at three, they lifted a nocturnal song, a weird and eerie chant, in which it was Buck's delight to join. (3.27)
Buck finds camaraderie with wild wolves because he recognizes his own primitivity in them.