Quote 4
He had been suddenly jerked from the heart of civilization and flung into the heart of things primordial. (2.1)
Buck experiences a violent shift from the civilized world to the primitive one. He recognizes the extremity of such a change.
Quote 5
The snow walls pressed him on every side, and a great surge of fear swept through him--the fear of the wild thing for the trap. It was a token that he was harking back through his own life to the lives of his forebears; for he was a civilized dog, an unduly civilized dog, and of his own experience knew no trap and so could not of himself fear it. The muscles of his whole body contracted spasmodically and instinctively (2.12)
Buck retains memories of lives not his own.
Quote 6
And not only did he learn by experience, but instincts long dead became alive again. The domesticated generations fell from him. In vague ways he remembered back to the youth of the breed, to the time the wild dogs ranged in packs through the primeval forest and killed their meat as they ran it down. It was no task for him to learn to fight with cut and slash and the quick wolf snap. In this manner had fought forgotten ancestors. They quickened the old life within him, and the old tricks which they had stamped into the heredity of the breed were his tricks. They came to him without effort or discovery, as though they had been his always. And when, on the still cold nights, he pointed his nose at a star and howled long and wolflike, it was his ancestors, dead and dust, pointing nose at star and howling down through the centuries and through him. And his cadences were their cadences, the cadences which voiced their woe and what to them was the meaning of the stiffness, and the cold, and dark. (2.25)
Buck is not just an individual, but also a part of a long and communal history. He is the sum of all dogs that came before him and all that will come after.