"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. . . . Thus, as token of what a puppet thing life is the ancient song surged through him and he came into his own again."
London is reinforcing one of the themes of this book, in that he is saying that underneath all of the civilization is the heart of a beast meant to survive. The same skills and instincts that preserved its ancestors are the same that guide Buck now.