3. 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. . . .

This quote, taken from Chapter II, shows that as Buck fights for survival in the harsh world of the Klondike, he relies increasingly on buried instincts that belonged to his wild forebears. The role of this atavistic development—“atavism” refers to the recovery by an animal of behaviors that belonged to its ancestors—points to one of the central themes of London’s novel, namely, the way that primitive instincts and urges persist beneath the veneer of civilization. Throw a soft, civilized creature (human or animal) into the wild, London suggests, and if he survives, he, like Buck, will come to depend on the same instincts that guided the life of his primitive ancestors. “The ancient song,” in his phrase, is only waiting for the right opportunity to emerge.