Foreshadowing in Call of the Wild.

As Buck sits by Thornton's fire, he bigins to have primal dreams. Buck begins to dream of the wild. He imagines a life free of ties to humans. Buck thinks he hears the distant howl of a dog. One night he hears it more clearly than ever before. He ventures into the forest and he sees a timber wolf. This is the wildness that beckons Buck towards his true calling.