The title of Chapter III of The Call of the Wild indicates that Buck, a mix of a Scotch shepherd dog and a St. Bernard, retains the atavistic instinct of the wolf in him, the "dominant primordial beast." In addition, Buck is
preeminently cunning, and could bide his time with a patience that was nothing less than primitive.
Buck's innate strength and cunning serve him well because the club has knocked much of the "rashness" from him, so he can be patient. When the inevitable duel for leadership between him and Spitz happens, Buck is eager for it since he has been "gripped tight by that nameless, incomprehensible pride of the trail and trace" that makes dogs die happily in the harness. It is a pride that Buck recognizes in himself.
One day a rabbit has the misfortune to be discovered by Dub and the others give chase as it flees. As Buck delights in the pursuit of living game, he "sounds the deeps of his nature" and rounds a bend as Spitz grabs the rabbit, who shrieks in the forest air. When the other dogs cry out, Buck does not; instead, he feels his instinctive desire rise,
it was nothing new or strange, this scene of old time. It was as though it had always been, the wonted way of things.
Buck plays out the survival of the fittest; he attacks Spitz, but the fight becomes desperate until Buck uses his imagination and dives in low, breaking two of the legs on Spitz. Finally, Spitz disappears as the other dogs drag him off. Buck, "the dominant primordial beast" who had made his kill and found it good." Buck exemplifies the survival of the fittest as his instincts and imagination have answered "the call of the wild" and conquered Spitz.