After seven days in Dawson, during which time the dogs recuperate, they begin the return trip. It is imperative, however, that they cover the distance as rapidly as possible, but this is partly facilitated by the fact that they are going to travel light, and that the mounted police have deposited supplies along the way, plus the fact that the trail that they have already broken will be easier to travel. In spite of these added conveniences, though, the trip is slowed by the dissension between Buck and Spitz, and this is due, in large part, to the fact that Buck has undermined the authority of Spitz, and this has caused the other dogs to rebel. It is now even more obvious that the inevitable fight between Buck and Spitz will take place — and soon.
This climactic fight occurs when one of the dogs, Dub, startles a snowshoe rabbit. As the rabbit attempts to escape, all of the other dogs join in the chase. The chase greatly excites Buck's primitive, ecstatic instincts — "the blood lust, the joy to kill"; he is impassioned to kill with his teeth and savor the fresh, warm blood. Spitz, however, while Buck is enjoying the chase, becomes cold and calculating; he cuts across a narrow neck of land and captures the rabbit. Buck is furious and is so frenzied by the wild desire for fresh blood, as well as so fiercely enraged over Spitz's actions, that he realizes that the time has finally come — that the time for the critical battle between him and Spitz has arrived.
Spitz is the more experienced fighter and is able to ward off most of Buck's charges. In fact, Buck almost exhausts himself in his repeated, vicious attacks, as Spitz slashes Buck's shoulder; Spitz is, for a while, seemingly the superior dog. But just as Buck reaches the point of exhaustion, he uses that "quality that made for greatness — imagination." Previously, Buck was fighting by instinct, but now he chooses to fight by imagination and intellect. He pretends to make one more half-hearted dash at Spitz's throat, but, instead, he quickly reverses himself and crunches down on Spitz's left foreleg, breaking it. After successive tries, repeating the same gesture, Buck is able to break Spitz's other foreleg. By now, Spitz is down, and the other dogs begin circling around them, waiting for the kill. In London's words, "Buck was inexorable. Mercy was a thing reserved for gentler climes." Thus, the bestial instinct has been proven in Buck's superiority to Spitz, and after he has defeated Spitz, we see that Buck a previously highly civilized dog from the South, has now defeated the strong, brave, uncivilized dog from the North. Buck has become, without question, "the dominant primordial beast."
