London begins this chapter by stating that the "dominant primordial beast was strong in Buck," meaning that the primitive will to survive was now the primary factor in Buck's life. Thus, the entire chapter is constructed to show Buck's will and determination to survive. Buck's survival instinct will be climaxed at the end of this chapter in a dramatic fight-to-the-death scene between Buck and his archrival, Spitz. Just as the chapter begins with an emphasis on "the dominant primordial beast," it ends with the same words, emphasizing the central concern of the chapter: the survival of the fittest.
At the beginning of the chapter, Buck has been avoiding fights whenever possible, but there still exists a bitter hatred between him and Spitz, and if we review Chapter 1, where Buck had his first encounter with the Spitzbergen (Spitz, in shortened form), we will remember that Spitz is the dog that first stole Buck's food. Furthermore, at the beginning of Chapter 2, Spitz is the dog who watched with pleasure when Curly was killed. And now, in this chapter, Spitz is the leader of the dogsled team, which arouses Buck's jealousy. Buck was accustomed to being the head dog at Judge Miller's estate, and now he has to take a subordinate position, and it is insulting to his pride. An ultimate confrontation between Buck and Spitz seems inevitable, and in this chapter, we are first given a short, but vicious encounter between Buck and Spitz, when Buck returns to his nest to find Spitz occupying his sleeping hole. This arouses the "beast" in Buck, and he springs upon Spitz "with a fury which surprised them both." There probably would have been a fearful, bloody fight at that time if it had not been for François and Perrault, who quickly surmise the situation and put an end to the fight. At the same time, however, the fight between the two dogs is delayed by the sudden appearance of "four or five score of starving huskies" who invade the camp. These starving huskies ignore the clubs which flail them, and they attempt to consume all of the food supplies.
In the meantime, the sled dogs have burst from their nests in order to protect their food. Then, however, all of the team dogs are attacked by the wild dogs and even the team dogs revert to primitive behavior. London tells us that they "fought the wild dogs with a fierceness." Buck is especially excited by the taste of blood after he sinks his teeth into one of the wild dogs. The taste of blood goads him to "greater fierceness." The sled team dogs eventually escape to the wilderness, but are seriously wounded. Some lose an eye, some lose an ear, and all have gaping wounds; eventually, though, they return to the camp site and find that François and Perrault have driven the wild dogs away. Half of the food supply, however, is gone. This key incident shows the wild, untamed fury of the starving animals who contain a fury which indicates the instinctual desire for survival. Furthermore, in the encounter with the wild huskies, Spitz, rather than fight against a common enemy, uses the opportunity to attack Buck — on two different occasions. The first time occurs when Buck is fighting a wild huskie, and Spitz viciously attacks him from the side; the second time occurs when Spitz rushes upon Buck in an attempt to throw Buck in the path of the wild huskies, an event which Buck realizes would have meant certain death. There have now been three encounters between Buck, the dog of the Southland, and Spitz, the opportunistic dog of the Northland.
With the food supplies half gone, and with four hundred miles of wild trail ahead, François and Perrault begin to lead the dogs on a dangerous journey. It takes the sled team six days to cross the thirty miles that make up the "Thirty Mile River" because the ice keeps breaking under them, and they have to stop and build fires to dry out or else they will freeze to death. At one time, the ice breaks and Spitz, the lead dog, falls in, dragging the entire team, except Buck, in with him. During the rescue, which requires most of the day, another day of travel is lost.
Being a dog from civilization, Buck's paws are not accustomed to the harshness of the trail, and François often has to bring Buck's food rations to him; finally, he makes a pair of moccasins which will fit over Buck's paws. They are a great relief to the dog. One morning, a surprising event occurs. The dog Dolly, a particularly mild-mannered dog, suddenly begins "a long, heartbreaking wolf howl," which indicates that she has suddenly gone mad. Frothing at the mouth and snarling, Dolly begins to chase Buck, who flees in confused terror from her. After plunging through the woods and through ice, with Dolly snarling only one leap behind him, Buck finally passes through the camp site, where François holds an ax, poised in his hand. Thus he does what he must: he smashes the ax down on the mad dog's head.
The incident is exhausting, and Buck staggers along the trail, completely worn out. The nasty tempered Spitz takes Buck's weakened condition as an opportunity to attack him once more, and twice he tears Buck's flesh to the bone. François, however, is close by, and using the whip, he beats Spitz away. But Buck can no longer forgive Spitz, and from this moment on, it is all-out war between Spitz and Buck. Buck constantly challenges Spitz's authority and uses every chance he can to undermine Spitz's authority. London writes that it was "inevitable that the fight for leadership should come." Buck's pride is such that he does not like to be subservient to any dog — most of all to Spitz.
All the way to Dawson, there is continual bickering between Buck and Spitz, and this conflict is important, of course, because ideally a dog team must work as a single unit, and when a dog mutinies, that dog ceases to be effective. More important, though, is London's focus on the struggle between the civilized dog from the South and the primitive dog from the North. London's point here is that the cunningness that Buck learned in civilization combined with his superior strength and his newly acquired primeval instincts make Buck the superior animal in what will be a coming, critical contest between him and Spitz.
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."