The deadly rivalry between Spitz and Buck grows. One night Buck makes his nest under a sheltering rock, but when Buck returns after eating his food, he finds that Spitz has taken it over. He attacks Spitz, but their fight is interrupted when the camp is invaded by about fifty starving huskies who have scented the camp from an Indian village. They try to steal all the food. Three of them attack Buck, and the other sled dogs are attacked, too. Francois and Perrault drive them off. All the nine team-dogs are injured. They regroup in the forest and return to camp at daybreak.
They set off again, down the partially frozen Thirty Mile River, covering only thirty miles in six days. The temperature is fifty below zero. Once, the sled falls through the ice on the river, and Buck and Dave are nearly drowned. Another time, Spitz goes through the ice, and only the strength of Buck and the others saves them all. Then the ice breaks and they have to escape by climbing a cliff. That day they cover only a quarter of a mile.
Weathering all the difficulties, they persevere. Buck's feet are not as tough as those of the other dogs, and get very sore. Francois makes moccasins for him out of the tops of his own moccasins. Later, Buck's feet get harder, and the moccasins are thrown away.
One morning, Dolly suddenly goes mad, and Francois kills him with an axe. Spitz attacks Buck but is driven off by Francois's whip. From that point on it is a war between Spitz, the lead dog, and Buck, since Spitz feels threatened by Buck. On another occasion, when Spitz is punishing the malingerer Pike, Buck attacks him, but this time it is Buck who is driven off by the whip wielded by Francois.
They arrive in Dawson, in the Yukon territory. They spend a week there, before making the return trip, with Perrault carrying important dispatches. They cover fifty miles in the first day. But the dog-team is not harmonious, since Buck's rebellion against Spitz has destroyed the solidarity of the team. In a breakdown of discipline, the other dogs also challenge Spitz.
As the dog-pack chases down a rabbit, with Buck in the lead, Spitz cuts him off and gets to the rabbit first. This initiates a fight between the two dogs. Buck is triumphant, and Spitz is killed.
This chapter vividly shows the difficulties and the dangers that the travelers encounter, both external and internal. (Madness afflicts one of the dogs.) As the strongest, Buck inevitably triumphs over Spitz. It is his destiny to become the "dominant primordial beast." He uses cunning as well as strength, and he is infinitely patient. He knows how to use his wiles to undermine Spitz's authority with the other dogs, and as a result there is general insubordination amongst the dog-team. Buck has left a human community, with tame animals, but has found himself in another, more ruthless kind of animal community in which the pack dominates, and every pack needs a leader.
In regressing to a more primitive state, and becoming leader of the pack, Buck discovers for the first time a kind of superior joy, greater than anything he could have known on Judge Miller's ranch. He is getting in touch with the energy of life itself as it surges up within him, not tempered by any civilized or moral values, which, from this point of view, dilute rather than enhance life. London makes this clear in eloquent words: "There is an ecstasy that marks the summit of life, and beyond which life cannot rise. And such is the paradox of living, this ecstasy comes when one is most alive, and it comes as a complete forgetfulness that one is alive." In the harsh conditions that prevail in Alaska, in which Buck must be constantly alert, and is constantly put to the test, he discovers his true nature.