The morning after the fight, Françoisnotices that Spitz is missing and that Buck is covered with wounds. He surmises that Buck has killed Spitz, but proceeds with harnessing the dogs. Buck walks to take Spitz's position at the head of the traces. Yet François harnesses Sol-leks there. Buck, indignant, lunges at Sol-leks, who stands back. François grabs Buck by the scruff of his neck, removing him from the traces, then threatens him with the club when Buck attempts to assume Spitz's position again. Buck eludes the club's reach, until François and Perrault relent by fastening Buck in the lead position.
Buck's unwillingness to be harnessed in any position but that of lead dog underlines his conviction that he is the rightful leader of the team. By standing firm in the traces, he affirms his dominance over the other dogs. Yet Buck's leadership is only confirmed when his human masters confer that honor upon him by harnessing him in the lead position. While Buck dominates the pack, his authority is predicated upon his human owners' approval.
On the trail, Buck proves to be an excellent lead dog. He conducts the difficulties of the trail with quick judgment, keeps his teammates in line, and sets high work expectations. The team makes record time from Thirty Mile River to Skaguay, covering an average of forty miles a day over fourteen days.
In Skaguay, François and Perrault become minor celebrities for the record timing of their run. They celebrate for a week's time, but official orders from the government force them to depart the town, thereby leaving Buck and his team behind. François weeps over Buck as he and Perrault exit Buck's life "for good."
François and Perrault's departure from Buck's life signals the end of one phase in Buck's relationship with man. They have been wise and just masters to Buck. That François weeps over Buck shows the level of deep respect and care he has towards him.
Another courier, called the Scotch half-breed, (also referred to as the Scotsman), takes charge of Buck's sled dog team, adding a dozen more dogs to the pack. Under a heavy load of mail, the dogs toil their way slowly towards Dawson to make the delivery. The labor is monotonous and wearing, but in the evenings, Buck begins to have visions of a "hairy," "short-legged" caveman, clad in animal skins, squatting by the fire.
Buck is sold again, bringing a new master into his life. Buck's team suffers under the Scotsman's care because his course is driven by society's insatiable demands for material goods. Buck's dream of a primitive man shows that his memory is receding into a primeval past, but also that he is looking for something more in a human master, an equality and cooperation that is impossible when he works for humans.
Buck awakens from his dream-like state to face the harsh realities of life on the trail. Dave comes down with a mysterious illness, becoming so weak that he can barely stand in the traces. The Scotsman removes Dave from the traces, putting Sol-leks in his place. Dave resents being displaced; he refuses to walk alongside the sled by firmly standing in his place at the rear. In a merciful gesture, the Scotsman harnesses Dave in the traces. They drive on, but the next morning Dave, collapsed, cannot even walk over to the sled that's pulling away. The Scotsmen halts the sleigh, retraces his steps, and the dogs hear a gunshot ring out.
Dave's unswerving determination to remain in the traces demonstrates his pride in and loyalty to his work. Dave's death also shows the interplay between man and the laws of nature. Although the Scotsman drives his animals to near exhaustion, he exercises some judiciousness in taking the laws of nature into his own hands. He shoots Dave out of mercy, to save him from a long and painful natural death on the trail, but he also respects the dog's dying wish to serve in the traces for as long as he can.