What do Thornton and his partners find after a long search? Why does this cause Buck to move toward the call of the wild?

Thorton and his partners finally find gold. Their discovery certainly occupies much of their time; as a result, Buck has little to do and lies around in this primordial wilderness with dreams of the "ape-like" man who first subdued him. After a while, he hears the sounds of an atavistic call in the forest, sensing an instinctual pull from this wilderness. 

After the massively strong Buck wins a huge bet for Thorton, the man is able to pay off his debts and follow his dream of going with his partners to the East country in search of a fabled lost mine. They trek through wild country, hunting for their food on the way.

To Buck it was boundless delight, this hunting, fishing, and indefinite wandering through strange places.

The men travel for over a year and still do not find the Lost Cabin of legend. However, in the Spring, they find a marker and look out at the valley "where gold shone like yellow butter." As the men toil assiduously each day, stacking bags of gold by trees, Buck lies dreaming as there is little for the dogs to do except haul meat for the men. So, as he muses by the fire, Buck recalls the memory of the ape-like figure of Perrault, who seemed more at home in trees than on the ground as he swung effortlessly from branch to branch. This memory of his introduction to the life of the wild, triggers "the call still sounding in the depths of the forest."

Further, Buck begins to have a sense of unrest, and he experiences new desires as he lies idle. 

It caused him to feel a vague, sweet gladness, and he was aware of wild yearnings and stirrings for he knew not what.

This is the "call of the wild." It awakens him sometimes in the night as he sleeps. On one occasion Buck decides to follow the sound and happens upon a lone wolf howling; he chases the wolf into "one blind channel" after another, although he intends the wolf no harm, only hedging him in with friendly advances. After some time, the wolf approaches in a submissive pose and they sniff noses, becoming friends. Then the wolf lopes off and Buck follows.

Buck was wildly glad. He knew he was at last answering the call, running by the side of his wood brother toward the place from where the call surely came.

Though Buck returns to camp and Thorton, he begins to spend more and more time in the woods, where his atavistic nature begins to dominate him. When he returns to camp one day, he finds his beloved master and the others slain by the Yee-hats. Buck ferociously attacks these cruel foes of his beloved master, and after fighting them for some time in which he kills some, the others flee in the belief that they have witnessed the Evil Spirit. As he stands in the middle of the ruined camp, Buck realizes that all ties to civilization are broken in him, and he responds now only to "the many-noted call of the wild."