What happens when Buck begins to feel the call of the wild in The Call of the Wild by Jack London?

When Buck first hears the call of the wild in the night, he springs to his feet and runs in the direction of the sound, plunging into the forest; he only goes so far, however, before returning to camp and Thornton. 

Some time later, Buck is seized by irresistible impulses during the day: 

He would be lying in camp, dozing lazily in the heat of the day, when suddenly his head would lift and his ears cock up, intent and listening, and he would spring to his feet and dash away, and on and on, for hours, through the forest aisles.

Each time Buck ventures further. One night he hears the call of the wild and pursues it. After he follows the sound into a thicket of trees, Buck nears the source of this sound cautiously. When he arrives at an open place among the trees, Buck spots a timber wolf, who is seated on his haunches with his nose pointed upwards to the night sky. 

Buck approaches the wolf in a semi-submissive posture — "the menacing truce that marks the meeting of wild beasts that prey" — but the wolf runs off. Pursuing the wolf, Buck chases it into a creek bed that is dammed at one end; feeling trapped, the wolf stands on its hind legs baring his teeth and snapping them rapidly. When the wolf has a chance, it runs. Although Buck follows the wolf, Buck shows no aggression towards it. Finally, the wolf sniffs his nose and they act in a friendly manner, maneuvering around each other playfully. For a while Buck runs with the wolf, but after they go some distance, Buck remembers John Thornton, and he sits down. The wolf returns to him, even running beside him as he heads back to camp. Soon, the wolf sits and howls mournfully while Buck continues on alone. Once in camp, Buck scampers to Thornton, demonstrating all his affection. After his return, Buck does not venture out of the camp for two days.