What happens to Buck after he leaves Judge Miller's home in Call of the Wild?

The theft and sale of Buck in The Call of the Wild completely alters the dog's life as it reverses the generations of domestication of his breeds of St. Bernard and the Scottish shepherd and returns him to the atavistic nature of his ancestors.

Once Buck is in the possession of his captors, his freedom and kind treatment by humans vanishes. He becomes no more than a caged animal, at first. Then, he finds himself harnessed to a team that pulls a sled in Alaska. "He swiftly lost the fastidiousness which had characterized his old life." He begins to gobble down food lest another take it from him, and he steals what he can. These actions marks his adaptability and his new owners are pleased.

Because the "dominant primordial beast was strong in Buck" certain traits emerge in him such as cunning and deliberateness. For instance, he avoids fights, and although he hates his rival, the lead dog Spitz, Buck does not display his feelings and avoids offensive acts that would spur Spitz to fight. But, one day when Spitz tries to steal the nest Buck has made under a rock, he feels the "beast in him" roar, and he fights the usurper. However, as they fight, "famished beasts" appear, trying to steal as much food as they can. Buck fights them fiercely. Finally, one day Buck kills Spitz and takes over as the lead dog.

Later, Buck finds himself on another team that carries heavy loads of mail. When he and the other dogs arrive, they are in a "wretched state, worn out and worn down." One of the dogs is shot because he is so weakened. Afterwards, the owners sell Buck and his team to three eager prospectors. Unfortunately, they have little knowledge of the climate or dogs, and they overload the sled and have planned poorly for the journey they undertake. The woman, Mercedes, overfeeds the dogs early on and they run out of food for the dogs. By the time that they arrive, out of fourteen only five have survived. When the man, Hal, tries to beat the dogs and force them to cross thin ice, Buck's instincts tell him to rebel because of danger. When he does so, Hal beats him fiercely, but John Thornton intervenes, rescuing Buck from his insensitive owners who fall through the ice.

With Thornton as his owner, Buck lives happily; in his devotion to the man who has saved him, he, in turn, saves his owner from drowning and from an attack by another man. Buck even wins over a thousand dollars for Thornton as he pulls a load weighing about half a ton. Over time, however, more of the primordial beast emerges in Buck and he wanders off to hunt real game such as moose. He also mingles with wolves, but he still returns to camp at night. One time, however, Buck returns to find his owner has been killed by Yeehat Indians. Fiercely, he kills several of the Indians and runs off the others. Afterwards, Buck charges into the wild,; there he hears yelps that act as that atavistic call which persists in his memory. 

It was the call, the many-noted call, sounding more luringly and compelling than ever before.

Now without his human owner, Buck answers this call. When a pack of wolves come one day, Buck kills the boldest one and fights the others. But, when an old wolf approaches and sniffs his nose peacefully, then turns up his head and howls at the moon, Buck, too, howls. After this night he runs with the pack of wolves, yelping as they do. In time Buck becomes a legendary figure as the Indians who yet fear him call him the Ghost Dog.