How does the narrator show that Buck is becoming more like his wild, untamed ancestors?

The author shows that Buck is becoming more like his wild, untamed ancestors by highlighting the gradual change in his attitudes towards his primal instincts and by illustrating the compounding ferocity that corresponds to his metamorphosis.

Accordingly, during the journey to look for a lost mine, John Thornton and his group of adventurers stumble upon gold, which they harvest into fifty-pound moose-hide bags. It is during this last journey with his beloved master that Buck becomes fully aware of 'wild yearnings and stirrings' calling to him from the depths of the forest. The strange call evokes in him both apprehension and happiness. He finds himself compelled to give way to 'irresistible impulses'; the author describes how Buck gives in to his urge to roam the forests in the 'dim twilight of the summer midnights' and how he constantly stays alert in listening for the strange beckoning sounds of the woods. Basically, the author uses visual and auditory imagery to alert us to Buck's new sensitivity to his surroundings.

The author then sets the stage for Buck's complete metamorphosis from domestic dog to untamed canine by highlighting Buck's eventual capitulation to the call of the wild. Buck answers the mysterious howling call of a timber wolf by following the wild creature. The two become canine friends, and the author asserts that, as Buck continues to spend more time with his wolf brother, he begins to recover long-forgotten memories of his wild ancestry. Buck thinks that he has roamed the wilderness before, free and independent of all civilized constraints, in a nebulous past.

The author also skillfully illustrates how Buck resolves the conflict within himself as his untamed nature is pitted against his domestic inclinations. Buck does return to his master for two days and two nights, but he soon feels restless again. It isn't long before he again tries to seek out his 'wild brother' of the wilderness. Eventually, the death of John Thornton at the hands of the Yeehats becomes the catalyst for Buck's complete transformation from sled dog to wild canine of the forest.

Heartbroken at John Thornton's senseless death, Buck gives in to his ferocious and violent nature, 'tearing, rending, and destroying' as many of the Yeehats as he can; he executes a vicious and brutal vengeance for his beloved master's murder. Using kinesthetic imagery, the author delineates the massacre for us in violent detail. With John Thornton's death, the last link to civilization is broken, and Buck becomes free to pursue his deepest desire to answer the call of the wild. The author basically shows Buck's transformation by illustrating Buck's gradual capitulation to his primitive nature and expertly using visual, auditory, and kinesthetic imagery to illustrate Buck's evolving character.