The title says it all. From the moment Buck is snatched up and sold he was thrust into a new life reminiscent to that of his ancestors. Though I could sit here and nit pick my way through the story, I like to think of Bucks physical journey as a conditioning. Everything from the first beating and the revelation of fang and claw, to the laborious work and hardships as a sled dog, he finds within himself a natural instinct that at first surprises him, but over time develops him. If it weren't for the harsh world in which he dwelt mixed with his own found ferocity he would not have survived. But as he was surviving he was growing and digressing. Like a kite takes to the wind, he tore through the layers of his being and found the dormant primordial beast that enveloped completely at the end.
During the finial chapters of the book Buck gives in to the pull of the forest. It takes him so vivaciously that it starts to become him. A real turning point comes when John T. is murdered. I have read some other threads and allot of you say it's with his decision to stay and hunt the moose, but it's just to premature. Admittedly though it was a major stepping stone. I believe he viewed the bull moose as a challenge, something that needed to be done to once and fore all lay down even the smallest of thoughts or doubts he may have had subconsciously. As the victor he takes time to bask in his glory. Remember the smaller moose he took down? He left it for sometime and chased a few wolverines away upon his return. But this his greatest kill he stayed next to it for the next couple of days. Basking.
But the love he had for John was the one thread that would always pull him back. And thus upon his arrival at camp and seeing what fate had befallen Thornton, that last thread of attachment snaps so surely and completely that he begins to savagely attack the Yeehat Indians. When it's over one of his last thoughts was he would never be afraid of man again, weary of club and arrow, but never afraid. And as the anger still boils within him and the finial match with the wolf pack Buck not so much as lets go rather than releases himself to the call. The call in which would run past his own mortality and through the voice of legend.