Which excerpt from The Call of the Wild best shows that Buck is entering into a new episode in his life?

Buck did not cry out. he did not check himself, but drove in upon spitz, shoulder to shoulder, so hard that he missed the throat. they rolled over and over in the powdery snow. spitz gained his feet almost as though he had not been overthrown, slashing buck down the shoulder and leaping clear. twice his teeth clipped together, like the steel jaws of a trap, as he backed away for better footing, with lean and lifting lips that writhed and snarled. read this passage. explain what the conflict shows about buck and spitz.