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Buck really doesn't trust men anymore. He has been betrayed by a human thought to be friendly. This begins to awake Buck's nature instincts as a dog separated from the word of man.
This event changes Buck’s relationship with humans. Up to this point, Buck has always been treated well by all of the
humans he has encountered; the paragraphs before say that he was the ruler of the humans, not the other way
around. This paragraph says that “he had learned to trust in men he knew…” Now, suddenly, someone is placing a
rope around his neck—and he learns that he cannot trust all men. Manuel was a human who Buck knew and did not
see as a threat. Because Buck knew him and trusted him, Manuel was able to steal Buck and sell him to a man at the
station. Now, all men are a threat and he reacts by growling threateningly—and then by reacting violently.