What does London say is the first time Buck harkens back to the life of his forebears?

The first time would be when Buck is trapped in the snow in Chapter Two. 

The snow walls pressed him on every side, and a great surge of fear swept through him—the fear of the wild thing for the trap. It was a token that he was harking back through his own life to the lives of his forebears; for he was a civilized dog, an unduly civilized dog, and of his own experience knew no trap and so could not of himself fear it.