What incident teaches Buck a very important lesson soon after he is off the ship?

Soon after the dogs are taken from the ship, Curly is killed simply for trying to make friendly advances towards a husky dog only half her size. The dog takes her down with efficient, quick technique. Once it is certain that Curly will not rise again, the rest of the dogs in the camp attack and kill her. Spitz, the lead dog of Buck's team, laughs. Buck hates him bitterly. He has learned that fair play does not exist, and he resolves that he will never go down.