what reason did jim give the map to keep him alive?

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Silver asks Jim to join the pirates. In response to Silver, Jim confidently explains that it is he who has been the downfall of the pirates - through the apple barrel, the cutting the schooner loose and killing the men on board, and bringing the ship to a place that the pirates will never find her. Jim concludes with, "I no more fear you than I fear a fly." He tells the pirates that they may choose whether to kill him or to spare his life, but if they let him live, he will try to help keep them from hanging. When Jim finally finishes his diatribe, none of the pirates move. Thinking that he is dead, Jim tells Silver to tell Dr. Livesy that Jim courageously stood up to the pirates and that Jim thought that Silver was the "best man here."