In Treasure Island, why are chapters 16,17,18 told by the Doctor?

The major difference between this chapter (ch 16) and the previous ones is the shift in narration, from Jim Hawkins to the Dr. Livesey. This shift was necessary in order to ensure that a first-hand account can be told of the events that Jim was not a witness to, because he was on land. Within the narration, there are certain clues that that Jim is no longer the narrator, including comments about places that might be likely breeding places for disease and comments concerning his time in the military. It is also interesting to contrast the two different narration techniques.