 Before they left England, FitzRoy gave Darwin a copy of the first volume of Lyell's "Principles of Geology" which explained features as the outcome of a gradual process taking place over extremely long periods of time.In his autobiography Darwin recalled Henslow giving advice at this time to obtain and study the book, "but on no account to accept the views therein advocated".Darwin's position as a naturalist on board was as a self-funded guest with no official appointment, and he could leave the voyage at any suitable stage.