Capt. F. wants a man (I understand) more as a companion than a mere collector & would not take any one however good a Naturalist who was not recommended to him likewise as a gentleman. …The Voyage is to last 2 yrs. & if you take plenty of Books with you, any thing you please may be done— You will have ample opportunities at command— In short I suppose there never was a finer chance for a man of zeal & spirit. (Letter from J. S. Henslow, 24 August 1831)
It was this letter from his friend and former teacher, John Stevens Henslow, Cambridge University Professor of Botany, that brought the 22-year-old Charles Darwin news of the offer of a place on board the Admiralty surveying vessel, HMS Beagle, on a voyage to chart the coast of South America. Charles returned on 29 August 1831 from a geological fieldtrip in Wales with another former teacher, Adam Sedgwick, to find the letter waiting for him at home in Shrewsbury. Henslow had been asked to recommend a young man as a companion to Robert FitzRoy, the Beagle's captain; Darwin was not the first choice for the trip, but a combination of engaging social skills and an already evident appetite for natural history, brought him to the top of the list when first Henslow himself, and then Leonard Jenyns, was forced to turn it down.
It took several weeks to pursuade his reluctant father, Robert Waring Darwin, to give his permission and finance the trip, and more delays were caused by the refitting of the ship and by bad weather, but when the Beagle finally sailed from Plymouth on 27 December 1831, Charles was on board.
What was originally planned as a two-year voyage around the world stretched to five. During this time Darwin exchanged letters not only with his family, in particular his sisters, but with Henslow and others who could give him advice on how to make the most of the opportunities he had to observe and collect.
See also The geology of the Beagle voyage.
All the letters have been published and are available in an illustrated book Charles Darwin: the Beagle Letters. To find out more about the books that Darwin read during the journey, see the article on the 'Books on the Beagle'.