Darwin Online. http://darwin-online.org.uk
All manner of Darwin’s publications and letters (including the Journal of Researches) which you can download as pdf files.
Darwin, C. 1845. Journal of Researches into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. Beagle Round the World, Under the Command of Capt. Fitz Roy, R.N. 2nd edition. John Murray, London.