1. 4 F Burkhardt, and S. Smith, Correspondence of Charles Darwin. Cambridge: Cambridge University Press, 1985 : 1: 81 , 88 . Darwin to W. D. Fox [1 April 1829] and [3 July 1829].
2. 5 On 27 December 1831, Charles Darwin embarked on a five year voyage on H.M.S.Beagle, commanded by Captain Robert FitzRoy. Darwin was the naturalist for the surveying voyage, which circumnavigated the globe.