William Darwin Fox was Charles Darwin's second cousin, a keen entomologist (he was particularly fond of beetles) with whom Charles had a close relationship throughout his life. From the many letters Charles wrote to Fox while on the Beagle voyage come the following:
From near Rio de Janeiro, May 1832
I am entirely occupied with land animals, as the beach is only sand; Spiders & the adjoining tribes have perhaps given me from their novelty the most pleasure.— I think I have already taken several new genera.— But Geology carries the day; it is like the pleasure of gambling, speculating on first arriving what the rocks may be; I often mentally cry out 3 to one Tertiary against primitive; but the latter have hitherto won all the bets.
And from Lima, August 1835
I look forward to the Galapagos, with more interest than any other part of the voyage.— They abound with active Volcanoes & I should hope contain Tertiary strata.— I am glad to hear you have some thoughts of beginning geology.— I hope you will, there is so much larger a field for thought, than in the other branches of Nat: History.— I am become a zealous disciple of Mr Lyells views, as known in his admirable book.— Geologizing in S. America, I am tempted to carry parts to a greater extent, even than he does. Geology is a capital science to begin, as it requires nothing but a little reading, thinking & hammering.— I have a considerable body of notes together; but it is a constant subject of perplexity to me, whether they are of sufficient value, for all the time I have spent about them, or whether animals would not have been of more certain value.—