Blackwall wrote four letters on the subject of spiders to
Charles Darwin, dated 12 February 1868, 18 February 1868, 10 August 1869 and 8 September 1869. They survive in the Darwin Archive at
Cambridge University Library. The first, second, and third letters are in direct response to communications from Darwin, though the whereabouts of these letters, presumably kept by Blackwall, are unknown. Their subject matter is, broadly, variation among spiders. The first letter begins ==Legacy==