Oliver’s letter has not been found. Ferdinand Julius Cohn had written an article on the function of the bladders of Aldrovanda and Utricularia (Cohn 1875a). Cohn had sent CD a galley proof of Cohn 1875a in October 1874 (see Correspondence vol. 22, letter from F. J. Cohn, 4 October 1874). CD’s annotated proof copy of Cohn 1875a (not including illustrations) is in DAR 58.2: 35–43.
Ferdinand Cohn has already sent a copy of his article, [possibly: "Über die Function der Blasen von Aldrovanda und Utricularia", Beitr. Biol. Pflanz. 1 (1870–5) pt 3: 71–89].