No reply from Bartlett has been found. It is not known why CD required the duck skins, but see also the letter to A. D. Bartlett, 15 September 1871, which suggests that CD was interested in the structure of the beak. CD may have been working on countering Mivart’s objections to the possibility of the evolutionary development of the baleen sieve in the mouth of the whale (see letter to Francis Darwin, [after 21 January 1871] and n. 5). Dabbling ducks such as mallards use lamellae (comb-like projections on the bill) to sieve food particles from pond water.