"Pass we therefore to the abnormal varieties of domestic fowls; respecting which one important generalization occurs to me, which is that all appear to have originated in those countries to which the species is indigenous, or in the immediate neighbourhood of those countries.  The crested Polish may be an exception; & this is one of the least abnormal among them.  Though now diffused over the whole habitable globe, & so thoroughly reclaimed that it does not appear to have anywhere returned to wildness, though subjected to the extremes of climatal and local influence, it does not appear that the latter have produced appreciable result; & from this and analogous facts which I could cite, I do think that those supposed influences have been immensely over-rated!  China, Indo-China, & the Malay countries, though more especially the two former, seem to me to be the regions in which the abnormal varieties of domestic fowls have originated, -- not India, because I know of none peculiar to it, & the Hindus have some absurd prejudice against common poultry (which I never could get explained), even the touch of them being contamination to these most inconsistent people.  E.g. With all their absurd nonsense about food, they scruple not to drink filthy water, &c &c.  Now the Chinese & Indo-Chinese are just the very people to be taken with any outlandish monstrosity, & to try & propagate it as a curiosity, & to keep such a breed up by selection of parents, the China-men evincing a most especial taste for monsters of all kinds (witness their carvings) & shewing the same perverted taste in their floriculture, dwarfs of trees, &c &c..."  1

1      Fan, Fa-ti  British Naturalists in Qing China: Science, Empire, and Cultural Encounter (Cambridge, MA and London: Harvard University Press; 2004), pp. 119-120: "My research has revealed that textual scholarship was more relevant to nineteenth-century natural history than had been assumed.  In fact, Darwin himself used many examples from Chinese texts to support his arguments in natural history.  Attempting to prove his hypothesis of natural selection, which he compared to breeding and artificial selection, Darwin endeavored to compile evidence about the domestication of animals and plants in history.  In 1855 he asserted that many abnormal varieties of domestic animals had originated from China and Indo-China, "the China-men...dwarfs of trees, &c&c."  Footnote 125 on pg. 215 cites this from Frederick Burkhardt et al, eds., The Correspondence of Charles Darwin (Cambridge, England: Cambridge University Press, 1989), vol. 15, 448.  Fan's reference brought the letter to RJB's attention;

"Letter 1761 - lyth, Edward to Darwin, C. R., [30 Sept or 7 Oct 1855]," Darwin Correspondence Project, http://www.darwinproject.ac.uk/darwinletters/calendar/entry-1761.html.    Note, however, that the actual letter as cited by this authoritative source was actually to Darwin, not from him as Fan mentions.