To Henry Denny 7 November 
Down near Bromley | Kent
I am much obliged for your note & have been greatly interested by the facts you mention of the identical parasites on the same species of birds at immensely remote stations. I am sorry to say I cannot think of any possible means of procuring the parasites of the S. American Mammifers to which you refer. Some surgeon, or officer, interested in Nat. Hist wd be the only means & I know none now there.
Are you aware whether the same parasites are found on any of our land birds in this country & in N. America. Some of the birds of Europe & N. America appear certainly identical; many form very closely related species or as some would think races: What an interesting investigation wd be the comparison of the parasites of the closely allied & representative birds of the two countries.
Should you chance to know anything of the parasites of the land-birds of North America, perhaps, sometime you kindly wd take the trouble to send me a line, as I am deeply interested in everything connected with geographical distribution, & the differences between species & varieties.— I hope you will turn in your mind the possibility of investigating closely the N. American land-bird-parasites.—
When the same bird in immensely remote countries, has the same parasite, do you never observe some slight difference in colour, size or proportions of such parasites? I have forgotten to answer your question, about the Aperea being identical with the guinea-pig, & I can only answer it by professing entire ignorance & doubt: I certainly shd disbelieve it, if you cd show the parasites were different. How is the parasite of the wolf with the dog, if the latter has one?
I hope you will excuse this long note & believe me dear Sir | Yours very faithfully | C. Darwin
Discusses HD’s information that same species of birds at remote stations have identical parasites. Urges him to investigate N. American land-bird parasites.
Is deeply interested in everything connected with geographical distribution, and the differences between species and varieties.
Please cite as
Darwin Correspondence Project, “Letter no. 787,” accessed on 25 April 2017, http://www.darwinproject.ac.uk/DCP-LETT-787