To Fritz Müller 10 August 
Down. | Bromley. | Kent. S.E.
I have been for a long time so ill that I have only just finished hearing read aloud your work on species.1 And now you must permit me to thank you cordially for the great interest with which I have read it. You have done admirable service in the cause in which we both believe. Many of your arguments seem to me excellent, & many of your facts wonderful. Of the latter nothing has surprised me so much as the two forms of males.2 I have lately investigated the cases of dimorphic plants, & I should much like to send you one or two of my papers if I knew how.3 I did send lately by post a paper on climbing plants as an experiment to see whether it wd reach you.4
One of the points which has struck me most in your paper is that on the differences in the air-breathing apparatus of the several forms. This subject appeared to me very important when I formerly considered the electric apparatus of fishes.5 Your observations on Classification & Embryology seem to me very good & original6 They shew what a wonderful field there is for enquiry on the development of Crustacea; and nothing has convinced me so plainly what admirable results we shall arrive at in Natural History in the course of a few years.
What a marvellous range of structure the Crustacea present & how well adapted they are for your enquiry! Until reading your book I knew nothing of the Rhizocephala; pray look at my account & figures of Anelasma; for it seems to me that this latter Cirrepede is a beautiful connecting link with the Rhizocephala.7
If ever you have any opportunity, as you are so skilful a dissector, I much wish that you wd look to the orifice at the base of the first pair of cirri in Cirripedes, & at the curious organ in it & discover what its nature is; I suppose I was quite in error, yet I cannot feel fully satisfied at Krohn’s observations.8 Also if you ever find any species of Scalpellum, pray look for complemental males; a German author has recently doubted my observations for no reason except that the facts appeared to him so strange.9
Permit me again to thank you cordially for the pleasure which I have derived from your work & to express my sincere admiration for your valuable researches. Believe me | Dear Sir with sincere respect | yours very faithfully | Ch. Darwin
P.S. I do not know whether you care at all about plants but if so I shd much like to send you my little work on the Fertilization of Orchids & I think I have a German Copy.10
Could you spare me a Photograph of yourself, I shd much like to possess one?—
Has read and admires FM’s work on species.
Observations on Crustacea are good and original; asks FM to dissect and check some of CD’s observations on cirripedes.
Has sent "Climbing plants" paper [J. Proc. Linn. Soc. Lond. 9 (1865): 1–118] and would like to send Orchids.