To Fritz Müller   10 August [1865]

Down. | Bromley. | Kent. S.E.

Aug 10

Dear Sir

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?—

LS(A)

1
The reference is to Müller 1864, a study of the Crustacea that was supportive of CD’s theory of transmutation. CD had received a presentation copy from Müller before 21 November 1864, but found the German difficult to understand and had it translated (see Correspondence vol. 12, letter to Ernst Haeckel, 21 November [1864] and n. 6). The translation was probably made by Camilla Ludwig, who had been governess at Down House between 1860 and 1863. CD’s Classed account book (Down House MS) records, under the heading ‘Science’, a payment to Ludwig of £5 11s. 6d. on 10 June 1865 for translation. An annotated copy of Müller 1864 is in the Darwin Library–CUL (see Marginalia 1: 609). CD’s annotated copy of the later translation by William Sweetland Dallas (Dallas trans. 1869) is in the Rare Books Collection–CUL (see Marginalia 1: 609).
2
In Müller 1864, Müller discussed the existence of two forms of male in some species of Crustacea and argued that this dimorphism could be seen as a result of natural selection. Müller’s example was a species of the genus Tanais. Müller found that one form of male developed enlarged olfactory filaments, making it better at finding females, while the other form had enlarged chelae (claws), giving it an advantage in clasping females (see Müller 1864, pp. 13–19 and Dallas trans. 1869, pp. 20–9). The absence of intermediate forms was explained by the lack of any advantage conferred by such hypothetical forms. CD added this information to Origin 4th ed., pp. 50 and 288.
3
CD sent ‘Two forms in species of Linum’, and ‘Three forms of Lythrum salicaria’; the last-named concerns trimorphism (see letter from Fritz Müller, 5 November 1865 and n. 2).
4
CD refers to ‘Climbing plants’, which was published on 12 June 1865 in the Journal of the Linnean Society (Botany). No presentation list for ‘Climbing plants’ has been found, but it is likely that CD sent Müller an author’s offprint. Müller resided at Destêrro on Santa Catarina Island off the coast of the state of Santa Catarina, Brazil. Destêrro is now Florianópolis.
5
Müller compared the structure of the air-breathing apparatus in certain non-aquatic species of two families of true crabs (Brachyura): shore crabs (Grapsoidae, now Grapsidae), and ghost and fiddler crabs (Ocypoda, now Ocypodidae). He observed that the structure and position of the apparatus differed in various ways in each family and argued that the differences could be explained only by assuming that each family adapted to an air-breathing existence independently (see Müller 1864, pp. 24–6, and Dallas trans. 1869, pp. 35–7). In Origin, pp. 193–4, CD wrote that the electric organs in fish of widely differing species appeared difficult to explain within the framework of his theory, but conjectured that they might be explained as analogous features, that is, structures with similar functions that have developed independently in response to similar habits. In the fourth edition of Origin, pp. 226–8, he added Müller’s observations on these crab families.
6
For Müller’s observations and remarks on crustacean embryology, see Müller 1864, pp. 31–65; his remarks on classification are on pp. 65–74 (see also Dallas trans. 1869, pp. 47–96, pp. 97–109). Müller noted that the use of embryological characteristics, although helpful in determining classificatory schemes, was by no means foolproof. He gave numerous examples of species or genera in which embryological similarity did not translate into adult similarity, or in which embryonic and adult stages were similar but the intermediate nauplius (larval) stages were disparate. He then devised a hypothetical classificatory scheme for the Crustacea to show the pitfalls of placing too great an emphasis on embryonic similarity (Müller 1864, pp. 73–4; Dallas trans. 1869, p. 109). In the fourth edition of Origin, p. 494, CD added a reference to Müller’s classificatory scheme.
7
Rhizocephala is the name coined by Müller (see Müller 1862a and 1862b) for a highly unusual group of crustaceans that as adults are naked and limbless parasites attached to their crab hosts by means of root-like tubes, and that appear from the outside as sausage-like sacs. Müller had noted their similarity, in pre-adult stages, to cirripedes, placing them, not within what Darwin had described as the sub-class Cirripedia (barnacles), but alongside it (Müller 1864, p. 59; Dallas trans. 1869, pp. 88–9). CD had described Anelasma as a ‘curious cirripede’ that lived ‘parasitic, with its peduncle imbedded in the skin of sharks’ (Living Cirripedia (1851), p. 170). When he described the species A. squalicola, he could not find a cement gland or ducts, but noted that the peduncle had root-like branched filaments that, he conjectured, helped the animal to remain embedded in the shark’s skin (Living Cirripedia (1851), p. 174). After reading Müller’s account of Rhizocephala, CD may have considered the branching filaments in Anelasma to be an intermediate form between the cement ducts of other cirripedes and the root-like tubes of Rhizocephala.
8
In Living Cirripedia (1851), pp. 53–5, CD had described the empty orifice, which seemed to contain a nerve at its mouth, as possibly an auditory sac. His description was criticised by August David Krohn, who identified the structure as a female genital aperture (Krohn 1859, pp. 362–3). In a letter to the Natural History Review, [before 10 October 1862] (Correspondence vol. 10), CD referred to Krohn’s paper, and asked whether any reader would ‘endeavour to discover ova within the so-called auditory sack’ (‘“Auditory-sac” of cirripedes’; Collected papers 2: 87; see also Correspondence vol. 8, letter to Charles Lyell, 28 [September 1860] and nn. 4–6). For a discussion of the problem of the orifice see Newman 1993, pp. 368–71.
9
CD refers to his discovery in 1848 of the small ‘complemental’ males attached to the bodies of hermaphrodites in the barnacle genus Scalpellum (see Living Cirripedia (1851), pp. 281–93, and letter from Henry Holland, 2 January 1865, n. 5). CD did not name in his published writings the German author who doubted this discovery; however, in 1863 the German zoologist Carl Eduard Adolph Gerstaecker had questioned CD’s account of the existence of complemental males in his Handbuch der Zoologie (see Gerstaecker 1863b, p. 410). CD referred to the criticism in a letter to Nature, 20 September [1873] (‘Males and complemental males of certain cirripedes’), and in his Autobiography, pp. 117–18. For CD’s discussion of complemental males in the genera Scalpellum and Ibla see Living Cirripedia (1851), pp. 281–93 (see also Newman 1993, pp. 377–82, for a discussion of CD’s work on complemental males). See also Correspondence vol. 12, letter to J. D. Hooker, 26 November [1864] and n. 4.
10
CD refers to Orchids, which was translated into German in 1862 by Heinrich Georg Bronn (Bronn trans. 1862).

Manuscript Alterations and Comments

7.1 Could … one?—-] added in CD’s hand

Please cite as “DCP-LETT-4881,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-4881