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Darwin Correspondence Project

To Fritz Müller   31 July [1867]1

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

July 31.

My dear Sir

I received a week ago your letter of June 2. full as usual of valuable matter & specimens.2 It arrived at exactly the right time, for I had just completed, & was enabled to correct a pretty full abstract of your observations on the plant’s own pollen being poisonous. I have inserted this abstract in the proof sheets in my chapter on sterility, & it forms the most striking part of my whole chapter.3 I thank you very sincerely for these most interesting observations, which however I regret that you did not publish independently. I have been forced to abbreviate one or two parts more than I wished—viz. about the fertility of the Epidendreae & on the advantage to the plant of the pollen being poisonous, for I was not quite sure that I understood parts of your letter.4 The seeds which you have sent are very valuable.5 Your letters always surprize me from the number of points to which you attend. I wish I cd make my letters of any interest to you, for I hardly ever see a naturalist & live as retired a life as you in Brazil. The terrestrial Orchis with the pollen-staff seems very curious; my first impression was that it was allied to Spiranthes. With respect to mimetic plants I remember Hooker many years ago saying he believed that there were many, but I agree with you that it wd be most difficult to distinguish between mimetic resemblance & the effects of peculiar conditions.6 Who can say to which of these causes to attribute the several plants with heath-Erica-like foliage at the C. of Good Hope?7 Is it not also a difficulty that quadrupeds appear to recognize plants more by their odour than their appearance? What I have just said reminds me to ask you a question: Sir J. Lubbock brought me the other day what appears to be a terrestrial planaria (the first ever found in the northern hemisphere) & which was coloured exactly like our dark coloured slugs. Now slugs are not devoured by birds like the shell-bearing species, & this made me remember that I found the Brazilian planariæ actually together with striped Vaginuli which I believe were similarly coloured.8 Can you throw any light on this? I wish to know, because I was puzzled some months ago how it wd be possible to account for the bright colours of the planariæ in reference to sexual selection. By the way I suppose they are Hermaphrodites?9

Do not forget to aid me, if in yr power, with answers to any of my questions on expression, for the subject interests me greatly.10

with cordial thanks for yr never failing kindness | believe me yours very sincerely | Charles Darwin


The year is established by the relationship between this letter and the letter from Fritz Müller, 2 June 1867.
CD described Müller’s observations in Variation 2: 134–5, in a section headed, ‘On certain hermaphrodite plants which, either normally or abnormally, require to be fertilised by pollen from a distinct individual or species’.
CD refers to discussions in the letter from Fritz Müller, 1 January 1867.
Müller had sent seeds of two species of Coccocypselum with his letter of 2 June 1867; he also sent seeds of Oncidium flexuosum resulting from a cross with a Cyrtopodium. These are still in a packet glued to the letter.
James Philip Mansel Weale evidently noted this resemblance in an excised portion of his letter of 7 July 1867 (see letter to J. P. M. Weale, 27 August [1867].
John Lubbock was a neighbour of CD’s; he documented his discovery in ‘Note on the discovery of Planaria terrestris in England’, Journal of the Linnean Society (Zoology) 10 (1870): 193–5. For CD’s discovery of planarias in association with Vaginulus (a genus of slug) at Valparaiso, Chile, see Correspondence vol. 1, letter to J. S. Henslow, 24 July – 7 November 1834; CD also mentioned finding planarias in the Brazilian forest in his Journal of researches, pp. 30–2. See also R. D. Keynes ed. 2000. He discussed the species he found, and their similarity to Vaginulus in ‘Planariae’.
CD later decided that the bright colours of planarias and other ‘lower’ animals, whether hermaphrodite or not, was not due to sexual selection (see Descent 1: 322).
Müller’s answers to CD’s queries about expression have not been found, but they were evidently enclosed in the letter from Fritz Müller, [8 October 1867] (see n. 2 of that letter). CD had sent a copy of the queries with his letter to Müller of 22 February [1867].


Has abstracted for insertion in his sterility chapter [Variation 2, ch. 18], FM’s observations of plant’s pollen being poisonous to itself.

Occurrence of mimetic plants.

Colouring of Planariae.

Letter details

Letter no.
Charles Robert Darwin
Johann Friedrich Theodor (Fritz) Müller
Sent from
Source of text
British Library (Loan 10: 17)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 5591,” accessed on 27 June 2017,

Also published in The Correspondence of Charles Darwin, vol. 15