To Fritz Müller   31 July [1867]¹

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 c^d 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 w^d 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 w^d 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 y^r power, with answers to any of my questions on expression, for the subject interests me greatly.¹⁰

with cordial thanks for y^r never failing kindness | believe me yours very sincerely | Charles Darwin