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Darwin Correspondence Project

From Fritz Müller   1 April 1867

Desterro, Brazil,

April 1. | 1867.

My dear Sir.

The last French Steamer brought me your two letters of Feb. 7 and 22, for which I am much obliged to you.1

As to your question about the sexual differences of lower animals, I can hardly give you any information.2 Among higher Crustacea I know of only one instance of males distinguished by bright colours from their modest females.


In a little Gelasimus of our coast the posterior half of the cephalothorax assumes often, in the adult male, a pure white colour. This white colour may, in a few minutes, change into dirty grey or even black, while at the same time the colours of the anterior half lose much of their brilliancy. The cephalothorax of the female is commonly of a nearly uniform greyish brown. This little gelasimus uses to run about in the sunshine and so is more able to exhibit its beauty to the female than are most other Crustaceans. The number of the males seems to be much larger than that of the females; just now, when I went to catch some specimens, I found a dozen males and only two females. I suspect, that the large pincers may serve to this and perhaps to some other male crabs for fighting for the female; at least, when several males are emprisoned in a glass, they often kill or mutilate each other.—3 Among lower Crustaceans some species of Sapphirina Thomps. offer a most interesting case in point. The males are known since the time of Cook for the brilliancy of their colours, while the females are colourless according to Gegenbaur and Claus. In these little Copepods also the colours are changing (See the enclosed extract from Claus’ book.).—4 From Gerstäcker’s Textbook of Zoology I see that the males of some spiders (Sparassus smaragdulus &c) are distinguished by different and more gaudy colours.—5 Among Annelids some Syllideæ with alternate generations show very marked sexual differences, so great, that the asexual form, the male and the female were placed in three distinct genera (Autolytus Grube, Diploceræa Gr. and Sacconereis J. Müll.)6 Some of the sexual forms have very large eyes and I have found some of these little worms having much prettier colours than is usual with Annelids. Perhaps these colours here also may be due to sexual selection.—

As to the expression of countenance of Negros, I shall keep the subject before my mind and may perhaps be able to answer some of your questions; but I have here no opportunity of observing native S. Americans.7

Oncidium flexuosum is a native of our district, and so are the other Orchids of my garden, all of which I collected either on our island or on the neighbouring part of the continent.8

I but once saw ramifying filaments of a parasitic cryptogam causing the decay of Orchid-pollen, when I had placed old pollen-masses of Lælia purpurata on the stigma of Brassavola fragrans.— In the case of self-impotent Vandeæ I never saw a trace of cryptogams, which might be considered as the cause of the decay.—9

To the list of self-impotent Orchids, in which pollen and stigma of the same plant are poisonous to each other, I can add a small Oncidium


(probably undescribed) from Theresopolis10 in which own pollen after three days’ stay in the stigmatic chamber was decayed and dark brown. This Oncidium grows in company of O. unicorne and though the flowers, as you see, are quite different, (belonging even to a different section of the genus), the pseudobulbs and leaves of the two species are almost undistinguishable. Perhaps this strange ressemblance may be due to mimicry.— I am now experimenting on the last (as to its flower-time), but not least of our Oncidia, the Oncidium crispum, which seems to be an exception among our other self-impotent species of that genus; at least own pollen, placed on the stigma March 23, I found to be quite fresh as well as the stigma and pollen-tubes, when I examined a flower this morning.11

As to the fertility of Epidendrum cinnabarinum being much lessened by self-fertilization, I can now give you one more striking case. I fertilized (Jan. 17) six flowers of a raceme with pollen of the same flowers and obtained six pods, the seeds of which weighed: 1.5— 2.— 1.5— 2.— 2,5— 2.5 grains; most of the seeds were bad.— On the same day I fertilized six flowers of a second raceme of the same plant with pollen of a distinct plant and obtained 5 pods, the seeds of which weighed 5.— 6.— 5.— 5.—5 grains, but few seeds being bad.—12

At reading Dr Hildebrand’s paper on Corydalis cava, which you have been so kind as to send me, I was struck by the circumstance that among the 29 seed-capsules obtained by crossing distinct plants, the seeds of which were counted, none had 4 seeds, while about half the number had from 5 to 7, and the other only from 1 to 3 seeds. May not this species be functionally dimorphic, the poor seed-capsules being the result of illegitimate unions?—13

⁠⟨⁠Drawing or specimen excised⁠⟩⁠

Here is another dimorphic Rubiacea;14 the pollen-grains of the short-styled plant are larger (about 112 mm diam.), than those of the long-styled (about 116 mm diam.). The plant has beautiful blue Fruits.

During the last months I have been making some experiments on the crossing of different species of Orchids; and I hope that these experiments, if continued on a larger scale, will lead to some interesting results. Some points may already be worth mentioning. It seems that much more widely distinct species of Orchids may be successfully crossed, than is usual with plants. Thus I have (as yet unripe) pods of Cattleya elatior var Leopoldi ♀ and Lælia purpurata ♂;—of Cattleya elatior var Russelliana, ♀, and Epidendrum cinnabarinum ♂;—of Brassavola fragrans Lem. ♀ and different species of Epidendrum ♂;—of Oncidium flexuosum ♀ and Cyrtopodium (Andersoni?) ♂;—of Notylia ♀ and Ornithocephalus ♂!—of Sigmatostalix ♀ and Ornithocephalus ♂!—of the small Oncidium from Theresopolis ♀ and Sigmatostalix ♂;—of Oncidium flexuosum ♀ and Gomesa ♂;—of Gomesa ♀ and Onc. flexuosum ♂ etc.—

But the case, which most surprised me, is the following: A Frenchman, Mr. Gautier,15 bade me to cross for him, in his garden, Zygopetalum maxillare and Miltonia cereola. I did so, without expecting any result; but both the species have now fine swelling pods!— There appears to be no close systematic affinity between the two genera; at least Miltonia is much more closely allied to Oncidium than to Zygopetalum. Mr. Reichenbach even cancels the genus Miltonia, uniting it with Oncidium;16 but I repeatedly fertilized several species of Oncidium with pollen of Miltonia, without obtaining a single pod.— The only point in which Zygopetalum maxillare and Miltonia cereola closely ressemble each other is the colour of the labellum!—

March 6, when I was fertilizing a Cattleya (probably a var. of C. elatior) with pollen of some other species of the genus and of Epidendrum, I had just at hand some pollinia of Oncidium micropogon and placed them on the stigma of one of the flowers of the Cattleya. Now this flower withered sooner than the other fertilized flowers, and has yielded a fine pod, which is now 36mm long, while the other pods are about 46mm long. Probably the pollen-tubes of Oncidium will not be able to fertilize the ovules of Cattleya and I think the pod will wither, when the time of fertilization arrives, but even so the case is curious.— If horticulturists contrived to raise Orchids from seeds, this facility with which widely distant species are crossed, would be a highly valuable quality.—

There is with Orchids the same unaccountable ⁠⟨⁠one-third line excised⁠⟩⁠ fertility of reciprocal crosses, as with other ⁠⟨⁠one-third line excised⁠⟩⁠ certain species seems to have a great fertilizing ⁠⟨⁠one-third line excised⁠⟩⁠ Epidendrum cinnabarinum, these same species are ⁠⟨⁠one-third line excised⁠⟩⁠ with difficulty. I fertilized with pollen of ⁠⟨⁠one-third line excised⁠⟩⁠

Epidendrum Zebra n. sp?17 6 flowers, ⁠⟨⁠    ⁠⟩⁠

— fragrans 2 ⁠⟨⁠    ⁠⟩⁠

— raniferum? 2 1.

— vesicatum 2 2

Cattleya elatior 2 1.

Brassavola fragrans. 4. 3.

18 13.

On the other hand I fertilized Epidendrum cinnabarinum with pollen of

Epidendrum Zebra 17 flowers, which yielded 0 pod

— fragrans 4 0.—

— raniferum 37 1 —

— vesicatum 6 0.—

Cattleya elatior 2. 0.

Brassavola fragrans 16. 0.

82 1.

The easity, with which fertilisation is effected and the number of seeds produced is not proportional. Epidendrum Zebra is easily fertilized with Ep. cinnabarinum, but the pods (not yet ripe) will be very poor. On the contrary 14 flowers of Epid. cinnabarinum fertilized with pollen of Ep. Schomburgkii(?) yielded only two pods, but one of these had more seeds than any pod fertilized with pollen of the own species, and also the other was very rich.—

The sterile unions between different species behave in most cases very differently from those of the self-impotent species. First discolours and withers the pedicel of the germen and this discolouring goes slowly on in an upwards direction and even, when the germen falls off, pollen and stigma are generally quite fresh. Generally the germen will fall off 1 or 2 weeks after fertilization; but a germen of Cirrhæa dependens? fertilized with Stanhopea Jan. 24 fell off only March 11; stigma, pollen and pollen tubes were fresh.— Sometimes the pollen grows brownish; but then this discolouring generally begins at the outer surface of the pollen masses, and but in very few cases at the inner surface, where it touches the stigma, whilst the latter is invariably the case, when a self-impotent Oncidium, Gomesa, Sigmatostalix or Notylia is fertilized with own pollen.—18

Hoping that this letter will find you in good health, Believe me, dear Sir, | very truly and respectfully yours | Dr Fritz Müller.

P.S. I forgot to thank you for the copies of my paper on Climbing plants.19


Claus, die freilebenden Copepoden. Leipzig, 1863; pg. 35; 21

Endlich mag an diesem Orte der merkwürdige Farbenschiller besprochen werden, welcher an der äusseren Körperbedeckung einiger Saphirinen-Männchen bereits älteren Forschern bekannt war. Schon Anderson beobachtete die Erscheinung auf Cook’s letzter Reise an seinem Oniscus fulgens, einer Thierform, die offenbar mit Banks’s Carcinium opalinum und Thompson’s Sapphirina identisch ist .... Meyen, welcher Sapphirinen in der Nähe der Azoren beobachtete, beschreibt dieselben als äusserst bewegliche Plättchen mit beständigem Farbenwechsel.22 In die Tiefe hinabgesunken zeige sich das Thier mit dem glänzendsten Violettroth, das einen purpurnen Kern einschliesse, eingefangen aber verliere es die Farben, welche nur durch die Brechung der Lichtstrahlen an der spiegelnden Oberfläche des Körpers hervorgerufen würden. An der letztern aber entdeckte er auf der Rückenseite 4-, 5- oder 6-seitige Schilder, die in stumpfen Kanten aneinander gereiht, eine glänzende Fläche zeigten; durch diese dachte sich Meyen wie durch aneinander gereihte Prismen das Licht gebrochen und so bei jeder Bewegung des Thieres den Farbenwechsel erzeugt. Eine bei weitem detaillirtere Beschreibung der erwähnten Farbenerscheinungen erhalten wir durch Gegenbaur (Müller’s Archiv 1858 p. 67).23 Auch dieser Forscher verlegt ihren Sitz in jene polygonalen . . Felder, die er auf eine zusammenhängende Schicht von platten Zellen unter der Cuticula, auf die Matrix der Chitinhülle, zurückführt. “Beim Weibchen”, sagt unser Forscher, “ist der Zellinhalt während des Lebens durchaus hell, das Männchen dagegen lässt im Leben beinahe dieselben Erscheinungen an jenen Zellen unter dem Mikroskope erkennen, wie man sie am frei lebenden Thiere beobachten kann. Bei durchfallendem Lichte sowohl als bei auffallendem ist der Wechsel des Farbenspieles von Zelle zu Zelle zu beobachten und während im letzteren Falle nur Metallglanz funkelt, so ist bei ersteren neben dieser Erscheinung noch ein dioptrisches Farbenspiel sichtbar. Oft grenzt sich eine Zelle von der benachbarten mit grösster Schärfe durch Farbe oder Metallschimmer ab, erscheint gelb, roth oder blau mit den verschiedensten Nüancirungen von einer Farbe in die andere übergehend, jedoch ohne alle Mittelfarben, ohne Grün, Violett oder Orange. Die beiden ersten Farben kommen jedoch bei dem katoptrischen Phänomen vor, bei welchem Blau die erste Rolle spielt. Betrachtet man die Erscheinung an einer einzelnen Zelle, so findet man den Übergang von Blau in Roth ohne die Mittelfarbe dadurch zu Stande kommen, dass an einem Theile der Zelle, etwa in einer Ecke derselben, das Blau erblasst, fast grau wird und dann plötzlich an dieser Stelle ein rother Saum auftritt, der breiter werdend über die Zelle in dem Maasse sich ausdehnt, als das Blau gewichen ist, so dass alsbald die ganze Zelle blau erscheint. Dasselbe gilt vom Gelb. Die Qualität der Farbe einer Zelle ist völlig unabhängig von den benachbarten Zellen. So erscheinen gelbe mitten im Roth, rothe mitten im Blau. Doch kann auch die Erscheinung auf benachbarte Zellen überschreiten; vom Rande einer blauen Zelle geht Blau auf die Nachbarzelle über, die eben noch roth war, und so dehnt sich zuweilen eine Farbe über eine grosse Strecke aus. Zuweilen tritt plötzlich in einer und derselben Zelle ein farbloser Fleck auf, in der Mitte oder am Rande, grösser oder kleiner, während der übrige Theil noch in voller Farbe prangt. Verwandelt man jetzt das durchfallende Licht in auffallendes, so leuchtet der Fleck in vollem Metallglanze, während die übrigen vorher und nachher gefärbten Partien dunkel sind. Die Zeiträume, innerhalb welcher diese Phänomene verlaufen, sind verschieden lang, oft wechselt in einer Secunde die Farbe dreimal, oft währt eine Farbe mehrere Secunden lang. Mit dem Tode des Thierchens, wo sich der feinkörnige Inhalt jedesmal gegen die Mitte hin zusammendrängt, ist die ganze Erscheinung erloschen.”

Ich habe die ganze Beschreibung Gegenbaur’s citirt, weil meine eigenen das Phänomen des Farbenwechsels bestätigenden Beobachtungen nicht bis in die von Gegenbaur erforschten Details für die Aufeinanderfolge der Farben eingedrungen sind. Indess kann ich mich nicht in allen Stücken mit Gegenbaur einverstanden erklären. Zunächst bilden nach meinen Beobachtungen die polygonalen Felder allerdings eine unter der Cuticula gelegene Schicht, aber sie sind 1, keine Zellen 2, liegen sie nur unter der Rückenfläche, 3, konnten sie im weiblichen Geschlechte nicht gesehen werden. Dass sie keine einfachen Zellen des Matricalepithels sind, geht nicht nur aus ihrer Grösse hervor, welche bei Sapph. auronitens 0,08 mm, bei Saph. fulgens 0,1 mm. im Durchmesser beträgt, also mit den kleinen Zellen der Matrix anderer Copepoden gar nicht verglichen werden kann, sondern vor Allem aus dem Verhalten der Begrenzung. Die polygonalen Platten sind nicht von einer festen Membran umgeben, sondern zeigen sehr feingezackte Umrisse. Man hat es mit dünnen Platten einer feinkörnigen Substanz zu thun, mit Platten, welche durch suturenartig in einandergreifende Ränder begrenzt sind, und (bei Sapph. nitens) häufig äusserst dichte und zarte Streifen ähnlich wie gewisse Lepidopteren-Schuppen darbieten. Kerne, wie sie Gegenbaur für eins der drei von ihm gezeichneten Felder abbildet, habe ich niemals deutlich und regelmässig beobachtet und ich kann mich auch aus diesem Grunde nicht dazu verstehen, die Felder für Zellen zu halten. Weit eher entsprechen dieselben ganzen Complexen von verschmolzenen und veränderten Zellen der Matrix, für die ich keine zweite tiefere Lage eines Epithels nachweisen konnte. Ferner habe ich hervorzuheben, dass der Farbenschimmer keineswegs mit dem Tode des Thieres erlischt, der nur den wunderbaren Wechsel der Farben, die Veränderung derselben Theile von Blau in Roth &c. aufhebt. Der goldgrüne Metallglanz (S. auronitens) sowohl als das grünlich violette Farbenspiel (S. fulgens) finde ich an einigen seit Jahren in diluirter Glycerinlösung aufbewahrten Formen prachtvoll erhalten. Auch bei Sapphirinella mediterranea treten unter der Cuticula die nämlichen polygonalen feinstreifigen Felder auf und zeigen bei auffallendem Lichte einen schwach violetten, bei durchfallendem einen blassgelblichen Schimmer.

Eine Erklärung der besprochenen Farbenerscheinungen wage ich nicht im Detail auszuführen. Von einem Vergleiche der polygonalen Tafeln mit aneinandergereihten Glasprismen, die das Licht in die Spectralfarben zerlegen, kann natürlich keine Rede sein, vielmehr haben wir es mit Interferenzerscheinungen zu thun, welche ihren Sitz in dem feinkörnigen zuweilen wie in Sprüngen und Rissen zerspaltenen Gefüge der Tafeln haben. Vollkommen dunkel aber bleiben die höchst merkwürdigen Farbenveränderungen während des Lebens in den einzelnen polygonalen Feldern, die, wenn auch nicht dem Willen des Thieres unterworfen, doch von Vorgängen des Stoffwechsels abhängig zu sein scheinen, in denen man auch die Ursache für das Leuchtvermögen der Sapphirinen zu suchen hat.

l.c. pg. 152. Das Männchen von Sapphirina nigromaculata Claus hat keinen Farbenschimmer.

CD annotations

1.1 The last … selection.— 3.23] crossed ink
4.1 As … Americans. 4.3] crossed pencil
5.1 Oncidium … bad.— 8.7] crossed blue crayon; enclosed in square brackets brown crayon; ‘See last pageadded ink
9.1 At … unions?— 9.6] crossed brown crayon
17.1 The sterile … plants. 19.1] crossed red crayon; after opening square bracket red crayon


Letters to Fritz Müller, 7 February [1867] and 22 February [1867].
In his letter to Müller of 22 February [1867], CD had asked whether Müller knew of any ‘lowly organized’ animals in which the male differed from the female in ‘arms of offence’ or ornamentation.
Gelasimus or Uca is the genus of fiddler crabs. CD cited Müller for this information in Descent 1: 333 and 336. Müller wrote a short paper on colour change in crabs that includes a description of this Gelasimus (F. Müller 1881). The species Müller observed was probably Uca leptodactyla (see Crane 1975, pp. 304–7).
CD used this information in Descent 1: 335–6. Müller refers to Captain James Cook, Carl Gegenbaur, and Carl Friedrich Claus. With his letter Müller enclosed a handwritten extract from Claus’s Die freilebenden Copepoden (Claus 1863, pp. 35–7); CD cited page 35 in Descent 1: 336. Gegenbaur discussed Sapphirina in Gegenbaur 1858, Claus in Claus 1863, pp. 149–53. See also n. 22, below.
Müller refers to Adolph Gerstaecker; Sparassus smaragdulus (now Micrommata virescens) is discussed in Gerstaecker 1863, p. 338, and Descent 1: 337.
Müller refers to the family Syllidae (marine worms). See Gerstaecker 1863, p. 439. See also Descent 1: 327, where CD repeats much the same information.
CD had enclosed a handwritten copy of his questionnaire on expression with his letter to Müller of 22 February [1867]. Destêrro (now Florianópolis) is on Santa Catarina Island off the coast of south-eastern Brazil.
CD had asked whether Oncidium flexuosum was native to Müller’s part of Brazil in his letters to Müller of 7 February [1867] and 22 February [1867]; see also letter from Fritz Müller, 4 March 1867.
See letter to Fritz Müller, 22 February [1867] and n. 3. CD noted Müller’s sole observation of a parasitic cryptogam in Variation 2: 134.
For Müller’s most recent report of self-impotent (self-incompatible) orchids whose pollen and stigma were poisonous to each other, see his letter of 4 March 1867. Müller had travelled to Theresopolis in January 1867 (see letter from Fritz Müller, 2 February 1867 and n. 14.
CD reported Müller’s observations of Oncidium species in Variation 2: 134. For Müller’s earlier discussions of Oncidium species, see Correspondence vol. 14, letter from Fritz Müller, 1 December 1866, and this volume, letters from Fritz Müller, 1 January 1867, 2 February 1867, and 4 March 1867.
Müller most recently mentioned experiments on Epidendrum cinnabarinum in his letter to CD of 4 March 1867. CD reported his results in Variation 2: 134.
Müller refers to Friedrich Hildebrand and to Hildebrand 1866a. CD mentioned this paper in his letter to Müller of 22 and 25 September [1866]; Hildebrand had concluded from his experiments that flowers of Corydalis cava were sterile with their own pollen, and most fertile with the pollen of any other individual plant of the species.
Müller had sent CD a specimen from the family Rubiaceae with his letter of 4 March 1867.
Hippolyte Gautier has not been further identified (see also letter from Fritz Müller, 2 February 1867 and n. 3).
Müller refers to Heinrich Gustav Reichenbach and possibly to Reichenbach 1852. For Müller’s earlier discussions of Epidendrum, see Correspondence vol. 14, letter from Fritz Müller, 1 December 1866, and this volume, letters from Fritz Müller, 1 January 1867, 2 February 1867, and 4 March 1867. See also Variation 2: 134.
CD paraphrased the information in this paragraph, and cited Müller, in Variation 2: 135.
CD had received copies of Müller’s paper on climbing plants on 1 January 1867, and sent Müller eleven of them (Correspondence vol. 14, letter to Fritz Müller, [late December 1866 and] 1 January 1867). The paper was composed of parts of three letters from Müller to CD, edited by CD, and published in the Journal of the Linnean Society (Botany) (F. Müller 1865; the paper is also published in Correspondence vol. 13 as the letter from Fritz Müller, [12 and 31 August, and 10 October 1865]).
For a translation of the text of this enclosure, see Correspondence vol.15, Appendix I.
In Descent 1: 335–6, CD cited Claus’s description of colour change in Sapphirina.
William Anderson’s identification of Oniscus fulgens is discussed in the account of Cook’s third voyage to the Pacific Ocean from 1776 to 1780, during which Cook was killed (Cook and King 1784, 2: 257). John Vaughan Thompson’s account of Sapphirina is in Thompson [1828–34]; Franz Julius Ferdinand Meyen’s is in Meyen 1834, pp. 153–5. Claus also refers to Joseph Banks.
Claus refers to the Archiv für Anatomie, Physiologie und wissenschaftliche Medizin, which was edited by Johannes Peter Müller between 1834 and 1858. The lengthy quotation given by Claus is from Gegenbaur 1858, pp. 66–8, with some brief omissions.


Claus, Carl Friedrich. 1863. Die frei lebenden Copepoden mit besonderer Berücksichtigung der Fauna Deutschlands, der Nordsee und des Mittelmeeres. Leipzig: Wilhelm Engelmann.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Crane, Jocelyn. 1975. Fiddler crabs of the world (Ocypodidae: genus Uca). Princeton, N.J.: Princeton University Press.

Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.

Gegenbaur, Carl. 1858. Mittheilungen über die Organisation von Phyllosoma und Sapphirina. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin (1858): 43–81.

Reichenbach, Heinrich Gustav. 1852. De pollinis orchidearum genesi ac structura et de orchideis in artem ac systema redigendis. Leipzig: F. Hofmeister.

Thompson, John Vaughan. [1828–34.] Zoological researches, and illustrations; or, natural history of nondescript or imperfectly known animals. Vol. 1, part 1. Cork: King and Ridings.

Variation: The variation of animals and plants under domestication. By Charles Darwin. 2 vols. London: John Murray. 1868.


Cites cases of difference in coloration between the sexes of some species of Crustacea, annelids, and spiders.

Discusses dimorphic plants and self-sterility.

Outlines some experiments involving the crossing of different species of orchids.

Encloses extract from Carl Claus, Die freilebenden Copepoden [1863].

Letter details

Letter no.
Johann Friedrich Theodor (Fritz) Müller
Charles Robert Darwin
Sent from
Desterro, Brazil
Source of text
DAR 110: B111–12; DAR 81: 167
Physical description
4pp inc †, encl 2pp

Please cite as

Darwin Correspondence Project, “Letter no. 5480,” accessed on 25 October 2021,

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