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Darwin Correspondence Project

From Albany Hancock   8 February 1868

St. Mary’s Terrace— Newcastle on Tyne

8th. Feby 1868—

My dear Sir,

I had much pleasure in receiving your letter this morning as it reminded me of our former correspondence; and now I shall be, as I was then, most happy to be of any service to you in your important scientific enquiries.—1

All the Nudibranchs as far as is known are androgynous; therefore their colouring can in no way be influenced by sex.— The vivid colouring of the Eolides is mainly due to internal organization; it is the liver, as you are aware, that gives the rich tints to the papillae— And perhaps these tints are too intimately connected with the structure to be modified by any secondary cause.—2 Be this however as it may it is certain that many of these delicate and richly coloured animals seek shelter under stones and in other dark places.— And hence according to Mr Wallace’s doctrine it might be inferred that they did so in consequence of their gaudy appearance;—or that only such of them as concealed themselves had been able to continue their races.—3 The species alluded to are Eolis coronata, E. rufibranchialis, E. nana, and E. gracilis.—4 Many of the white Nudibranchs have also the same habit; namely Doris aspera, D. repanda, Goniodoris nodosa and Ancula cristata, all of which may be considered conspicuous species.—5 But on the other hand inconspicuously coloured animals do so likewise; as for instance Eolis papillosa, E. concinna, E. Drummondi, E. olivacea, Ægires punctilucens, Doris Johnstoni, D. tuberculata, and D. pilosa, which latter is as frequently black as white and is very commonly of an obscure brown.—6 In fact the greater number of the Nudibranchs are driven to seek shelter in dark places on account of their unprotected condition whether they are finely coloured or otherwise.— There are a few however which are usually found in exposed situations.— The most remarkable of these is Doris bilamellata.—7 This is commonly found exposed on the surface of rocks, and its colour is certainly favorable to concealment.— Eolis aurantiaca,8 though frequently hidden under stones is not uncommonly seen feeding on Zoophytes whose colour somewhat harmonizes with that of the mollusk. Doto fragilis also resembles in colour the horny, flexible corallines on which it usually occurs, and Hermæa dendritica is as green as the vegetable on which it feeds.—9 But then again Limapontia nigra is black though it lives on green Algæ.— Polycera quadrilineata, a white and frequently brilliantly coloured animal, is found on the dark seaweeds, and cuts a very conspicuous figure in the broad day light.— Eolis pellucida which is as highly coloured as any of its congeners lives exposed on Zoophytes; so likewise does Triopa claviger, a white species.—10

Besides the few instances above alluded to there is another, perhaps the most remarkable of all which I must not omit to mention— I allude to Doris depressa, the colour of which so closely resembles the Retepora on which it nests and feeds that it requires a practised eye to detect it.—11 But,—there is still a but in the way, I have always taken this species under stones where little or no light could penetrate, and where, of course, this protecting colouring, if so it be, could really be no use to the animal.—

Other instances might be cited pro and con, but perhaps the above will be sufficient to enable you to form some opinion on the subject.— But should you require further information or explanation and will kindly specify the same I will have much pleasure in doing my best to assist you.—

On the whole it appears to me that Mr. Wallace will not find much in this group of animals to support his views as to the use of colour in the animal kingdom.— But at the same time I must state that I have made no especial study of this subject, and that what I have written above is just what has occurred to me on the spur of the moment.—

I know of no “secondary sexual characters” in the Mollusca, except that relating to the shell of the Argonauta.— The true species of Nautilus have also, I believe, been stated to be merely the opposite sexes.—12

I remain, My dear Sir, | Your’s ever truly | Albany Hancock

Ch: Darwin Esqre

I enclose my carte—13 Yours in return I shall esteem a great favour.—

CD annotations

1.1 I had … cause.— 2.5] crossed pencil; ‘Used’ added pencil
2.3 it is … papillae— 2.4] scored red crayon
2.9 The species … species.— 2.13] ‘White & conspicuous species concealedadded pencil
2.24 Doto … feeds.— 2.26] scored red crayon
2.27 Polycera … seaweeds, 2.28] scored red crayon
2.28 and cuts … species.— 2.30] scored pencil
3.5 and where, … animal.— 3.7] scored red crayon
6.1 I know … Argonauta.— 6.2] double scored pencil
6.1 “secondary sexual characters”] ‘of the kind we [illeg]added pencil
9.1 carte] underl pencil
Top of letter: ‘Detailed information | in detail with names); (Hectocyli)’14 pencil
End of letter: ‘Most valuable letter, full of clearest information & quite sufficient for my purpose— It is surprising to me that such highly organised animals as the naked Cephalopoda shd not [‘show’ del] at least in some cases display conspicuous differences [interl] in colour of the 2 sexes—’15 pencil crossed pencil


CD’s letter to Hancock has not been found. He had previously corresponded with Hancock about cirripedes (see Correspondence vol. 5).
CD cited information from Hancock on the coloration of nudibranch molluscs (sea slugs) in Descent. He concluded that the colours ‘were probably a direct result … of the nature of the tissues’ (Descent 1: 326).
Wallace had published several papers on the protective role of colour in various animals; for example, ([A. R. Wallace] 1867b and A. R. Wallace 1867d (see also Correspondence vol. 15). CD discussed Wallace’s views on protective coloration extensively in Descent (see Descent 1: 403–10, 416–17, 2: 166–80, 223–5).
Eolis coronata is now Facelina coronata; E. rufibranchialis is now Flabellina rufibranchialis or F. verrucosa; E. nana is now Cuthona nana; E. gracilis is now Flabellina gracilis. Information on nudibranch systematics and nomenclature (see nn. 5–11, below) is taken from Thompson and Brown 1976, Thompson and Brown 1984, and Gary McDonald, personal communication.
Doris aspera is now Onchidoris muricata; D. repanda is now Cadlina laevis; Ancula cristata is now Ancula gibbosa.
Eolis papillosa is now Aeolidia papillosa; E. concinna is now Cuthona concinna; E. drummondi is now Facelina bostoniensis; E. olivacea is now Cuthona foliata; Doris johnstoni is now Jorunna tomentosa; D. tuberculata is now Archidoris pseudoargus; D. pilosa is now Acanthodoris pilosa.
Doris bilamellata is now Onchidoris bilamellata.
Eolis aurantiaca is now Cuthona gymnota.
Hermaea dendritica is now Placida dendritica.
Limapontia nigra (a European marine snail) is now Limapontia capitata; Eolis pellucida is now Flabellina pellucida; Triopa claviger is now Limacia claviger.
Doris depressa is now Onchidoris depressa; Retepora (a genus of Bryozoa) is now Reteporella.
Argonauta, commonly known as paper nautiluses, are in the order Octopoda. The paper-thin white calcareous ‘shell’ is an egg case made by the females. Nautilus (order Nautilida) are sexually dimorphic.
Hancock’s carte de visite has not been found in the Darwin Archive–CUL.
Hectocotyle: ‘A modified arm in male dibranchiate Cephalopods, which serves as a generative organ, and in some species is detached and remains in the pallial cavity of the female’ (OED). CD remarked that this structure should be classed ‘a primary rather than a secondary sexual character’ (Descent 1: 325–6).
CD noted the absence of secondary sexual characters in the Cephalopoda in Descent 1: 325.


On the colours of the nudibranch Mollusca. [See Descent 1: 326.]

Letter details

Letter no.
Albany Hancock
Charles Robert Darwin
Sent from
Source of text
DAR 82: A51–4
Physical description
7pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5852,” accessed on 19 April 2019,

Also published in The Correspondence of Charles Darwin, vol. 16