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 M^r 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 M^r. 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.—