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Darwin Correspondence Project

To Albany Hancock   25 December [1852]1

Down Farnborough Kent

Dec. 25th.

My dear Sir

You will probably remember that you called my attention to the following facts, that Verruca (=Clisia &c)2 (1st) has the power of excavating a slight depression for itself; but that (2d) epidermis on a shell quite stops this process; & (3d) that under its middle there is sometimes a hollow sometimes with chalky matter. I have just been at work on the genus & find these three facts occurring in three different species from different quarters of the world. My object in writing is to ask you to look to one point in your collection; but first I will mention what results, I have come to: I began with a very strong leaning to the view, which you advocate, that the excavation must be due to mechanical agency, but unwillingly I have been driven to hypothetical chemical action. My grounds of belief are as follows, & I should be grateful for your opinion,—viz;—

(1) I can discover no sort of boring contrivance on margin of shell or on under side of basal membrane: & there is no difference in appearance in these parts, when an individual has bored and has not in the least bored: I have examined the single shell, & cleaned with potash, & after acid, with all powers.

(2d) Either the shell or basal membrane must, on mechanical theory, be the wearing agent; & certainly, as far as the central hollow, it must be the basal membrane: but the basal membrane is united to the shell & animals body by (besides corium & epidermis) only by a circle of fibres which Prof. Quekett,3 after most careful testing says are only ligament: hence I think it impossible that the basal membrane can be moved, (at least near the circumference, where the animals cirri cannot reach) or again that the shell can be moved, if we look at the basal membrane as the fixed point.—

(3d) When a central hollow has been formed, the basal membrane (in this case generally brittle & cracked) is loose over this middle part, but was once certainly attached, as I have found the prehensile larval antennæ in the middle, surrounded by the ordinary cirripedial cement, which certainly would require considerable mechanical power to separate from any object of attachment, & yet there is nothing whatever over this central portion of the basis, but the open sack: dissolution of the shell, on the other hand, to which the cement was attached, would perfectly explain the appearance.

4th. As you state the epidermis of shells quite prevents the wearing, except where abraded or cracked: & I further find the epidermis of Bal. lævis (of which I send a valve not to be returned) is equally protective, now this membrane is so weak, that I cannot believe it could resist mechanical wear & tear, sufficient to wear into solid shell: so again Laminaria, (when not uneven, & so slightly ploughed up, like cracked epidermis) though not hard, is not at all excavated: again I have specimens on two pieces of slate rocks (one rather soft) which contained no calcareous matter, & were not in the least, affected; whereas a third specimen of hard marble was excavated.4

(5) The cement-ducts might pour out an acid over any part of the basis; but that they do so is a mere hypothesis: in Lepas fascicularis they must, I think, secrete some gas, [reverse question mark]carbonic acid gas? I should have remarked that owing to the generally reticulated state of the cement round the central hollow, lime dissolved under the central hollow might easily escape.—

This is the state of the case, as far as I can make it out: will you forgive the length of this letter, & tell me what you think?5 And further will you see whether you have specimens of Verruca attached to any softish rocks or substances, without calcareous matter, & look & see if they act on them?6

In two weeks’ time, I shall positively at last, after a quite ridiculous lapse of time look at your Alcippe, which I have never done yet!7 Have you anything new (or any fresh specimens to spare) on this most curious genus? I shd. be pleased to hear that time or inclination had led you to look at what I have said on the sexes of Ibla & Scalpellum, about which I remember once writing to you; & which facts appear to me curious.—8

Again I beg forgiveness for the length of this letter & remain. My dear Sir | Your’s very faithfully | Charles Darwin A. Hancock Esq

I do not think my wretched, school-boy M.S. on the outlandish Mollusca9 has been returned? Has it??


Dated on the basis that the last entry for 1852 in CD’s journal (‘Journal’; Correspondence vol. 5, Appendix I) recorded that he ‘Began Verruca’.
Hancock was one of the first naturalists to draw attention to the boring powers of molluscs (A. Hancock 1848). Hancock had sent CD specimens of Verruca (Clysia and Clitia were the names previously used for Verruca) in connection with their discussion of the boring powers of Lithotrya (see Correspondence vol. 4, [26 January – March 1850]). Hancock’s observations on the excavating powers of Verruca are described in Living Cirripedia (1854): 512.
John Thomas Quekett was professor of histology at the Royal College of Surgeons.
Verruca strömia, described in Living Cirripedia (1854): 314.
Hancock apparently gave a cautionary reply (see CD’s letter to Albany Hancock, 10 January [1853]). In Living Cirripedia (1854): 512–18, CD repeated the arguments he had given in this letter and described his own chemical explanation of boring in Verruca. In a footnote (p. 516) he rejected Hancock’s suggestion of how, on the mechanical theory, the basal membrane might excavate without itself moving if its epithelial scales were transformed into cutting agents. CD stated he could find no such scales.
See letter to C. S. Bate, 10 January [1853], in which CD asked Charles Spence Bate to look for such specimens. Bate reported that he could find no impressions on the slate-rocks from which he had removed specimens of Verruca (Living Cirripedia (1854): 514).
CD had requested specimens of the new genus of Cirripedia that Hancock had discovered in 1849 (see Correspondence vol. 4, letter to Albany Hancock, [21 September 1849]). Believing it would form a separate family, CD deferred examining it until 1853, when he had finished the common pedunculated and sessile barnacles. See the correspondence with Hancock early in 1853, especially letter to Albany Hancock, 30 March [1853].
See letter to Albany Hancock, 10 January [1853].
See Correspondence vol. 4, letter to Albany Hancock, [26 January – March 1850]. CD’s notes on Mollusca were made during the Beagle voyage (see Porter 1985, p. 1013).


Discusses capacity of some cirripedes to bore into rock.

Mentions Alcippe specimens borrowed from AH.

Relation of sexes in Ibla and Scalpellum.

Letter details

Letter no.
Charles Robert Darwin
Albany Hancock
Sent from
Source of text
Archives of the New York Botanical Garden
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 1495,” accessed on 29 April 2017,

Also published in The Correspondence of Charles Darwin, vol. 5