To Albany Hancock 25 December 1
Down Farnborough Kent
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??
Discusses capacity of some cirripedes to bore into rock.
Mentions Alcippe specimens borrowed from AH.
Relation of sexes in Ibla and Scalpellum.
Please cite as
Darwin Correspondence Project, “Letter no. 1495,” accessed on 5 December 2016, http://www.darwinproject.ac.uk/DCP-LETT-1495