To John Stevens Henslow   [1 April 1848]

Down Farnborough Kent

Saturday night.

My dear Henslow

Thank you for your note & giving me a chance of seeing you in town; but it was out of my power to take advantage of it, for I had previously arranged to go up to London on Monday. I should have much enjoyed seeing you. Thanks, also, for your Address,1 which I like very much. The anecdote about Whewell & the tides,2 I had utterly forgotten; I believe it is near enough to the truth.— I rather demur to one sentence of yours, viz “however delightful any scientific pursuit may be, yet if it shall be wholly unapplied it is of no more use than building castles in the air”. Would not your hearers infer from this that the practical use of each scientific discovery ought to be immediate & obvious to make it worthy of admiration? What a beautiful instance Chloriform is of a discovery made from purely scientific researches, afterwards coming almost by chance into practical use.3 For myself I would, however, take higher ground, for I believe there exists, & I feel within me, an instinct for truth, or knowledge or discovery, of something same nature as the instinct of virtue, & that our having such an instinct is reason enough for scientific researches without any practical results ever ensuing from them.—4 You will wonder what makes me run on so, but I have been working very hard for the last 18 months on the anatomy &c of the Cirripedia (on which I shall publish a monograph) & some of my friends laugh at me, & I fear the study of the cirripedia will ever remain “wholly unapplied”5 & yet I feel that such study is better than castle-building.

Talking of Cirripedia, I must tell you a curious case I have just these few last days made out: all the Cirripedia are bisexual,6 except one genus, & in this the female has the ordinary appearance, whereas the male has no one part of its body like the female & is microscopically minute; but here comes the odd fact, the male or sometimes two males, at the instant they cease being locomotive larvæ become parasitic within the sack of the female, & thus fixed & half embedded in the flesh of their wives they pass their whole lives & can never move again. Is it not strange that nature should have made this one genus unisexual, & yet have fixed the males on the outside of the females;—the male organs in fact being thus external instead of internal.—7

I am delighted to hear good accounts of Hooker: if he should write to you any letter which might be forwarded,8 I should be very glad to see it: I seldom hear any news of him, as I am so little in town, & never see Sir W. Hooker. I do hope his great undertaking will well answer to him.—

Pray remember me very kindly to Mrs. & Miss Henslow. How pleasant the meeting at Oxford was;9 it is a white week10 in my memory. We are all well here, & a sixth little (d)11 expected this summer: as for myself, however, I have had more unwellness than usual.12

Believe me, my dear Henslow. | Ever most truly yours | C. Darwin

If you are ever starting any young naturalist with his tools, recommend him to go to Smith & Beck of 6 Colman St. City for a simple microscope: he has lately made one for me, partly from my own model & with hints from Hooker, wonderfully superior for coarse and fine dissections than any I ever before worked with. If I had had it sooner, it would have saved me many an hour.—

1
Henslow 1848a.
2
‘And to what profession ought a correct theory of the Tides to be of more importance than to the Navy?— and yet a scientific friend of mine was nearly laughed out of countenance by some brother officers, for having asked them … to assist in making certain observations that had been called for by the Master of Trinity College, Cambridge (Rev. Dr. Whewell), who has greatly occupied himself for several years in perfecting this branch of astronomical science. But when, afterwards, a printed copy of instructions and an account of the object in view was sent out to my friend, his sceptical comrades immediately and readily acknowledged that the Master of Trinity knew more about the theory of the Tides than all the officers of the Navy together; and they heartily set themselves to work in making those observations which were called for.’ (Henslow 1848a, p. 6).
3
The use of chloroform as an anaesthetic in obstetric and surgical cases was pioneered by James Young Simpson in November 1847. For CD and Emma’s use of chloroform see letter to Francis Boott, 20 August 1848.
4
In this sentence, ‘of the’ was added in pencil after ‘something’ and closing double quotation marks were added at the end of the sentence, possibly by Henslow.
5
Despite CD’s fear, his work on cirripedes is considered to have been of ‘enormous practical benefit … for those studying marine fouling in which barnacles have played such a prominent and commercially important role’ (Crisp 1983, p. 82).
6
CD used the term ‘bisexual’ to mean hermaphrodite, although he occasionally also used the term to denote species with both male and female individuals, as in the letter to Joseph Dalton Hooker, 10 May 1848. In Living Cirripedia (1854): 23, CD explained that: ‘Cirripedes are commonly bisexual or hermaphrodite, but in Ibla, Scalpellum, and Alcippe, members of the Lepadidæ in the order Thoracica, and in Cryptophialus in the order Abdominalia, the sexes are separate.’ See also Natural selection, pp. 33–4.
7
The description corresponds to that of Ibla cumingii in Living Cirripedia (1851): 189–203. Since the majority of cirripedes, in contrast to other members of the class Crustacea, are hermaphrodites, CD’s discovery of separate sexes in Ibla was an important one. There were, moreover, few examples in the animal kingdom of males of a species differing to such an extent in size and structure from the female, and of males living parasitic on the female (see Living Cirripedia (1851): 200 and n.).
8
Joseph Dalton Hooker had left for India in November and arrived at Calcutta on 12 January 1848. The ‘you’ is underlined because Henslow’s daughter Frances was engaged to Hooker.
9
The meeting of the British Association, June 1847. The Henslow family, J. D. Hooker, and CD had gone on several outings together (‘Journal’, Correspondence vol. 4, Appendix I; Allan 1967, p. 158).
10
White: ‘(Chiefly of times and seasons) Propitious, favourable; auspicious, fortunate, happy’ (OED).
11
Henslow referred to CD’s children as little ‘d’s (see letter to J. D. Hooker, 10 May 1848 and Correspondence vol. 2, letter from J. S. Henslow, 9 October 1843).
12
From July to the end of the year CD complained of being ‘unusually unwell, with swimming of head depression, trembling & many bad attacks of sickness’ (‘Journal’; Correspondence vol. 4, Appendix I).

Manuscript Alterations and Comments

1.8 practical] interl
1.11 almost by chance] interl
1.12 ground] after del ‘gru’
1.18 the study] after del ‘I’
2.6 thus fixed] after del ‘here’
2.6 & half embedded in the flesh of their wives 2.7] interl

Please cite as “DCP-LETT-1167,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-1167