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 M^rs. & 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.—