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Darwin Correspondence Project

To J. D. Hooker   10 December [1856]1

Down Bromley Kent

Dec 10th

My dear Hooker

I thank you sincerely about the trees: it must have cost you much more trouble than I anticipated. Now that the result is what it is in N. Zealand (I find trees occur in 38 Families, & in exactly half of these the trees have separated sexes; so that it is not an attribute related to any particular systematic structure) & with the much feebler case of England, & with the great number of trees in Persoon’s Synopsis2 in the mono- & dioicous classes (though I have here no accurate standard of comparison), I think it pretty certain there is a relation between trees & separation of sexes; whether or not my explanation is correct.—3 I have had lately some hard blows against my crossing theory, & hardly know what to think.4 I shall be very glad to hear about Tasmanian trees, & perhaps you will find the result worth a paragraph in your introduction.— It is a most tiresome drawback to my satisfaction in writing, that though I leave out a good deal & try to condense, every chapter runs to such an inordinate length: my present chapter on causes of fertility & sterility & on natural crossing has actually run out to 100 pages M.S., & yet I do not think I have put in anything superfluous.—5

My wife is going on capitally in every respect.— Give my best thanks to Mrs Hooker for her kind note.—

What a funny story about Lyell & the Review (which I have not yet seen); but really I think the most probable solution is that he forgot at moment that the Review was laudatory of yourself; you are the very last man whom he would suspect of praising yourself.—6

I have for last 15 months been tormented & haunted by land mollusca, which occur on every oceanic island; & I thought that the double creationists or continental extensionists had here a complete victory. The few eggs which I have tried both sink & are killed. No one doubts that salt-water wd be eminently destructive to them; & I was really in despair, when I thought I would try them when torpid; & this day I have taken a lot out of sea-water, after exactly 7 days immersion. Some sink, & some swim; & in both cases I have had (as yet) one come to life again, which has quite astonished & delighted me. I feel as if a thousand pound weight was taken off my back.7

Adios my dear kind friend | C.D.

P.S. | I must tell you another of my profound experiments! Franky said to me, “why shd not a bird be killed (by hawk, lightning, apoplexy, hail &c) with seeds in crop, & it would swim.” No sooner said, than done: a pigeon has floated for 30 days in salt water with seeds in crop & they have grown splendidly & to my great surprise even tares (Leguminosæ, so generally killed by sea-water) which the Bird had naturally eaten have grown well.— You will say gulls & dog-fish &c wd eat up the carcase, & so they wd. 999 out of a thousand, but one might escape: I have seen dead land bird in sea-drift.—8

Keep in mind some difficulty about distribution of F. W. Plants, which you alluded to at Kew when I was there; & I had not time to ask you about.— I do not mean write it, but for when we meet.—

I wonder whether Dr. Harvey would think it a bore to illuminate me about separation of sexes, how far general, in Algæ?9 What do you think?


Dated by the relationship to the letter from J. D. Hooker, 7 December 1856.
CD discussed this point in Natural selection, pp. 61–2.
See letter to George Bentham, 26 November [1856], letter from H. C. Watson, 26 November 1856, and letter to George Bentham, 30 November [1856]. The difficulties that CD felt in relation to the theory that all organic beings occasionally cross are given in Natural selection, pp. 63–71.
CD was preparing chapter 3 of his species book, ‘On the possibility of all organic beings occasionally crossing, & on the remarkable susceptibility of the reproductive system to external agencies’ (Natural selection, pp. 35–91). It was completed on 16 December 1856 (‘Journal’; Appendix II).
See letter to P. H. Gosse, 28 September 1856, n. 4, and the letter from T. V. Wollaston, [11 or 18 December 1856]. CD’s experiment, dated 3 December 1856, is recorded in his Experimental book, p. 17 (DAR 157a).
CD’s experiment was entered in his Experimental book, pp. 16–17 (DAR 157a). CD mentioned Francis Darwin’s suggestion in Origin, p. 361, where he proposed the floating carcases of birds as one of a number of ‘occasional’ means of dispersal.
William Henry Harvey was an expert on Algae. CD had sent him Algae specimens from the Beagle voyage (Correspondence vol. 4, letter to W. H. Harvey, [7 April 1847]).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Natural selection: Charles Darwin’s Natural selection: being the second part of his big species book written from 1856 to 1858. Edited by R. C. Stauffer. Cambridge: Cambridge University Press. 1975.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

Persoon, Christiaan Henrik, ed. 1805–7. Synopsis Plantarum, seu enchiridium botanicum, complectens enumerationem systematicam specierum hucusque cognitarum. 2 pts. Paris and Tubingen.


CD is convinced of relation between separation of sexes and tree-habit.

Recent hard blows against crossing theory.

CD long tormented by land molluscs on oceanic islands; found transport possible experimentally.

Letter details

Letter no.
Charles Robert Darwin
Joseph Dalton Hooker
Sent from
Source of text
DAR 114: 186
Physical description
ALS 6pp

Please cite as

Darwin Correspondence Project, “Letter no. 2018,” accessed on 1 March 2024,

Also published in The Correspondence of Charles Darwin, vol. 6