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Darwin Correspondence Project

To J. H. Gilbert   16 February 1876

Down | Beckenham | Kent

Feb 16. 76

My dear Sir

When I met you at the Linn: Soc you were so kind as to say that you would aid me with advice, and this will be of the utmost value to me & my son.1 I will first state my object and I hope that you will excuse a long letter. It is admitted by all naturalists that no problem is so perplexing as what causes almost every cultivated plant to vary, and no experiments as yet tried have thrown any light on the subject. Now for the last ten years I have been experimenting in crossing and self-fertilising plants; & one indirect result has surprised me much namely that by taking pains to cultivate plants in pots under glass during several successive generations, under nearly similar conditions and by self-fertilising them in each generation the colour of the flowers often changed, and what is very remarkable they became in some of the more variable species, such as Mimulus, Carnation &c, quite constant like those of a wild species. This fact & several others have led me to the suspicion that the cause of variation must lie in different substances absorbed from the soil by these plants when their powers of absorption are not interfered with by other plants with which they grow mingled in a state of nature. Therefore my son & I wish to grow plants in pots in soil entirely, or as nearly entirely as is possible destitute of all matter which plants absorb; and then to give during several successive generations to several plants of the same species as different solutions as may be compatible with their life & health. And now can you advise me how to make soil approximately free of all the substances which plants naturally absorb. I suppose white silver sand, sold for cleaning harness &c is nearly pure silica, but what am I to do for alumina?2 Without some alumina I imagine that it would be impossible to keep the soil damp and fit for the growth of plants. I presume that clay washed over & over again in water would still yield mineral matter to the carbonic acid secreted by the roots. I should want a good deal of soil, for it would be useless to experimentize unless we could fill from 20 to 30 moderate sized flower-pots every year. Can you suggest any plan; for unless you can it would I fear be useless for us to commence our attempt to discover whether variability depends at all on matter absorbed from the soil. After obtaining the requisite kind of soil my notion is to water one set of plants with nitrate of potassium another set with N of sodium, & another with N of lime, giving all as much phosphate of ammonia as they seemed to support, for I wish the plants to grow as luxuriantly as possible. The plants watered with nitrate of Na & of Ca would require I suppose some K; but perhaps they would get what is absolutely necessary from such soil as I should be forced to employ and from the rain water collected in tanks.3 I could use hard water from a deep well in the chalk but then all the plants would get lime.4 If the plants to which I give nitrate of Na & of Ca would not grow I might give them a little alum.

I am well aware how very ignorant I am and how crude my notions are, and if you could suggest any other solutions by which plants would be likely to be affected it would be a very great kindness. I suppose that there are no organic fluids which plants would absorb & which I could procure. Do you think Hopkins and Williams would be good men from whom to get the salts?5

I must trust to your kindness to excuse me for troubling you at such length & I remain | Dear Sir yrs sincerely | Charles Darwin


CD and Francis Darwin had been in London to vote for Edwin Ray Lankester’s membership application at the meeting of the Linnean Society on 3 February 1876 (Proceedings of the Linnean Society of London (1875–6): iii). Francis was assisting CD in his experiments for Cross and self fertilisation.
Silica is silicon dioxide (SiO2); alumina is aluminium oxide (Al2O3).
The chemical symbol for nitrogen is N; sodium is Na; calcium is Ca; potassium is K; phosphorus is P. Potassium nitrate is KNO3; sodium nitrate is NaNO3; nitrate of lime is calcium nitrate or Ca(NO3)2. There are several forms of phosphate of ammonia, but diammonium phosphate ((NH4)2HPO4) is the most commonly used fertiliser.
Hard water is water with a high mineral content as a result of percolating through mineral deposits such as chalk (calcium carbonate).
Hopkin & Williams were manufacturers of fine laboratory and photographic chemicals, with offices at 10 Cavendish Street, London, and a factory in Wandsworth, Surrey (Post Office London directory 1875). CD had obtained chemicals from them when he was working on Insectivorous plants (see Correspondence vol. 21, letter to W. W. Baxter, 4 September 1873).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Cross and self fertilisation: The effects of cross and self fertilisation in the vegetable kingdom. By Charles Darwin. London: John Murray. 1876.

Insectivorous plants. By Charles Darwin. London: John Murray. 1875.

Post Office London directory: Post-Office annual directory. … A list of the principal merchants, traders of eminence, &c. in the cities of London and Westminster, the borough of Southwark, and parts adjacent … general and special information relating to the Post Office. Post Office London directory. London: His Majesty’s Postmaster-General [and others]. 1802–1967.


Describes self- and cross-fertilisation experiments.

Asks JHG’s advice on setting up an experiment designed to test whether the cause of variation in cultivated plants lies in different substances absorbed from the soil when absorption is not interfered with by other plants in a state of nature. Can JHG suggest how he can get soil free of all the substances which plants naturally absorb?

Letter details

Letter no.
Charles Robert Darwin
Joseph Henry Gilbert
Sent from
Source of text
Rothamsted Research (GIL13)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10399,” accessed on 2 July 2020,

Also published in The Correspondence of Charles Darwin, vol. 24