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Darwin Correspondence Project

From J. S. Burdon Sanderson   12 February 1875

49, Queen Anne Street. | W.

Feb. 12, 75

Dear Mr. Darwin,

Potassium phosphate is certainly decomposed in the animal body.1 A double decomposition no doubt takes place between it and common salt which is always present, by virtue of which Potassium chloride & sodium phosphate are produced. It appears to me likely that the reason why potassium phosphate does not act upon Drosera is this—that it is present in considerable proportion in the juices of plants & consequently probably in Drosera.2 I think it would be found that to those salts which are present in the plant it is indifferent, even though it may be excited by others which are chemically closely allied to them.

With reference to the subject of experiments on animals, I have after conversation with Mr Huxley prepared a memorandum intended to serve as the basis of a Petition.3 It is now in his hands. I have lately been considering very carefully the whole question and am of course very anxious that if anything is done, it should be done in such a way as to promote rather than to hinder the progress of science.

Believe me, Dear Mr Darwin | Very truly yours | JB Sanderson


Potassium is one of the elements essential for the growth of plant cells and is therefore always present in plant tissue. In his letter to Burdon Sanderson of 10 February 1875, CD had noted that phosphates other than potassium phosphate caused a powerful reaction in the leaves of Drosera (sundew).
See letter to J. S. Burdon Sanderson, 10 February 1875 and nn. 2 and 3. Mr Huxley: Thomas Henry Huxley. For more on the background to the debate on vivisection and CD’s role, see Feller 2009.


Feller, David Allan. 2009. Dog fight: Darwin as animal advocate in the antivivisection controversy of 1875. Studies in History and Philosophy of Biological and Biomedical Science 40: 265–71.


Discusses the experiments with phosphates on Drosera and animals.

Considers the question of preparing a petition on the question of animal experiments, with the aim of promoting rather than hindering science. [Response to 9849.]

Please cite as

Darwin Correspondence Project, “Letter no. 9853B,” accessed on 4 August 2021,

Also published in The Correspondence of Charles Darwin, vol. 23