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Darwin Correspondence Project

To F. J. Cohn   24 August 1875

Down, | Beckenham, Kent. | Railway Station | Orpington. S.E.R.

Aug 24 75

My dear Sir

I am very much obliged for your letter which has gratified me extremely, as I value your good opinion on the subject of this book, more than that of almost any other man in Europe.1

I assumed that the matter which becomes aggregated was protoplasm, as I did not believe that any other known substance had the power of such peculiar amœboid movements. You will find that I describe the protoplasm that flows round the walls to be colourless, and that the granules which it contains are ultimately united with the central masses.2 After the process of aggregation these masses float in colourless or almost colourless fluid, and I assumed that the colouring matter of the fluid was filtered or sifted out of it by the protoplasm, as it coalesced or aggregated. Do not the grains of chlorophyll consist of protoplasm coloured by this matter; & why should not the aggregated masses be tinted in the same way by the red colouring matter? But you will know all this far better than I do.

Pray again accept my thanks & believe me Dear Sir | Yours sincerely | Charles Darwin


In his letter of 21 August 1875, Cohn had queried whether CD had shown that the substance aggregated in cells of the tentacles of Drosera rotundifolia (common or round-leaved sundew) was protoplasm. In Insectivorous plants, p. 38, CD described colourless circulating protoplasm lining the cell walls with a homogeneous purple fluid in the centre.


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


Responds to FJC’s criticism regarding "aggregation" as it occurs in protoplasm [see 10131].

Letter details

Letter no.
Charles Robert Darwin
Ferdinand Julius Cohn
Sent from
Source of text
DAR 185: 99
Physical description
LS(A) 4pp

Please cite as

Darwin Correspondence Project, “Letter no. 10134A,” accessed on 5 February 2023,

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