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Darwin Correspondence Project

From E. E. Klein   24 July 1874

The Brown Institution, | Wandsworth Road, S.W.

July 24, 1874.

Dear Mr. Darwin,

I am afraid I cannot give you a direct answer to your question, but it may be perhaps of some value to you to know, that the smallest micrococci which are distinctly discernable under a high power as 800 diam., and which micrococci belong to the smallest objects, are estimated to be 0.0005–0.0002 and less of a millimetre.1

very sincerely yours | E. Klein.

CD annotations

End of letter: ‘The specific gravity of the Phosphate is really 1.678’2 ink, enclosed in box blue crayon


CD’s letter has not been found, but he evidently asked about sensitivity to minute amounts of a substance (see letter to F. C. Donders, 7 July 1874). CD had observed that a very small amount of phosphate of ammonia applied to a gland of Drosera rotundifolia (common or round-leaved sundew) caused the tentacle bearing the gland to inflect. CD had asked Donders for an analagous case of extreme sensitivity and would have asked Klein a similar question. In Insectivorous plants, p. 173 n., CD referred to Klein’s information in his discussion of the sensitivity of glands of Drosera. Micrococcus is a genus of bacteria.
CD’s annotation relates to his question to Edward Frankland about the specific gravity of phosphate of ammonia (see letter to Edward Frankland, 22 July 1874 and n. 1).


Gives details of the smallest micrococci distinctly discernible under a microscope.

Letter details

Letter no.
Edward Emanuel Klein
Charles Robert Darwin
Sent from
Brown Institution
Source of text
DAR 57: 130v
Physical description
1p †

Please cite as

Darwin Correspondence Project, “Letter no. 9564,” accessed on 15 July 2019,

Also published in The Correspondence of Charles Darwin, vol. 22