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Darwin Correspondence Project

From Edward Frankland   17 July 1873

Royal College of Chemistry | Kensington Museum

July 17/73.

My dear Sir

I am sorry to learn that you cannot send the washings of the Drosera glands earlier than a fortnight hence, because this laboratory closes on the 26th. inst. & I leave for Switzerland a few days later. I shall be back & the laboratory reopened on the 1st. of October. Will that be too late?1

I am inclined to think that your suggestion about the Pepsin is probably correct especially as the secretion is slightly acid. The identification of Pepsin by analysis of washings would be exceedingly difficult if not impossible, because Pepsin is not a definite compound the composition of which is known, it is rather of the nature of a ferment & as such is probably continually undergoing chemical change.2

It appears to me that the most hopeful way of ascertaining if the gland liquid is of a pepsin character would be to operate with the concentrated liquid detached from the hairs & to ascertain lst. If its digestive powers are obliterated by the neutralisation of its acidity & again restored by the addition of hydrochloric acid. 2d. If its digestive powers are permanently destroyed by the addition of strong alkalies, and impaired by strong acids. 3rd. If the liquid is most active at blood heat.3

So far as the washing is concerned I am of opinion that a solution of carbonate of soda is preferable to one of carbonate of ammonia.

Believe me | Yours sincerely | E. Frankland

Charles Darwin Esq. F.R.S.

CD annotations

Top of letter: ‘2d letter’ red crayon circled red crayon


What Frankland and CD describe as a ferment contained some of the elements that would later be explained by the concept of enzyme. For a history of pepsin and the transition from ferment to enzyme, see Fruton 2002. Pepsin is only active in an acid environment, and it has a different chemical composition when it is not active.
In his letter to Edward Frankland, 21 September [1873], and Insectivorous plants, pp. 94–6, CD describes his experiments to ascertain the presence of pepsin in the digestive secretions of Drosera.


Fruton, Joseph S. 2002. A history of pepsin and related enzymes. Quarterly Review of Biology 77: 127–47.

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


Is going to Switzerland, so analysis of Drosera washings will be delayed. Agrees with CD that solvent is probably pepsin.

Letter details

Letter no.
Edward Frankland
Charles Robert Darwin
Sent from
R. Coll. Chem.
Source of text
DAR 164: 207
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 8981,” accessed on 31 July 2021,

Also published in The Correspondence of Charles Darwin, vol. 21