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Darwin Correspondence Project

To Lawson Tait   5 May 1876

Down, | Beckenham, Kent.

May 5. 76

My dear Sir

I have at last heard about the Physiological Referee, but my informant was directed not to communicate to me the exact language, but only the sense of the Referee’s report.1 I will now copy what my informant says:—2

“The referees report that the modifications which Mr L. T has introduced into Brücke’s process for isolating pepsin consist in neglecting certain precautions without which the method is useless.3 He relies on neutralisation for separating his droserin.4 This process can have no diagnostic value, seing that innumerable substances would behave in this manner. The hygroscopic quality of his azein on which he insists is also unimportant since it is common to many derivitives of proteids, ex: gr. peptones5   His method of determining the nature of the acid by comparative trials is valueless, because he has reduced them to a standard strength & Brücke has shown that different acids act equally effectively at different strengths. His method however of estimating the degree of acidity of the different acids is in itself defective in the absence of any evidence as to the purity & constant quality of the litmus used”.6

I am extremely sorry to be compelled to convey the above information to you my dear Sir | Yours very faithfully | Charles Darwin


After the Royal Society of London had rejected his paper on the tropical pitcher-plant, Nepenthes, Tait asked CD about the objections of the physiological referee (see letter from Lawson Tait, 25 April [1876]). The physiological referee was Michael Foster, and CD’s informant was probably Joseph Dalton Hooker (see letter from J. D. Hooker, [18 April 1876] and n. 3, and letter to W. T. Thiselton-Dyer, 26 April [1876]).
The communication from which CD copied the report has not been found.
In 1861, Ernst Wilhelm von Brücke had described a process for isolating the digestive agent pepsin (see Brücke 1861, pp. 618–19; Fruton 2002, p. 131).
Tait had used Brücke’s process to isolate a nitrogenous substance in the digestive fluid of Nepenthes that he named droserin (see Correspondence vol. 23, first letter from Lawson Tait, 12 June [1875]). The paper he submitted to the Royal Society has not been found, but Tait later described his modification of the process and his method of neutralisation in a paper on the digestive principles of plants that was published in the Proceedings of the Birmingham Philosophical Society (L. Tait 1879, pp. 126–7).
Azerin was a second digestive fluid isolated by Tait; according to Tair, dried azerin could absorb more than forty times its bulk of water from the air (see Correspondence vol. 23, letter from Lawson Tait, 16 November [1875]). Proteids was the term used in Britain from 1871 to designate protein bodies or substances after it had been shown that there were more than one, but in Germany the term was used to signify more complex protein compounds; the confusion continued until 1907, when, on the recommendation of the Committee on Proteid Nomenclature, ‘proteins’ replaced ‘proteid’ as the term for protein bodies (OED s.v. protein). Proteins that can be broken down by gastric juices form peptones, which are then absorbed. Ex: gr. (exempli gratia): for example.
Tait was already aware that his litmus paper might not be reliable when, contrary to the findings of other researchers, he found acid in the fluid of an unopened pitcher (see Correspondence vol. 23, letter from Lawson Tait, 23 November [1875]; L. Tait 1879, p. 128).


Brücke, Ernst Wilhelm. 1861. Beiträge zur Lehre von der Verdauung. Sitzungsberichte der Akademie der Wissenschaften in Wien. Mathematisch-naturwissenschaftliche Klasse 43 (2d part): 601–23.

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

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

OED: The Oxford English dictionary. Being a corrected re-issue with an introduction, supplement and bibliography of a new English dictionary. Edited by James A. H. Murray, et al. 12 vols. and supplement. Oxford: Clarendon Press. 1970. A supplement to the Oxford English dictionary. 4 vols. Edited by R. W. Burchfield. Oxford: Clarendon Press. 1972–86. The Oxford English dictionary. 2d edition. 20 vols. Prepared by J. A. Simpson and E. S. C. Weiner. Oxford: Clarendon Press. 1989. Oxford English dictionary additional series. 3 vols. Edited by John Simpson et al. Oxford: Clarendon Press. 1993–7.

Tait, Lawson. 1879. Researches on the digestive principles of plants. [Read 22 May 1879.] Proceedings of the Birmingham Philosophical Society 1 (1876–9) pt 2: 125–39.


CD sends the gist of an extremely negative report from the [Royal Society’s] physiological referee on the value of RLT’s modifications of Brücke’s process for isolating pepsin [see 10470].

Letter details

Letter no.
Charles Robert Darwin
Robert Lawson (Lawson) Tait
Sent from
Source of text
Shrewsbury School, Taylor Library
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10497,” accessed on 26 October 2021,

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