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Darwin Correspondence Project

From H. C. Watson   10 March 1857

Thames Ditton

March 10th/57

My dear Sir

I am not quite sure of understanding your question about “variable genera”.1 To explain my uncertainty, I will endeavour to define or state the differences for choice.

1. Genera, of which the species are close, & difficult to distinguish by reason of their similarity;—but the species themselves not remarkably variable. Ex: Carex & Ranunculus (excluding Batrachium)

2. Genera, of which the quasi species are so close that it becomes highly difficult to say whether the genus is composed of a comparatively few extremely variable species, or of many very close species. Ex: Rubus & Hieracium.

3. Genera, the species of which are themselves so variable, & approximating, that it becomes difficult to say where one species ends & the next begins. Ex: Viola & Saxifraga, at least in certain sections or subgenera—

It seems to me that Dr. A. Gray may have inclined to the first, while you perhaps yes certainly intend 2 or 3. I will copy the three categories, that you maybe

CD annotations

crossed pencil
Top of first page: ‘Please return to me’ink; ‘1)’ink 2


CD had asked Watson to comment on the list of ‘protean’ genera (in which the species present a great amount of variation) included in the letter from Asa Gray, 16 February 1857. CD discussed protean genera in Origin, p. 46.
The annotations relate to a later occasion when CD forwarded Watson’s letter to Asa Gray. See letter to Asa Gray, [after 15 March 1857].


Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.


HCW is trying to define what CD means by "variable" genera.

Letter details

Letter no.
Hewett Cottrell Watson
Charles Robert Darwin
Sent from
Thames Ditton
Source of text
DAR 181: 35
Physical description
inc †

Please cite as

Darwin Correspondence Project, “Letter no. 2063,” accessed on 22 July 2024,

Also published in The Correspondence of Charles Darwin, vol. 6