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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].


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

Please cite as

Darwin Correspondence Project, “Letter no. 2063,” accessed on 19 July 2019,

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