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Darwin Correspondence Project

From Asa Gray   [c. 24 May 1857]1

Now about Protean genera, and Mr. Watson’s very proper discriminations.2 For want of these discriminations, my hasty list of “variable genera” can be of little use.

As to Mr. Watson’s three Categories,3 the 2d & 3d differ only in degree, and I see not how to draw any clear line of distinction between them. Where “species are so close that it is highly difficult to say whether … etc.”—there it must “become difficult to say where one species begins and another ends”. This, certainly i.e. Categories 2 & 3,—is what I had in view, with also an eye to the Category no 1.— For, see you, Mr. Watson cites Carex & Ranunculus under this head. Now Carex, viewed according to Dr. Boott, is a good case in point.4 The species very difficult to distinguish by reason of their similarity, but not remarkably variable; because he regards almost every definable form as a separate species. But if Dr. Hooker were to elaborate the genus, how would it be? Would it not fall at once into no. 3? Potamogeton, would be placed by Mr. Tuckerman under no 1.5 —by me under no. 3. etc—

As to Ranunculus—what I call R. repens (though, I wish I were surer it is the European sp.) figures as a dozen in books; and where I limit it, I am not clear about its boundaries, on one side. Several of the other species vary a good deal, but none are so very proteiform. Dr. Hooker, who refers our R. abortivus (I doubt if rightly) to R. auricomus, would rank Ranunculus under no. 2 or no. 3, incontinently.

So you must take my list, especially the underscored names, in the rough, as including those genera that I find most difficulty with here, in the complete limitation of the species,—or some of them, either, because of the great variability of a certain species, or the very close approximation of a greater number of species.—

I should say the greater part of those underscored in my list were of the former sort.—excluding, Liatris, Eupatorium, Solidago? Salix, & Populus,—& Carex.

I have not time just now to put my mind on the subject, however, and must close my rambling letter without reading it over.

Kindly give me any remarks that strike you, on reading over my last article,—which I sent you Journal sheets of.6 Your letters are always most instructive to me, and I only regret I have not the opportunity to think and write more upon the topics they bring up.—

Kindly post the 3 enclosed letters, and believe me to remain

Ever Yours | A. Gray

CD annotations

crossed pencil
Top of first page: ‘Ch. 4’7 pencil; ‘June 9th—1857’ink


The date of the letter is conjectured from Gray’s reference in letter from Asa Gray, 1 June 1857, to having ‘despatched’ a letter to CD ‘last week’ and from CD’s endorsement, which probably records the date of receipt (see CD’s annotations).
CD had returned Gray’s list of protean genera (letter from Asa Gray, 16 February 1857); he had also sent Hewett Cottrell Watson’s comments on this list to Gray (see letters from H. C. Watson, 10 March 1857 and from H. C. Watson to Asa Gray, 13 March 1857).
Francis Boott was a specialist on the genus Carex.
Edward Tuckerman had worked on Potamogeton in 1849 (Tuckerman 1849).
Gray had sent CD the third part of A. Gray 1856–7 (see letter to Asa Gray, 9 May [1857]).
CD refers to the chapter of his species book on ‘Variation under nature’ (Natural selection, pp. 92–171).


Gray, Asa. 1856–7. Statistics of the flora of the northern United States. American Journal of Science and Arts 2d ser. 22: 204–32; 23: 62–84, 369–403.

Natural selection: Charles Darwin’s Natural selection: being the second part of his big species book written from 1856 to 1858. Edited by R. C. Stauffer. Cambridge: Cambridge University Press. 1975.

Tuckerman, Edward. 1849. Observations on American species of the genus Potamogeton, L. American Journal of Science and Arts 7: 347–60.


Discusses difficulties involved in deciding which genera are protean in the light of some comments by H. C. Watson.

Letter details

Letter no.
Asa Gray
Charles Robert Darwin
Sent from
Source of text
DAR 165: 97
Physical description
ALS 2pp inc †

Please cite as

Darwin Correspondence Project, “Letter no. 2104,” accessed on 26 September 2022,

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