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 2^d & 3^d 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 D^r. 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 D^r. 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. D^r. 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