From H. C. Watson   23 March 1858

My dear Sir

As you write about not publishing for a couple of years, it is needless to send you the proof to which I alluded. It will be better sent in volume, when ready.¹

I do not quite understand your query about the comparative range of species in large or small genera,—in its application to Cybele Britannica.² The geographic area of almost every reliable species, found in the British Isles, is wider than those isles; the space over which it is spread in Britain being only a fragment or portion of its total area.

The Spiranthes cernua of South Ireland is the only exception, in addition to disputed species. Some of the most local of British plants have very wide geog-raphic areas, ex: gr: Cynodon Dactylon. Thus, whether widely or locally spread in Britain per se, is no test of the real area of a species.— Nor does frequency of repetition much avail; for some plants frequent in Britain, have rather restricted areas elsewhere, ex: gr: Erica cinerea.—

A much wider geographic area should be taken, than our Isles, to try whether the species of large genera, or of large orders, are more or less widely spread.—

I should quite expect that the species of small genera would be most widely spread, if this were tried by books;—because they are the species of large genera that Authors find most difficult to identify. If we believe our books, there are several species of Rubus, Rosa, Hieracium, Salix, &c. peculiar to Britain;—but these are likely enough species (true or false) not identified elsewhere, as the same, though existing elsewhere.—

One of my lists in the Volume printing will be a Census of species, with them all placed in sequence according to frequency of repetition & area; taking as thetest or measure the number of Provinces—Subprovinces—Counties & Vice-Counties.³ It has been an immense consumer of time, & is not quite done.

The number of provinces given in the 5 ^th. edition of the London Catalogue of British Plants is nearly correct & complete.⁴ Those numbers show the areas better than the smaller & more numerous subdivisions. The latter show the comparative frequency better.

I give these explanations, that you may judge whether I have aught in hand likely to meet your wants in details.—

But as I look upon the orders & genera of plants as purely conventional arrangements, not natural groups,—that is, not as groups in nature, but only as groups in books & herbaria;—the question whether the species of large or small orders are more common seems to me like a question of law,—one of human arrangement only.—

Very truly | Hewett C. Watson C. Darwin Esq