From H. C. Watson   14 December [1857]¹

My dear Sir

I will return your list of varieties in British Plts with some notes in course of few days.² But it is to be feared that I can very imperfectly meet your object.

1. As to the actual commingling:—it is seldom I can say aught of this. Many of the var^s. or subspecies I have seen only in gardens & herbaria. And of those which I have myself picked wild,—in divers instances I cannot now recollect whether mingled with the type or not.

2— As to difference of situation:— This is pretty often the case. For instance, there are maritime & alpine forms of many inland & lowland plants. In their appropriate places (shores & hills) they remain so different, as usually to be retained in lists for distinct species. And where the differences are less obvious, or transition forms frequent, they are set down as species & vary.— Corresponding instances occur with plants of relatively dry & wet places. But as the cause of the variety is here usually obvious, it is less frequently deemed a species, & often passed by.

Thus, Polygonum amphibium diag α natans

β terrestreramme are two states of a species, which no botanist would have united into one species, if they had arrived in England from 2 different countries. They are not in your list, just because they are known to be varieties from situation;—the very fact you want to reach thus causing their exclusion.

3. Area. Within our limited Country the area or geographic space of varieties is usually an included & considerably smaller portion of the area of the type species. (On a wider view, there are varieties outside the area of the type species. For instance, there are boreal or arctic forms of temperate species.) Alpine forms with us are usually not included in the area of the type species;—but they overlap somewhat.—

I wish your list had been taken from the 1857 edition of the London Catalogue.³

In answer to your question about the Cybele Britannica,—I quite hope to commence printing Vol. 4 by or before Spring. But the result of so much labour is so often compressed into two or three figures (no^s.) that progress is slow.—⁴

Yours very truly | Hewett C. Watson To | C. Darwin | Esq