My dear Sir

Allow me to suppose your letter divisible into four questions, instead of two.

1^st. Sir Charles Lyell’s wish or advice seems sound, that you should not postpone publication of your views to a distant date;—rather, look forward to perfect or improve at the future time. ¹

2^d. My own volumes on distribution of British plants, prior to Cybele Britannica, were either three forms or three editions—expanding onwards.² The third went very much into detail, but only one part (3 orders) was printed as an experiment, & not published. Had that third form been proceeded with, the information you want as to the species common to Europe & America would have been found. But as only 39 species were treated, the mere fragment would be practically useless. A Table or List in the second form (“Remarks”) attempts to give the information, but incompletely & too condensedly.³ At its date, the published data for America were far less ample & reliable than is now the case; & the distribution of the species beyond Britain was restricted to a single line for each. ⁴

3^rd. America certainly possesses very few European species, which are absent from the Scandinavian Peninsula & Russia northward of 55^o. Perhaps it cannot be said positively that no exceptions are found. But among the few which are reported to be so, some are highly questionable in respect to the identity of the species, or to the certain nativity on both continents. I will look out a few examples, to show what is here meant, & write them on a separate paper.⁵

4^th. In England some 40 or 50 species of plants may be held as restricted, or almost restricted, to tillage ground (ploughed or dug); & about as many more are prevalent chiefly on such ground, altho’ more or less extending to road sides, sands, commons, old gravel pits, etc. I think these 50 or 100 species do not run into varieties more than other species of the same genera or same orders, which grow in other situations. If any difference in this respect, it may be that they are less widely variable, on an average, than are the non-agrarian species. But there are some striking instances of variability among the agrarians; and especially among those which grow also in other situations. For example, the varieties of Polygonum aviculare & Viola tricolor are numerous & wide; the agrarian forms differ much from those which grow in other situations; and the agrarian forms vary among themselves also. It would be easy to pick out three, or six, or perhaps more numerous varieties of either of these two species, which would have been received as so many good & distinct species, if they had been brought from different countries, & unaccompanied by those intermediate forms, which seemingly connect them into single species.⁶

Assuming that agrarian weeds differ less widely than others, on an average,—how far may this assumed fact be connected with another fact, that they are almost all of them Annuals? Ask of British botanists, which genera are their greatest opprobria in technical description,—in which is it most difficult to define & describe the Species, & to decide as to which forms should be called Species, & which should be called varieties?— They will answer, Salix, Rubus, Rosa, Mentha, Batrachium (sub-genus of Ranunculus), Potamogeton, Poa, Festuca, Viola, Galium,—Genera exclusively or chiefly of perennial Species.

You can make such use as you deem fit of answers to questions, which you pay me the compliment of asking;—but I fear the replies are unavoidably of that vague & unsatisfactory kind which must render them of no really available use—

Very faithfully | Hewett C. Watson To C. Darwin Esq

Examples of alleged American & European plants, not extending far north in Europe—⁷ Chrysosplenium oppositifolium. Said to occur in middle Russia & extending eastward to Altai. Absent from Sweden, but found in Denmark & the more southern portion of Norway. In Britain plentiful, extending to Orkney, & high on the Grampian Mountains. Absent from Shetland, Faroe, & more northerly isles. Was formerly held to be an American plant, found in various parts of Canada, &c. But in the Flo. Bor. Am. of Hooker the Eastern American plant is described as a distinct species, under name of Americanum.⁸ Still, Hooker keeps a Western form as a Variety of C. oppositifolium, viz. “β Scouleri”, from Columbia river; & he remarks on the resemblance of this Western-American form to the C. Nepalense of N. India, in its leaves, tho’ more like C. oppositifolium by its inflorescence. Thus, we have here a species which would have been deemed common to both Continents, tho’ not extended far northward in either, if the question had been asked some years ago. Now, they are either different “Species”, or different “varieties”. Looking beyond these two (“oppositifolium” & “β Scouleri”) we find Chamisso joining to them also the C. Nepalense & C. Kamtschaticum, as other varieties of the type species C. oppositifolium;⁹ thus tracing this last, but only in its alleged varieties, round the earth in latitudes which attain a culminating point northwards in Orkney. Isnardia palustris. Less of a boreal plant than the preceding. In Europe, attaining to the South of England, North of Germany, & S.E. of Russia. In America, to the Saskatchawan, & far southward.— Perhaps a true example,—but a semi-aquatic, widely distributed. Astragalus Hypoglottis. Absent from the Scandinavian Peninsula & N. Russia. Occurs in Scotland, Denmark, Island of Oesel, Middle Russia (say to lat 58^o), Siberia. In British America, northward to the Saschatchawan river. Thlaspi alpestre.— England, Forfarshire, Gottland, Germany, &c; Absent from Russia, & Scandinavian peninsula. A single authority for its occurrence in Canada (Hooker Flo. Bor. Am.) but whether the author had seen specimens, or recorded on statement only, does not appear. Torrie & Gray simply copy, & add “Introduced?”¹⁰ But it is an unlikely plant to be introduced. The European “T. alpestre” has been latterly subdivided into various species by Jordan &c.¹¹ Arenaria verna. Not admitted as a plant of Scandinavia, in the Summa Vegetabilium of Fries.¹² It occurs on some few hills in Scotland, northward to Aberdeenshire. Hooker (Flo. Bor. Am.) says that the specimens from Canada & Western America correspond with the A. Verna of Britain. It is to be noted, however, that Ledebour (Flora Rossica)¹³ combines with this a number of so-called species, & thus makes the grouped forms, or the name, represent a species extending into arctic latitudes. Lythrum hyssopifolium— In Europe, northward to England & Lithuania; not in Denmark, Baltic Isles, or Scandinavian peninsula. Locally in the American States, & perhaps introduced. These half dozen species are perhaps as good examples as can be cited, of species alleged to be common to Europe & America, but having an early limit northward, comparatively with most species certainly common to both continents.