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Darwin Correspondence Project

From H. C. Watson to Asa Gray1   13 March 1857

13 March 1857

First, with regard to a point in Mr. D’s last letter.2 The categories 2 & 3 are different chiefly by the manner of viewing & grouping the facts, rather than by the natural facts of themselves;—& they even pass into 1 from the like cause; altho’ 1 is essentially more different. Suppose, that 200 forms under one genus can be recognized by the eye, & described by words. Suppose, that one botanist groups these into 60 species; and that another groups them into 20 species;—in the latter case the 20 species will seem to be, on an average, thrice as variable as the 60 species of the former case. Such differences, & even greater differences, are found among botanical systematists; and they bear importantly on investigations such as Mr. D’s. Thus, in arranging a collection of British Hieracia into one dozen species (Hudson, 12)3 we readily distinguish the type examples from each other; but then there still remain many other examples so aberrant from these types, & more or less intermediate, as to render their assignment among the twelve species very difficult. In arranging the same collection into nearly three dozen species (Backhouse, 33),4 the difficulty of assignment is found more in the closeness of the presumed species, than in the number of aberrant & intermediate examples. There is left less space for varieties (so to write) between the type forms of the more numerous species, than between those of the less numerous & less similar species. Not only does the same genus thus vary in regard to species in the works of different Authors;—but further, by a sort of conventional consent of the let-alone kind, some genera are habitually less divided into species than others. It is only of late years that Rubus & Hieracium have been so greatly subdivided into species. On the contrary, Rosa and Mentha are now less so subdivided than formerly. Or, to take an example from Species merely:—Ranunculus aquatilis & Polygonum aviculare, two common British plants, were held single species by Linneus & his early successors. The former is now divided into a dozen or upwards; the latter left entire. Both are proteiform; & as far as external form goes, either might be made into a dozen species; the subdivisions of Polygonum aviculare quite as describably so divided, as are those of Ranunculus aquatilis. Now, Mr. D. wants the facts of Nature for his investigations, but is thus forced on conventional interpretations & arrangements instead. I find the difference between these two things enormous impediments.

European genera, including several variable species, or some few very variable species; the species more or less gliding into each other by the varieties.— SALIX — RUBUS — ROSA — MENTHA — HelianthemumCirsiumHieraciumViolaFestucaPoa — Triticum — SaxifragaPotamogetonAtriplex — Chenopodium — Verbascum — Polygala — BATRA-CHIUM (section of Ranunculus) Ulmus — Lastrea — Galium — Epilobium — Taraxacum — Fragaria — Polygonum — Potentilla — Campanula — Betula — Sedum — Erythræa — Euphrasia — Lamium. Ribes — Plantago — Cerastium — Phagnalon — Teucrium. Stachys — Galeopsis — Cochlearia — Ophrys — Daucus. The above names are set down as they came into recollection. In general, the earlier names may be considered to indicate genera in which the species most run together by their varieties. The underscoring is an attempt to imitate Dr. Gray, but without perfectly understanding his signs.5 There are other large genera, with close species, having one or few variables among them,—but not entered above, because the variables are proportionably few. Thus, the species of Silene are very numerous, many closely alike, but not variable as a characteristic, although among them one or more species have been subdivided (wrongly) into several; Silene anglica, gallica, lusitanica, & 5-vulnera, for example, being forms likely of a single species.

I wrote down the above names without having the list from Dr. A. Gray before me. I find the following are in his list also,—Viola, Polygala, Ribes, Galium Cirsium, Plantago, Stachys, Polygonum, POTAMOGETON, Salix,—only one of them strongly underlined.

Remarks on the European genera in the list by Dr Gray.— Ranunculus:—The species are numerous, & close; but not remarkably variable in Europe after deducting the subgenus Batrachium. R. Flammula however is very proteiform. Lathyrus:—Numerous Europeans, not variable, unless in colour, & breadth of leaves. Ribes has few European species; the Gooseberry is now held a single species, though deemed 3 species by Linneus; the red Currant has been divided into 4, but is perhaps only 1 species; the alpine Currant has had two sub-species carved out of it. These changes show a considerable variability. Galium is perhaps an unsettled & imperfectly understood genus, & thus its species may seem more variable than should be. Xanthium:—doubtful whether 3, 4, or 5 species in Europe. Artemisia:— species numerous & close, rather than variable if taken singly; but A. camphorata is a group of species according to some botanists, a varying species according to others. Senecio:—same remark, three or four confused or variable species. Cirsium:—many intermediates, supposed hybrids. Potamogeton is the most strongly underlined, & its species differ a good deal in Europe, running into varieties as the water is deep or shallow, stagnant, still, or running, &c. Quercus:—variable in the form of leaves, & position of fruit; the English forms being assigned to 1, or 2, or 3 species by different botanists. Juncus:—some of the species in very close pairs (conglomeratus & effusus—compressus & cœnosus—&c.) but the species separately not very variable. Carex:—species numerous, many of them close; but not much variable, unless we adopt the views of Læstadius that several alpine Carices are reduced states of the lowland species. Scirpus:—several pairs & trios are very close, or some species sport into strong varieties, undecided which, tho’ botanists in general call them species, not varieties. Several of the other genera mentioned in Dr. Gray’s list are chiefly or exclusively American;—& on the other side, several of those in my European list are chiefly European, with few American representatives.

On the whole, I fear that nothing satisfactory can be got out of these lengthy & rather vague notes.—

My impression is, that some species tend more to vary than others, apart from external influences,—though such influences may call out or augment the tendency.6

But let me repeat the remark that species may be made to appear more or less variable, according as a genus is divided in books into few or many species. How very variable, for instance, is the Rubus fruticosus (of Linneus) when including about 50 modern species! Or, the Helianthemum variabile (Spach),7 formed by the re-union of a dozen species, & several subspecies; many of them long supposed, or still supposed, quite distinct.

H. C. W.

CD annotations

2.3 SALIX] cross added pencil
2.3 RUBUS] cross added pencil
2.3 ROSA] cross added pencil
2.3 MENTHA] cross added pencil
‘Those with pencil cross have been mentioned in your former letter’8 added pencil
2.4 Hieracium] cross added pencil
2.4 Saxifraga] cross added pencil
2.10 Daucus.] ‘42 genera’ added pencil
triple scored pencil
Top of first page: ‘Please return to me’ink; ‘2’ink 9


The manuscript of the letter indicates that Watson began by writing a letter to CD; he subsequently changed ‘you’ to ‘Mr. D.’ throughout, evidently with the intention that CD could send it to Asa Gray. The letter provides information on variable genera in Britain that would enable Gray to make comparable remarks about the same genera in the United States (see letter to Asa Gray, [after 15 March 1857]).
CD’s letter has not been found, but he had evidently questioned the categories listed by Watson in the letter from H. C. Watson, 10 March 1857.
See letter from Asa Gray, 16 February 1857. The underscoring has been printed as italics for one underline, bold for two underlines, and small capitals for three underlines.
CD discussed this point in Natural selection, pp. 105–8.
Edouard Spach specialised in the taxonomy of Cistaceae, to which Helianthemum belonged.
The pencil crosses and written remarks refer to a later time when CD forwarded this letter to Asa Gray (see letter to Asa Gray, [after 15 March 1857]).
These notes are for Asa Gray (see n. 8, above).


Backhouse, James. 1856. A monograph of the British Hieracia. York: William Simpson.

Hudson, William. 1762. Flora Anglica: exhibens plantas per regnum Angliæ sponte crescentes, distributas secundum systema sexuale. London.

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.


Describes problems of classifying species in highly variable genera. Lists highly variable genera. Comments on the list of Asa Gray. Says species may be made to appear more or less variable according to whether a genus is divided into few or many species.

Letter details

Letter no.
Hewett Cottrell Watson
Asa Gray
Sent from
Source of text
DAR 181: 36
Physical description
AmemS 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 2065,” accessed on 24 October 2021,

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