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Darwin Correspondence Project

From H. C. Watson   13 August 1855

Thames Ditton

13 Augt. 1855

My dear Sir

On attempting to mark the list of British Plants, I find myself still in some doubt as to the kind of species, real or reputed, that you wish to be marked.1

Enclosed is a paper giving a sort of classification under four heads. Am I to understand that it is No 2 you require? They are not at all numerous. The great bulk belong to No. 1, & No. 4.—

The grand difficulty for naturalists or botanists of our turn of thought, is, that the use of the word “species” by technical describers is indefinite & variable.—2 Theoretically, it is supposed to mean objects actually & essentially distinct,—so existing as productions of nature, & reproducing only their own selves or similitudes. Practically, it means only an idea of the mind, with no more real restriction in its application to objects, than have the words “genus” or “order”. Taking J. D. Hooker & Jordan3 as representative men for the opposite factions in botany,—‘lumpers & splitters’, the former would reduce the species of Vascular plants to three score thousand, or perhaps much fewer;—while Jordan would raise them to three hundred thousand. With this difference of idea practically, we could never say,—

Species of Hooker = Species of Jordan. We should be as near the fact, possibly, in saying,—

Species of Hooker = Genus of Jordan.

In all my attempts to advance geographical botany, I am stopt by the application & signification of the word “Species”. Where I seek to effect precise comparisons of objects & numbers & proportions,—that word constantly frustrates & makes vague & indefinite—

Sincerely yours | Hewett C. Watson C. Darwin | Esq


Categories of Species4


1. Plants distinguishable from each other by positive characters, & generally received as true species.

2. Same as No 1; but so closely resembling each other as to be frequently mistaken one for the other, & by botanists even of some experience.

3. Same as No 1; & not liable to be mistaken in their typical forms; but accompanied by intermediate or transition forms, approximating so much to each or both, as not to be quite satisfactorily assigned to either. (NB. The primrose & cowslip would be in this category, but it has been there proved that the intermediate produces both the alleged species from the same year’s seed)

4. Plants deemed true species when their typical & most general forms only are looked at; but the limit of the species is rendered uncertain by the existence of forms closely allied, deemed varieties of the type by some botanists, distinct species by other botanists. These varieties or quasi-species (3, 4) being generally5 much more rare or local than the type species. They differ from No 3 only in so far as the varieties or quasi species cluster around one,6 instead of linking together two supposed genuine species.

Altho’ four such categories are easily defined on paper, & illustrated by selected examples, they glide together by other examples; & thus, as groups, are different in degree rather than kind.7 To show this by garden species, &c.8

1. The Apricot, plum, & Cherry are commonly placed under one genus, Prunus; & as species these are very readily distinguished by any body.

2. But there are two Cherries spontaneous in England, an arborescent & a fruticose, which by most botanists are deemed two real though very similar species, & between which in a wild state we can hardly point out any connecting links.

3. Many botanists deem the wild sloe of England to be quite a distinct species from the cultivated & probably imported plum-tree of the gardens. Nevertheless, between the plum-tree of the garden & the sloe-bush of the hedges, there exist numerous intermediate forms or links, which render it highly difficult to say, ‘here ends the sloe & its varieties, there begins the plum & its varieties.’ If we hold the intermediate Bullace a good species, this also passes insensibly down to the Sloe, & improves almost as insensibly into the plum, so numerous & fine are the steps or links either way.

4. Linné described the fruticose bramble as a species, under name of Rubus fruticosus; but various modern botanists make out 50 to 100 supposed species of Bramble, which others call varieties of R. fruticosus, & others again group into a small number of species, say half a dozen.

CD annotations

Top of first page: ‘It might be worth while to compare this list with a list from Babington of the genera, which present most varieties on same principle as [over full stop] I have worked out A. Gray’s lists & Manual’ ink; ‘(1)’ pencil
Whole letter: crossed pencil
Enclosure: 0.1 Categories of Species] del pencil
3.3 (NB… . seed) 3.5] crossed pencil
scored brown crayon, scoring del pencil
4.4 being] del pencil; ‘are’ added pencil
4.5 in so far as] del pencil
4.6 cluster] ‘ing’ added pencil
5.3 kind.] ‘in’ interl pencil before
5.3 show this by garden species, &c.] del pencil; ‘give examples of the four categories’ added pencil


CD had asked Watson go through a copy of Watson and Syme 1853 and mark those species that he considered to be distinct but nevertheless extremely ‘close’ to other species. Similar requests had been made of John Stevens Henslow and Asa Gray. As explained in his letter to J. S. Henslow, 21 July [1855], CD was interested in whether there is more variation in large or in small genera and whether the closely allied species are more frequent in the large genera. CD copied Watson’s markings onto a list headed: ‘List of Genera from London Catalogue, marked by Mr H. C. Watson, *(August 1855) [interl pencil] showing the species, resembling each other, perhaps only 1 species,/ those more probably 2 species/ probably 2 species but connected by intermediate forms./ Genera Rubus, Rosa, Hieracium [interl] Salix & Carex, omitted from doubts in some cases & large numbers in others. The greater number of marks refer to species probably really distinct.—’ The list is in DAR 15.2: 10.
CD’s description of Watson’s views on the differences among authors over the definition of ‘species’ is in Natural selection, pp. 112–13.
The French botanist Alexis Jordan was renowned for his tendency to make a great number of species from what others might consider a single species. Joseph Dalton Hooker had criticised Jordan in J. D. Hooker and Thomson 1855, introductory essay, pp. 2–4 n. A copy of Jordan 1852 is in the Darwin Library–CUL.
The enclosure is preserved in DAR 9: 15A, in the manuscript version of the chapter of Natural Selection on ‘Variation under nature’. When CD came to write up this chapter, he evidently intended that the printers should copy Watson’s list directly. The list is printed in Natural selection, pp. 102–3.
CD and Watson subsequently altered the beginning of this sentence to read: ‘As is the case with the intermediates of no. 3, so these varieties or sub-species of no 4 are usually’.
CD and Watson subsequently altered this part of the sentence to read: ‘They differ from the intermediates of No 3 only as varieties or quasi species clustering around one’.
CD and Watson subsequently altered the end of this sentence to read: ‘they are different in degree rather than kind.’
CD and Watson subsequently altered this sentence to read: ‘To give examples of the four categories [‘by trees and shrubs very familiar to most bo’ del].


Jordan, Alexis. 1852. De l’origine des diverses variétés ou espèces d’arbres fruitiers et autres végétaux généralement cultivés pour les besoins de l’homme. Mémoires de l’Académie Impériale des Sciences, Belles-lettres et Arts de Lyon 2d ser. 2: 65–161.

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.


Is having difficulties marking close species on the list of British plants.

In all his attempts to advance geographical botany he is stopped by the "application and signification of the word ""species"" " the use of which is both "indefinite and variable". He encloses his list of "Categories of Species".

Letter details

Letter no.
Hewett Cottrell Watson
Charles Robert Darwin
Sent from
Thames Ditton
Source of text
DAR 98: A5–A6, DAR 9: 15A
Physical description
ALS 4pp † Amem 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 1740,” accessed on 25 July 2024,

Also published in The Correspondence of Charles Darwin, vol. 5