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Darwin Correspondence Project

From H. C. Watson   23 August 1855

Thames Ditton

August 23d /55

My dear Sir.

It is highly probable that I should have marked the Catalogue of British Plants1 differently, if previously aware of your object. I think my leading idea was to select examples of close species or quasi-species, rather than to make the list numerically exact.

Even if aware that numerical comparison entered into your object, the Catalogue used would hardly have afforded the means for reaching much precision, because various of the genera with few species in Britain, are nevertheless large genera in Europe, or elsewhere.

In the question of close species, I should prefer the testimony of Fries before that of Hooker & Bentham united. And apart from personal authority, I incline to think, without worked-out conviction, that the fact is what Fries intimates.2

I cannot off-hand say, whether in large genera the extremes differ more than they do in small genera; but it is likely they do so to some extent.

First, suppose the extremes about equally different or distant in both. It would seem to follow as a logical necessity, that the species are closer in the large genera: diag 1 2 3 4 5 6 7 8 9 Many sps.

1 - 3 - 5 - 7 - 9

1 - - - 5 - - - 9 Few sps.

1 - - - - - - - 9ramme

But, secondly, suppose the extremes different, on an average, in proportion to the number of species (which I do not believe); & that a representation would be thus:—3

1 2 3 4 5 6 7 8 9 Many sps

1 2 3 4 5 6 7 - -

1 2 3 4 - - - - - few sps

1 2 - - - - - - - The question will arise, on this supposition, Is this approximation of extremes real or conventional?—in nature, or in science only?

Whatever it may be in Zoology, I cannot think that in botany the groups are formed on any uniform principle; either orders or genera. The so-called ‘natural system’ in botany is to a great extent artificial or arbitrary; both orders and genera being (so to write) capriciously joined or separated. Hence, when we compare the number of genera in different orders, or the number of species in different genera, we are not truly comparing the facts of nature, but the capriciously technical arrangements of those facts by man. In some instances, Several very similar species are grouped together under one generic name. In other instances, very dissimilar species are grouped under one generic name. Is this natural, or is it only technical? Surely the latter! If so, what is the value of statistics founded on numbers in these groups? They are exponents of human science, rather than of nature’s facts or laws.

Still, as technical arrangements are intended to represent the resemblances among objects in nature, & doubtless do so in a considerable degree, altho’ not with minute exactness,—a large floral field, such as Europe or the American States, should afford tolerably good data towards your quest. In a botanical list so small as that for Britain, where genera are very unequally represented by the numbers of their species which are native here, statistical results are little to be relied upon except as mere indications,—something between possibilities & probabilities.

On the P.S. of your letter I may have some little to say another day. Saxifraga Andrewsii, a newly found or made species, may be a very striking instance of your apparent rule.—4

Sincerely Yours | Hewett Cl. Watson To | C. Darwin Esq

CD annotations

2.2 much precision 2.3] underl ink
scored ink; ‘I think any small country may be viewed independently. The Flora of world is only fragment of what has lived. Submerge all world Except England & Australia & S. America Then we shd. be forced to take England as foundation of statistics for most northern plants. Now this supposed case I cannot doubt is real picture of world.—’ added ink; ‘3’ added pencil, circled pencil
5.2 species are closer in large genera:] ‘Hooker says that he thinks large genera really closer species’ added pencil
double scored pencil
‘Any change wd be generally to make genera smaller. & so make result more in my favour?’ added ink
‘In the small genera, if divided strictly by structural differences, then the number would be increased.—’ added ink
scored pencil
scored pencil; ‘[reverse question mark]added pencil


H. C. Watson and Syme 1853. See letter from H. C. Watson, 17 August 1855.
CD had evidently told Watson of his interest in Elias Magnus Fries’s remarks on close species in Fries 1850, p. 188. CD’s notes on the paper are in DAR 73: 118–19. In Natural selection, pp. 146–7, CD stated: I was strengthened in my expectation of finding more varieties in the larger genera by a remark of Fries, that, ‘in genera containing many species, the individual species stand much closer together than in poor genera: hence it is well in the former case to collect them around certain types or principal species, about which, as around a centre, the others arrange themselves as satellites.’ In his notes on Fries 1850 (DAR 73: 118), CD wrote after this quotation: ‘This very important, it shows that extinction has not [interl] been at work in the large genera.— But some of the small growing [interl] genera ought to have close species.—’
See Natural selection, p. 147, for CD’s discussion of Watson’s diagram illustrating this point.
This may refer to CD’s notion that individuals of one or more species in a genus may vary in some of the characters by which the genus is defined. See letter to H. C. Watson, [26 August 1855].


Fries, Elias Magnus. 1850. A monograph of the Hieracia; being an abstract of Prof. Fries’s ""Symbolæ ad Historiam Hieraciorum"". Botanical Gazette 2: 85–92, 185–8, 203–19. [Vols. 5,7]

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.


Close species in large and small genera.

Artificiality of botanical classification.

Letter details

Letter no.
Hewett Cottrell Watson
Charles Robert Darwin
Sent from
Thames Ditton
Source of text
DAR 181: 29
Physical description
ALS 4pp ††

Please cite as

Darwin Correspondence Project, “Letter no. 1747,” accessed on 22 March 2023,

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