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Darwin Correspondence Project

From H. C. Watson   8 November 1855

Thames Ditton

Nov 8. 1855

My dear Sir

All investigations have an interest in my eyes, which bear upon the grand question, whether species are or are not absolutely & always distinct;—or on the various secondary questions connected with or arising out of the former. Thus, you may correctly believe that I do take an interest in your investigations about the numerical evidences or indications of variability. There seems to be this difference between us, that you regard the proportions as approximating more towards evidences of a general law, while I see them only as fainter indications of it. The technical groups of botany, Orders & genera, appear to me to be formed in a mode that is too arbitrary & multi-form to admit of your inquiry being worked out to very reliable results.1 This would still introduce much uncertainty into the results, even supposing Botanists better agreed than they are likely to be in our day, as to what (or which) are species & what are varieties. Really, they can neither define what species are, nor can they empirically agree in selecting species from varieties, or varieties from species.— Jordan would make a million species. J. D. Hooker would perhaps allow 50,000.2

Looking to the sources of doubt & difficulty, I should rather seek the proportions by bringing together the three categories of quasi-species (or quasi-varieties) into which I marked the London Catalogue. Uniting the 69, 195, 32 into 296 debateable species you obtain a goodly group of ‘forms’ which are in fact held species by some, and varieties by other botanists. Their disseverance into three groups, say + probable species = possible species – probable varieties is matter of opinion; & if you tried half a dozen botanists among those most attentive to such points, likely no two of them would concur in their grouping of the forms into the three sets.

The London Catalogue would of course only show one set of varieties. My purpose in marking also the more debateable species, & adding Babington’s varieties3 (being also quasi-species) was as much that of augmenting the total group, as of showing three grades.

There are some species of plants which vary so much that botanists seldom attempt to name & describe the varieties. Such species consequently stand in lists with no varieties or very few, altho’ in fact they have them most numerous. Polygonum aviculare is an example in point. Say that three or six of its varieties had been brought hither from as many different & distant Countries, instead of being all home-grown, they would surely have been duly named & described as three or six species. As it stands in the London Catalogue, Polygonum is a large genus (13 species) with one variety only. Looking to nature’s facts, it might as truly have had half a dozen or upwards.

I must confess a pretty strong bias towards the view, that species are not immutably distinct;4 —altho’ in our time-narrowed observation of the individuals they seem to be so.

Very truly yours | Hewett C. Watson To | C. Darwin | Esq

CD annotations

crossed pencil
‘Ask Mr Watson to mark species, which have never been debated.—’ added pencil
double scored pencil; double scored brown crayon
double scored pencil
Top of first page: ‘13’brown crayon, circled brown crayon; ‘(6 [over ’2‘])’ pencil, del pencil; ‘(6)’ pencil


See letter from H. C. Watson, 11 October 1855. CD referred to several of Watson’s doubts about the validity of his statistical inquiry in Natural selection, p. 160.
Watson’s views on ‘progressive development’ were first expressed in Watson 1845. CD recorded this work in his reading notebook (Correspondence vol. 4, Appendix IV, *119: 20v). See Natural selection, p. 126, where CD summarised Watson’s attempt to follow changing characteristics of a species through several generations of cultivation in which one species sometimes exhibited features of another species.


Babington, Charles Cardale. 1851. Manual of British botany, containing the flowering plants and ferns arranged according to the natural orders. 3d edition. London: John van Voorst.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

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.

Watson, Hewett Cottrell. 1845. On the theory of "progressive development," applied in explanation of the origin and transmutation of species. Phytologist 2: 108–13, 140–7, 161–8, 225–8.


Artificiality of orders and genera in botany.

Difficulties in numerical analysis of close species in large and small genera.

HCW has "pretty strong bias towards the view that species are not immutably distinct".

Letter details

Letter no.
Hewett Cottrell Watson
Charles Robert Darwin
Sent from
Thames Ditton
Source of text
DAR 181: 31
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 1775,” accessed on 23 September 2020,

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