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Darwin Correspondence Project

From H. C. Watson   [19 November 1854]1

I am not able to refer you to any list of species, as the flora of a single & sterile soil, that would fully meet your question.2 Perhaps the nearest approach to an answer would be found in a list of plants observed on Ailsa Craig, Phænogamic 79, Cryptogamic 68, by Prof. J. H. Balfour, Phytologist Vol. II, part 1, p. 261. 1845.3

In Phytologist Vol. III, part 1, page 267, 1848, I printed some brief remarks on the number of species to a square mile in Britain.—4 In the Flora of Hertfordshire, the county is divided into 12 sections, varying in extent from 39 to 80 square miles, & the number of species ascertained in each section is tabulated, being 461 to 810.— But these estimates do not properly meet your question; the diversity of soil & situation being doubtless considerable in each, & mostly fertile.

Regretting that I must say so little, in writing so much,— | I am yours faithfully | Hewett Cl. Watson To | Charles Darwin | Esq


The year of this and the letter from H. C. Watson, 20 November [1854], is surmised from the formal style of the letters, which suggests that they pre-date the extensive correspondence between CD and Watson in 1855, and from CD’s interest in acquiring data on the diversity of organic types in restricted areas (see n. 2, below). The precise day is fixed in relation to the letter from Watson, 20 November [1854].
CD was gathering information on the diversity of types in restricted areas. The origin and advantages of diversification was a main concern from late 1854. In a note dated November 1854 (DAR 205.5: 149), he commented: It is indispensable to show that in small & uniform areas there are many Families & genera. For otherwise we cannot show that there is a tendency to diverge (if it may be so expressed) in offspring of every class, & so to give the diverging tree-like appearance to the natural genealogy of the organised world. CD’s facts on the diversity of organic beings, and his division-of-labour argument to account for how competition generates diversity, are given in Natural selection, pp. 227–35.
Balfour 1845. CD’s notes on this paper, recording the range of genera and families to which the plants of Ailsa Craig belong, are in DAR 205.4: 58. These, together with related notes in DAR 205.4 and DAR 205.2, appear to have been made around March 1855.


Balfour, John Hutton. 1845. Account of a botanical excursion to Ailsa Crag, in July, 1844. Phytologist 2, pt 1: 257–63.

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. 1848. On the number of botanical species to a square mile of ground. Phytologist 3, pt 1: 267–8.


In response to CD’s query, HCW says he cannot supply "any list of species as the flora of a single and sterile soil". Suggests a possible source of information, and provides some figures for Britain, but these apply to diverse soils.

Letter details

Letter no.
Hewett Cottrell Watson
Charles Robert Darwin
Sent from
Source of text
DAR 205.9: 402
Physical description
ALS 2pp inc

Please cite as

Darwin Correspondence Project, “Letter no. 1604,” accessed on 30 May 2023,

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