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Darwin Correspondence Project

From G. H. Darwin   [2 December 1869]1

14 Arlington St. | Piccadilly


My dear Father,

After grinding away for some time at the calculation of the red & white animals it suddenly occurred to me that I could find without any bother at all the ratio of the reds to the browns at any period, that is to say the percentage of the whole no of animals existing at that period which are brown. In fact the death rate has no effect whatever on this percentage, for this is the very meaning of the death rate being the same for browns & reds. If this is all you want I can do it in five minutes, but if you want the actual numbers of reds and browns at each period—I can easily calculate it, I had in fact actually done it for the first period when I perceived this thing wh. had been staring me in the face all the time.—2 Will you write me a line to Cambridge & let me know what you want. I have just been dining here with a man that I have known for a good time at Cambridge called Hammond, & he tells me that his Uncle was a lieutenant on the Beagle.3 As I thought you might like to know what had become of him I found out that he is banking down in Norfolk or Yorkshire I forget which.4

I also find that he knows Sullivan a little.5 Again I fd out today that Whiteford (a man at Tathum’s) old Col. H Smith of Plymouth’s grandson knows Mrs Clem. Wedgwood very well;6 how marvellously small the world is. I am going to take a holiday on Saturday & Monday & I am going down to Camb. tomorrow night.

Balfour & Strutt I there & it is the last of Balfour that I shall see at Camb. as he has just been in for his degree & goes down next term.7 I went to Nilsson’s concert last night & thought it extremely good.—8

Your affectionate Son | G. H. Darwin


The date is established by the reference to Arthur James Balfour’s taking his degree (see n. 7, below) and by the reference to Christine Nilsson’s evening concert (see n. 8, below). In 1869, 2 December was a Thursday.
George was making calculation about a hypothetical population of red and brown animals. A statement of the problem that George was working on is in George’s hand in DAR 85: B17: Let there be a number of young animals all born brown & let them turn red (the operation being supposed instantaneous for each animal) at such a uniform rate that (if none died) half of the whole no: wd be red at the end of 8 quarter years; also let them perish irrespective of colour at such an uniformly diminishing rate that 2/3 the original no: are left after 4 quarter years & 1/2 orig: no: are left after 8 quarter years— It is then required to find the nos: of red & browns at any period. George’s calculations are in DAR 85: B1b–17.
George refers to Thomas Astley Horace Hamond. His uncle was Robert Nicholas Hamond.
Robert Nicholas Hamond appears at Tun Street, Fakenham, Norfolk, in the Post Office directory of Cambridge, Norfolk, and Suffolk 1869.
George refers to Arthur James Balfour and John William Strutt. The examination for the moral sciences tripos took place on 29 November 1869 (Cambridge University calendar 1869).
Nilsson appeared in a series of evening concerts at Exeter Hall in London in 1869; the last two were on 1 and 15 December (The Times, 10 September 1870, p. 1).


Cambridge University calendar: The Cambridge University calendar. Cambridge: W. Page [and others]. 1796–1950.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Post Office directory of Cambridge, Norfolk, and Suffolk: Post Office directory of Cambridge, Norfolk, and Suffolk. Kelly’s directory of Cambridgeshire, Norfolk and Suffolk. London: W. Kelly; Kelly & Co. 1853–1900.


Discusses some calculations which he is doing for CD on the ratios of red and brown colouring in some animals.

Letter details

Letter no.
George Howard Darwin
Charles Robert Darwin
Sent from
London, Arlington St, 14
Source of text
DAR 210.2: 17
Physical description
ALS 8pp

Please cite as

Darwin Correspondence Project, “Letter no. 7158,” accessed on 29 May 2023,

Also published in The Correspondence of Charles Darwin, vol. 17