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Darwin Correspondence Project

To George Howard Darwin   [1866]

Dear George

Can you or any of your friends answer me this.1 In a family of 10 5 of the children squinted & 5 did not & they came alternately—.2 But you must understand it is indifferent whether a squinter or a non squinter comes first. What I want to know is what are the chances against their coming alternately.3

Yours affec | Ch. Darwin | Ch. Darwin | Ch. Darwin4

P.S This case of the squinting & other such cases are real.


George Darwin was studying mathematics at Cambridge University (Alum. Cantab.).
A case of squinting in five of ten siblings was reported in Streatfield 1857–9, and was referred to in Sedgwick 1861, which CD cited (in relation to a different subject) in Variation 2: 328 n. However, Streatfield stated that squinting affected only the boys among ten siblings, not that alternate children squinted. CD briefly discussed squinting in Variation 2: 9, in the chapter on inheritance, but relied exclusively on evidence provided by William Bowman.
At the top of the letter, George calculated a 1:126 chance of squinters and non-squinters being born alternately in a family of ten, by dividing twice the square of the factorial of five by the factorial of ten. At the end of his working, he multiplied 126 by two; the reason for this is unclear. George’s reply to CD has not been found.
The letter is in the hand of CD’s daughter Elizabeth. The signatures are copied from CD’s own.


Asks GHD what the chances are against squinting and non-squinting children coming alternately in a family of ten.

Letter details

Letter no.
Charles Robert Darwin
George Howard Darwin
Sent from
Source of text
DAR 210.1: 1
Physical description
( S ) 3pp †

Please cite as

Darwin Correspondence Project, “Letter no. 4961,” accessed on 26 June 2017,

Also published in The Correspondence of Charles Darwin, vol. 14