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Letter 4961

Darwin, C. R. to Darwin, G. H.

[1866]

    Summary Add

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

Transcription

Dear George

Can you or any of your friends answer me this. In a family of 10 5 of the children squinted & 5 did not & they came alternately—. 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.

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

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

    Footnotes Add

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    f1 4961.f1
    George Darwin was studying mathematics at Cambridge University (Alum. Cantab.).
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    f2 4961.f2
    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.
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    f3 4961.f3
    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.
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    f4 4961.f4
    The letter is in the hand of CD's daughter Elizabeth. The signatures are copied from CD's own.
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