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Darwin Correspondence Project

To William Farr   17 July [1870]1

Jul 17

My dear Sir

I am very much obliged for the papers.— I feel certain that your judgment on all parts in regard to the Census is incomparably safer than mine. Firstly My impression is that it wd be better to confine enquiry to 1st cousins.2 Secondly, I incline to think that your second form of enquiry is the best, that is if both forms are not included; in as much as second will give the proportion out of the whole population of cousins who marry & who have children.

The object of the enquiry is to ascertain whether with mankind, consanguineous marriage within the degree of cousinship is in any way injurious, & The result of such an enquiry is almost equally important whether negative or affirmative.

In England & any part of Europe the marriages of cousins are objected to by fear their supposed injurious consequences; but this belief rests on no direct evidence. It is therefore [manifestly] desirable that this belief shd either be proved to be false, or shd be confirmed so that in the latter case the marriage of cousins might be discouraged. The returns gained if the proper queries are inserted, would show whether married cousins have in their households on the night of the census as many children, as parents who are not related; & shd the number prove fewer, we might safely infer either lessened fertility in the parents, which is more probable lessened vitality in the offspring.

If the proportional number of cousins who marry in the whole population was once ascertained, there cd exist a standard by which to judge whether the proportion, (already tabulated in some cases) of persons in asylums for the dumb & deaf the blind & insane who are the offspring of cousins is in excess of the proportion of cousin-offspring in the whole population.—

I fear I have written at too great length, but I have not felt sure, how fully you wished me to write.—

It is moreover much to be wished that the truth of the often repeated assertion, that consanguineous marriages lead to deafness & dumbness, blindness &c shd be ascertained; & all such opinion cd be easy tested, by the returns from a single census.—

It is beyond my power of filling out all the detail, to see which of your 2 forms of enquiry wd be best, if both are not included.– I could insert the words “married to—” with a blank, showing that the word cousin ought to be introduced; when this is the case.


The year is established by the relationship between this letter and the letter from William Farr, 16 July 1870.
CD refers to the memorandum that Farr sent with his letter of 16 July 1870; it has not been found. CD evidently returned it with his suggestions. For the final version of the proposed amendment to the Census Bill, see letter from William Farr, 16 July 1870 and n. 2.


Writes concerning the questions on consanguineous marriages which CD wishes to have inserted into the Census. Discusses the form the questions might take and the value of the information that would be gained from them.

Letter details

Letter no.
Charles Robert Darwin
William Farr
Sent from
Source of text
DAR 96: 76–7
Physical description
Adraft 4pp

Please cite as

Darwin Correspondence Project, “Letter no. 7282,” accessed on 21 September 2021,

Also published in The Correspondence of Charles Darwin, vol. 18