Dear Sir

My Son George Darwin (who is a good Mathematician, having been 2^nd. Wrangler at Cambridge)² has been at work for nearly a year in trying, by several independent statistical methods, to discover what proportion of all marriages are between first cousins; & he now feels pretty confident that his results will not be subject to an error of as much as one per cent. He is being greatly aided in this part of his inquiry by D^r. Farre the Registrar General.³

He now wishes to utilize his results by the discovery of the proportion of the offspring of 1^st. cousin marrriages amongst the insane, idiotic, deaf & dumb &c.

I have for 30 years considered an answer to this inquiry of great importance. We have here I think, the rare case of an inquiry, the answer to wh. will be of value whatever it may be; for we shall either find that such marriages are injurious, or that we may persevere in them with impunity

Dr Maudsley has informed me that he thinks it very likely that you will consider the investigation of sufficient importance to be willing to help my son by asking the patients in the large asylum under your charge (thro’ your assistants), whether or or not they are the offspring of first cousins, & recording the answers in each case.⁴ The question which my son proposes sh^d. be asked is simply,—

‘Were your father & mother first cousins or not?’

The questioner will doubtless be the best judge of the most prudent way of approaching the subject & of explaining the question in each individual case & will be able to judge of the trustworthiness of the answer from the known character of each patient.

If you sh^d. be willing to undertake this laborious task, you w^d. be conferring a great favour both on my son & myself; & I beg you will send the tabulated results to my son at Trinity College Cambridge, where he is a resident fellow.

I hope you will pardon the liberty I take in thus addressing you on this subject.—

I beg leave to remain | Dear Sir | Yours faithfully | Charles Darwin

P.S It w^d. give my son also a further body of statistics on wh. to base his conclusions, if in the cases of 1^st. cousin marriage, (i) the surname of the father & maiden-surname of the mother were tabulated (for these will probably be learnt by the questioner), & (ii) further if any cases were recorded where the father & mother of a patient, not the offspring of a 1^st. cousin marriage, bore the same surname.

I w^d. not think of asking you to make this further inquiry, if you think the answer to the main point (to wh. this is entirely independent) would be thereby rendered less trustworthy, or if it w^d. at all seriously increase the labour of the investigation