My dear Sir

If you should think the subject of which I wish to speak in this letter important, I wish earnestly to beg your assistance.

My son Mr. George Darwin, who is a good mathematician (having been Second Wrangler at Cambridge)¹ has taken a very great deal of trouble in trying, by several independent methods, to discover what proportion of all marriages are marriages between first cousins; and he now feels pretty confident that his results will not be subject to an error of more than 1 per cent. He wishes to utilise these results by the discovery of the proportion of the offspring of first cousin marriages amongst the insane, deaf and dumb, and blind, &c.² I have for 30 years been desirous of seeing this point determined, and it still seems to me of high importance. We have here, I think, the rare case of an inquiry, the answer to which will be valuable, whatever it may be:—for we shall thus be informed either that such marriages are injurious, or that we may persevere in them with impunity. I have been consulted more than once on this point, but could only say that on general principles it seemed best to avoid such marriages.

Now if you are willing to give your invaluable help, would it be possible for you, by means of your assistants to ask the patients in your asylum whether their parents were first cousins or not, and to record the answers. I presume the questioner would be able to a great extent to judge of the sanity of the answer; and you will doubtless be the best judge of how it will be expedient to approach the subject with them so as neither to rouse their suspicions, nor to give the question a leading turn. Can you suggest any energetic and obliging doctors having the care of the insane, deaf-mutes, blind, &c., who, if I were to use your name, as an introduction would perhaps be inclined to aid my son. I shall be in London shortly, and shall call on D^r. Maudsley and speak to him on the subject.³ If I knew the address of D^r. Nichol (who helped me on ‘Expression’) I would write to him too; and I shall write to D^r. Lauder Lindsay, D^r. Scott of Exeter, and the Rev. Mr. Blair of Worcester—and I have good hopes from their former obliging conduct to me, that they will be willing to help.⁴ These names form my list. Now will you have the kindness to think over my son’s proposed investigations and aid him with any suggestions which may occur to you, and if you possibly can with detailed information about your patients. From your repeated acts of kindness to me I feel sure that you will forgive my troubling you at so great length.

Pray believe me my dear Sir | Yours very sincerely | Charles Darwin.