My dear Lubbock, As I hear that the Census will be brought before the House to-morrow, I write to say how much I hope that you will express your opinion on the desirability of queries in relation to consanguineous marriages being inserted. As you are aware, I have made experiments on the subject during several years; AND IT IS MY CLEAR CONVICTION THAT THERE IS NOW AMPLE EVIDENCE OF THE EXISTENCE OF A GREAT PHYSIOLOGICAL LAW, RENDERING AN ENQUIRY WITH REFERENCE TO MANKIND OF MUCH IMPORTANCE. IN ENGLAND AND MANY PARTS OF EUROPE THE MARRIAGES OF COUSINS ARE OBJECTED TO FROM THEIR SUPPOSED INJURIOUS CONSEQUENCES; BUT THIS BELIEF RESTS ON NO DIRECT EVIDENCE. IT IS THEREFORE MANIFESTLY DESIRABLE THAT THE BELIEF SHOULD EITHER BE PROVED FALSE, OR SHOULD BE CONFIRMED, so that in this latter case the marriages of cousins might be discouraged. If the proper queries are inserted, the returns would show whether married cousins have in their households on the night of the census as many children as have parents of who are not related; and should the number prove fewer, we might safely infer either lessened fertility in the parents, or which is more probable, lessened vitality in the offspring. It is, moreover, much to be wished that the truth of the often repeated assertion that consanguineous marriages lead to deafness, and dumbness, blindness, etc., should be ascertained; and all such assertions could be easily tested by the returns from a single census. Believe me, Yours very sincerely, CHARLES DARWIN. [When the Census Act was passing through the House of Commons, Sir John Lubbock and Dr. Playfair attempted to carry out this suggestion. The question came to a division, which was lost, but not by many votes. The subject of cousin marriages was afterwards investigated by my brother. ("Marriages between First Cousins in England, and their Effects.' By George Darwin. 'Journal of the Statistical Society,' June, 1875.) The results of this laborious piece of work were negative; the author sums up in the sentence:-- "My paper is far from giving any thing like a satisfactory solution of the question as to the effects of consanguineous marriages, but it does, I think, show that the assertion that this question has already been set at rest, cannot be substantiated."] CHAPTER 2.VII. -- PUBLICATION OF THE 'DESCENT OF MAN.' WORK ON 'EXPRESSION.' 1871-1873. [The last revise of the 'Descent of Man' was corrected on January 15th, 1871, so that the book occupied him for about three years. He wrote to Sir J. Hooker: "I finished the last proofs of my book a few days ago, the work half-killed me, and I have not the most remote idea whether the book is worth publishing." He also wrote to Dr. Gray:--