My dear M^r Greg.

Many thanks for your notes.² I am very glad indeed to read remarks made by a man who possesses such varied & odd knowledge as you do, & who is so acute a reasoner. I have no doubt that you will detect blunders of many kinds in my book.³ Your M.S. on the proportion of the sexes at birth seems to me extremely curious, & I hope that someday you will publish it.⁴ It certainly appears that the male are decreasing in the London districts, & a most strange fact it is. M^r. Graham, however, I observe in a note enclosed does not seem inclined to admit your conclusion.⁵ I have never much considered the subject of the the causes of the proportion. When I reflected on queen-Bees producing only males when not impregnated, whilst some other parthenogenetic insects produced, as far as known only females, the subject seemed to me hopelessly obscure. It is, however, pretty clear that you have taken the one path for its solution. I wished only to ascertain how far with various animals the males exceeded the females, & I have given all the facts which I could collect. As far as I know no other data have been published. The equality of the Sexes with race-horses is surprising. My remarks on mankind are quite superficial, & given merely as some sort of standard for comparison with the lower animals. M. Thury is the writer who makes the Sex depend on the period of impregnation. His pamphlet was sent me from Geneva.⁶ I can lend it you if you like. I subsequently read an account of experiments which convinced me that M. Thury was in error; but I cannot remember what they were, only the impression that I might safely banish this view from my mind. Your remarks on the less ratio of Males in illegitimate births strikes me as the most doubtful point in your M.S.—requiring two assumptions, viz. that the fathers in such cases are relatively too young, & that the result is the same as when the father is relatively too old.

My son George,⁷ who is a mathematician & who read your M.S. with much interest has suggested, as telling in the right direction, but whether sufficient is another question that many more illegitimate children are murdered & concealed shortly after birth, than in the case of legitimate children; & as many more males than females die during the first few days of life, the census of illegitimate children practically applies to an older age than with legitimate children, & would thus slightly reduce the excess of males. This might possibly be worth consideration. By a strange coincidence a Stranger writes to me this day, making the very same suggestion.⁸

I am quite delighted to hear that my book interests you enough to lead you to read it with some care

Pray believe me | Yours very sincerely | Ch Darwin