My dear Mr. Darwin

Don’t be afraid that I am intending to bore you with my ideas, & don’t be terrified at the broad packet I am sending you today, & don’t think of even acknowledging the receipt of any Mem^a I may send you.² But as I am reading your book³ slowly (and I need not say with what deep interest) I think you may perhaps not be sorry to receive any memoranda of actual facts I may have met with in the course of my life or reading, bearing upon your speculations & confirmatory of them.

I. The packet I send contains the result of some elaborate calculations I made some time since with a view of seeing if I could arrive at some conclusions as to the cause of the varying proportions of male & female births.⁴ I think some of them are worth consideration—especially those showing the much slighter excess of males in the London Districts & in recent years. I believe all the figures may be relied upon.

The only remark I have to add is the observation I have made how often a large family, exclusively of daughters, is finished off by a boy in later years:—ie—6 or 7 girls successively, & then a son.— I have no idea what this points to; but it may suggest a line of thought or inquiry to you.

I fancy from one sentence (at vol I. p        ) you may have met with a paper which struck me much at the time, in I think the Journal of Agriculture (or Agricultural Science) N^o.          :— in which the writer, a man of large breeding experience, is satisfied from, he says, the experiments & observations of many years, that the sex of the offspring (in cows at least) is determined by the period (relative to menstruation or its bovine equivalent) of impregnation.—⁵ Now, on reading this it occurred to me whether there may not be some relation to this fact or some confirmation of this theory, in the circumstance that the excess of males is less in illegitimate than in legitimate births. In the former copulation occurs quando Dio commenda.⁶ as Italians say:—in the latter, I believe, marriage & consequently copulation takes place pretty soon after menstruation. At least I imagine women mostly arrange their wedding-days with reference to the date when they are certain to be accessible. Conception is supposed to take place in the very early days of the monthly period.— Whether the (supposed) fact that the first child is most usually a girl, may have any thing to do with this;—or whether the whole theory is not knocked on the head by D^r. Duncan’s⁷ figures, which show that the first child on an average is not born till 15 months after marriage (if, indeed, previous miscarriages do not vitiate his conclusion),—I cannot say. I only thought that there might be some connection between the cases,—or some suggested elucidation to be got out of them.

II. At vol I. p.          you speak of moral sentiments in animals & of chastisement administered to peccant ones.⁸ I once saw (as I think I mentioned to you the other day vivâ voce) a ewe act in a manner clearly indicative of something of the sort. Two of her lambs, male & female (I think) which she was feeding were playing about her, frisking as lambs do, & one of their amusements was for one to pretend to be tupping the other. As often as it did this she paused from her feeding & butted it off pretty sharply. Whether from prudery, sanitary ideas, moral sense, or a dim perception of the ecclesiastical law of prohibited degrees,⁹ this deponent sayeth not. But of the distinct disapproval of the mother & the prompt indication of it, there was no doubt whatever.

III. The fact (which I also mentioned) of the case of an old toothless hunter, a great favourite with its master, having his oats masticated for him & deposited in the manger by two fellow horses—who no doubt had had their meal first—I cannot now recover my authority for. But I am certain of having read it in a book (not a newspaper), & I think one on Sporting or Horse Nat^l. History. Probably a question in “Notes and Queries”, would elicit the origin of the recorded Story.

IV. In re the continuance of the Sagittal Suture down the frontal bone in mature life.—¹⁰ At one time, long years ago, I attended a good deal to craniology, and had collected a considerable number of skulls, both animal & human. The collection I parted with many years since. Among its contents were a quantity of human skulls I had obtained from two very ancient & sacred burial places near Killarney—places so revered by the Irish of Kerry that every one wished to be buried there; & as the area was limited every fresh corpse displaced a number of ancestral ones, which lay scattered about the inclosure.— Now, of these (as far as my recollection serves me—& I noted the fact & drew my inference at the time, the adult skulls had the frontal bone divided by a suture as distinct as any of the others, in nearly one case out of three:—the normal proportion in Europeans being, I believe, one in twelve.— It would be curious to know whether the same abnormal proportion holds in Irish heads now: (these were probably 300 or 400 years old):—whether other Celtic races are similarly differentiated,—and whether Negroes & other savage tribes show the same peculiarity, as you say the very ancient, cave & drift skulls do,—and as my Muckross & Aghadoe skulls¹¹ assuredly did.— I, in my impertinence, drew the conclusion (which I have held ever since) that the Irish remained always children, or assimilated more than we do to the lower animals.

Yours very sincerely | W. R. Greg

PS. Someday I should like to have my calculations back, though I am not likely to have leisure for pursuing the inquiry. But perhaps Galton¹² would.