An excerpt from the INTRODUCTION: EARLY in the year 1887 my attention was first called to the fact that the great problem of the Causation of Sex in Mankind was still unsolved. The inquiries which I then began to make soon showed me how much in the dark the medical profession is on the subject. The whole question of the Causation of Sex in Mankind had been hedged around, encumbered and obscured with observations ad nauseam on the eggs of the invertebrata; on worms and tadpoles; on sponges, and plants; on bees, and water-fleas; and lastly, on hens' eggs, to which nothing more dissimilar could be found than the human egg or ovum. If ever it be true that "the proper study of mankind is man," it is in this study of the causation of sex, and therefore I have made it chiefly a clinical study. Among the large number of theories which had been advanced, the great majority were quite untenable, and were propounded without any clinical evidence or facts to support them; several others I found had been suggested which were diametrically contradictory to a theory which some former writer had advocated. A few of these I mention in the text. As a general practitioner of medicine, the daily round of work, the ever-present necessity of earning one's living, sadly interfered with, and was hardly conducive to, close study of and inquiry into such an engrossing subject, and so progress was slow; but we must remember, as Dr. Samuel Johnson said, " it is dangerous to quiet our uneasiness by the delusive opiate of hasty persuasion"; for the answers to most great questions have only been arrived at after much patience combined with persistence and sustained work. It was not, therefore, till some thirteen years after first beginning the study of the Causation of Sex that I ventured, in December 1900, to bring the subject before the Obstetrical Society of London: it is remarkable, but it was the first time that the subject of the Causation of Sex in Mankind had ever been discussed by the Society! Innovations, however, rankle with many, and, as was to be expected, but little knowledge of the subject was shown, and much of the criticism of the paper was irrelevant and inaccurate. The reception of the paper did not discourage me, and the following eulogistic notice of it from the pen of the then President of the Obstetrical Society, Mr. Alban Doran, F.R.C.S., appeared in vol. xliii. 1901, pp. 49 and 50, of the Society's Transactions: "A very remarkable monograph on ' The Essential Factor in the Causation of Sex: a New Theory of Sex,' was read in December by Mr. E. Rumley Dawson. This communication was prepared after long study of cases of removal of one ovary, and of families where one sex predominated or prevailed entirely. . . . The boldest theory in this singular monograph was the assertion that the sex of the child depends upon which ovary supplied the ovum fertilised. This paper was strongly criticised in a very active discussion by several obstetrical and gynaecological authorities; but the author, who showed great dialectical ability both in his written monograph and in his reply to his critics at the meeting, stoutly maintained the scientific value of his views. This memorable discussion on a sex problem-a subject always of interest, though on the borderland of the insoluble - was further remarkable as being the last piece of work done by the Obstetrical Society in the nineteenth century."