III. That sex is not determined until after fertilization, during the early stages of the development of the embryo; the determining causes are supposed to be various factors capable of influencing the developing organism during this period, and more particularly the nutritive conditions of the mother.
IV. That the determination of sex is not dependent solely upon the action of any single one of the factors above enumerated, but arises as a resultant effect of the operation on the germ of all three of these acting in temporal succession.
Modern physiology has endeavoured to solve this problem by statistical investigations, by anatomical demonstration, and finally by experiment.
I. Statistical Investigations.
Statistical data have been collected showing the ratios between male and female births in the most varied conditions possible, and from these data the attempt has been made to draw valid conclusions regarding the causes of the determination of sex. Now in the first place it is above all necessary to bear in mind that such statistical data cannot possibly have any value unless they relate to very numerous instances, and even then they are liable to be invalidated by various sources of fallacy. We may with advantage quote in this connection the remarks of Hensen in his work on the “Physiology of Reproduction:” “Each individual instance is rendered unique in kind by the interaction of certain incommensurable elements; for instance the state of health of the individual organs in their innumerable combinations, variations in the general health of the parents, the frequency of coitus and the time at which it took place, the desire of the parents to have a son and then no more children, their social position—in these ways innumerable complications are introduced into the problem, and the difficulty of drawing valid conclusions is rendered almost insuperable, unless the number of instances dealt with is enormously large.”
One fact definitely established is that more boys are born than girls, the proportion between the two, known as the sexual ratio, being 106 : 100. Statistics relating to the half of Europe (Oesterlen) and dealing with 59,350,000 births, showed a ratio of 106.3 male to 100 female births; in individual countries variations from this mean are found to occur, but these are not very extensive, the highest ratio being 107.2 : 100, and the lowest ratio 105.2 : 100.
From the works of Hofacker (“Ueber die Eigenschaften welche sich bei Menschen und Tieren von den Eltern auf die Nachkommen vererben”—Concerning the Qualities transmitted from Parents to Offspring in Men and Animals—Tuebingen, 1828) and Sadler (“Law of Population,” London, 1830) conclusions have been drawn regarding the effect of a variation in the age ratio of the parents on the determination of sex. The deductions in question, known as Hofacker and Sadler’s law, are as follows:
1. If the husband is older than the wife more boys are born than girls.
2. If husband and wife are the same age somewhat fewer boys are born than girls.
3. If the wife is older than the husband the excess of female births is larger still.