Part 59

Broadly speaking, the earlier theories may be said to diverge in two main directions, some holding that the sex of the infant was in some way determined by the mode of intermixture of the male and the female elements in the act of generation, and others maintaining that sex was already inalterably predetermined at the time of intercourse either in the male or in the female sexual elements.

Pari passu with the modern development of the theory of evolution, and with the enormous increase in recent days in anatomical and physiological knowledge, the theory of the determination of sex has been very widely extended. The rival views may be briefly arranged in the four following categories:

I. That sex is already inalterably predetermined in the ovum, upon the constitution of which it solely depends; there are therefore male and female ova, and the process of fertilization exercises no influence whatever upon the determination of sex. The alternative theory to the above, that sex is determined solely by the constitution of the fertilizing spermatozoon—i. e., that the spermatozoa, and not the ova, are male and female, respectively—is one which in recent years has tended more and more to disappear from the field.

II. That sex is determined in the moment of fertilization by the reciprocal interaction of male and female, of zoösperm and ovum. One variant of this theory maintains that each reproductive element strives for the reproduction of its own sex; that a struggle takes place and that the victor in the contest stamps its own sex-likeness upon the fertilized product. According to another view, however, sex is not directly transmitted in this manner; it is supposed that the more powerful the proper reproductive element (according to this theory the ovum) the more strongly does it tend to determine the reproduction of a stronger, i. e., a male organism; thus the greater potency of the female element in the act of reproduction tends to favour the determination of the male sex.

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.

Hofacker’s actual figures were the following:

Father younger than mother 90.1 boys to 100 girls. Father same age as mother 93.3 boys to 100 girls. Father 4 to 6 years older than mother 108.8 boys to 100 girls. Father 6 to 9 years older than mother 124.7 boys to 100 girls. Father 9 to 12 years older than mother 143.7 boys to 100 girls.

Sadler’s results were closely similar:

Father younger than mother 86 boys to 100 girls. Father same age as mother 94 boys to 100 girls. Father 1 to 6 years older than mother 103 boys to 100 girls. Father 6 to 11 years older than mother 126 boys to 100 girls. Father 11 to 16 years older than mother 147 boys to 100 girls. Father 16 years and more older than mother 163 boys to 100 girls.

Goehlert found that the offspring of marriages in which the husband was younger than the wife were 71 boys and 86 girls; of marriages in which husband and wife were of the same age, 263 boys and 282 girls; and of marriages in which the husband was older than the wife, 2,017 boys and 1,865 girls.

Wappaeus, combining the data supplied by these three investigators, Hofacker, Sadler and Goehlert, obtained the following sexual ratios for the entire 8,000 cases (i. e., the number of boys born to each 100 girls born): When the husband was younger than the wife, 88.2; when husband and wife were of the same age, 93.5; when the husband was older than the wife, 113.0.

It will be observed that the mean sexual ratio of these 8,000 cases is 109.6; whilst, as we saw above, when a sufficiently large number of instances is taken, the sexual ratio always closely approximates to 106.3. From this it appears that the numbers dealt with by Hofacker, Sadler and Goehlert in their investigations were too small for the deduction of trustworthy averages.

The same criticism is applicable to the observations of Ahlfeld, Breslau and Noirot, whose results conflict with those just given. According to Ahlfeld, in the case of 1,376 births where the father was at least 10 years older than the mother, the sexual ratio was only 98.2. According to Noirot’s data, in cases in which the father was older than the mother, this ratio was 99.7.

Oesterlen gives the following brief summary of the researches made for the establishment and confirmation of the law of Hofacker and Sadler:

═══════════════╤══════════╤══════════╤══════════╤══════════╤══════════ │ Father │Father and│ Mother │ Sexual │Number of AUTHOR. │older than│ mother │older than│ ratio. │instances. │ mother. │same age. │ father. │ │ ───────────────┼──────────┼──────────┼──────────┼──────────┼────────── Hofacker │ 117.8│ 92.0│ 90.6│ 107.5│ 1,996 Sadler │ 121.4│ 94.8│ 86.5│ 114.7│ 2,008 Goehlert │ 108.2│ 93.3│ 82.6│ 105.3│ 4,584 Noirot │ 99.7│ │ 116.0│ 103.5│ 4,000 Legoyt (Calais)│ 109.9│ 107.9│ 101.6│ 107.9│ 6,006 Legoyt (Paris) │ 104.4│ 102.1│ 97.5│ 102.9│ 52,311 Breslau │ 103.9│ 103.1│ 117.6│ 106.6│ 8,084 ═══════════════╧══════════╧══════════╧══════════╧══════════╧══════════

The law of Hofacker and Sadler cannot be regarded as possessing universal validity, although the figures on which it is based seem to show pretty clearly that we are justified in regarding the mutual interaction of the male and female reproductive elements at the moment of fertilization as effective in the determination of sex. In the investigations to which we have hitherto alluded it is only the relative ages of husband and wife that have been taken into account; but other researches have shown that the absolute age alike of the husband and of the wife has an influence in the determination of sex.

The influence of the absolute age of the mother in the determination of sex has been very clearly established. Ahlfeld was the first to draw attention to the fact that among the children of elderly primiparæ there was always to be found an excess of boys, and that there was an increase in this excess proportionate to the greater age of the mother. Among 102 children born to primiparæ over 32 years of age the sexual ratio was 137 : 100; and a later investigation made by the same author in conjunction with Schramm showed that among 1,038 children born to primiparæ over 28 years of age the sexual ratio was 124 : 100.

Hecker obtained similar results. Among 432 children born to primiparæ over thirty years of age the sexual ratio was 133 : 100. Winckel, dealing with primiparæ of the same ages, found a sexual ratio of 136.8 : 100.

Düsing, examining the records of the lying-in hospitals of Leipzig, Dresden and Jena and thus obtaining a very large number of instances whereon to base his conclusions, confirmed the view that elderly primiparæ give birth to an excess of boys, and further that the older they are the larger the excess of male births. He drew up the following table: