| Age of Mother. | 30 to 35 Years. | 25 to 30 Years. | 20 to 25 Years. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Age of Father. | Boys. | Girls. | Sexual ratio. | Boys. | Girls. | Sexual ratio. | Boys. | Girls. | Sexual ratio. |
| 15 to 30 years | 8,525 | 7,887 | 108.1 | 27,389 | 25,843 | 106.0 | 21,560 | 20,330 | 106.0 |
| 30 to 35 years | 23,283 | 21,823 | 106.9 | 23,394 | 23,486 | 103.9 | 7,954 | 7,469 | 106.5 |
| 35 to 40 years | 17,885 | 17,070 | 104.7 | 10,272 | 9,838 | 104.2 | 2,426 | 2,416 | 100.4 |
| 40 to 45 years | 7,972 | 7,681 | 103.8 | 3,165 | 3,058 | 103.5 | 1,154 | 1,100 | 105.0 |
| Over 45 years | 4,220 | 3,997 | 105.6 | 1,734 | 1,525 | 113.8 | |||
In this table we find the births arranged in relation to varying ages of the fathers and in relation to mothers whose ages are tabulated in three classes, the ages of the latter being those at which they are most prolific. The table shows clearly that the excess of boys is larger at the beginning and at the end of each column. Thus, the age of the mother remaining constant, young fathers and elderly fathers procreate a larger proportion of boys than do fathers of intermediate age.
But I find in this table, which is based upon a very large number of instances indeed, a confirmation of the thesis which I stated above, for the highest sexual ratio of 113.8 is in this table found in the case of fathers over forty-five years of age who are married to mothers of ages twenty-five to thirty years; this is, as I said, the case in which “the husband is at least ten years older than the wife, and the latter is at the age at which her reproductive capacity is at its maximum.”
From such figures as these, which seem to show the influence of the absolute age of the father upon the determination of sex, it has by many been inferred that the man exercises a preponderating influence upon the determination of the sex of the embryo, impressing his own sex upon it, and that the greater the sexual potency of the begetter the more powerful also is the influence exercised by the latter; that the point of first importance in this connection is the sexual capacity of the man; and that the excess of male births increases pari passu with the increase in the potency of the procreating male.
Two additional considerations have been adduced to demonstrate the influence of the father in determining the sex of the offspring. The first of these is a comparison of the ratio between male and female births in towns and in rural districts, respectively; and the second is a comparison between the ratio of males to females in the offspring of married and of unmarried parents, respectively.
In towns the excess of male births is smaller than it is in the country. The average sexual ratio in Prussia during the five-year period, 1875 to 1880, is given by Düsing as follows:
| In Berlin | 105.70 |
| In other large towns | 105.72 |
| In medium sized towns | 105.44 |
| In small towns | 106.17 |
| In rural districts | 106.62 |
The indisputable fact that in towns more girls are born than in rural districts is referred to the fact that in the country the husband usually enters on marriage with his virile powers completely unimpaired, whereas in towns many men only marry after they have for many years expended their best forces in irregular sexual intercourse, and thus reserve for their wives only the dregs. But, as we shall show presently, the difference between town and country in this respect is susceptible of a different interpretation.
It has also been asserted (Horn) that extra-conjugal sexual intercourse is favourable to the procreation of boys, the suggestion being that the greater sexual vigour in the former case determines a preponderance of males in the offspring; but this assumption is invalidated by the statistical evidence which now accumulates, that among illegitimate offspring there is a smaller excess of boys than among legitimate offspring. Babbage, for instance, came to this conclusion as a result of the comparison of 1,000,000 illegitimate births with 14,000,000 legitimate births. On the other hand, the following utterance of Ploss appears extremely artificial: “In a country in which illegitimate births are very numerous, in which the illegitimate children are for the most part begotten by enervated debauchees, the excess of male births is smaller; but in a country in which the illegitimate births are less numerous, and in which the illegitimate infants are for the most part the fruit of love and are begotten by youthful lovers, the excess of male offspring is larger.”
Next to the age of the progenitors, their nutritive condition is shown by statistical investigations to exercise an important influence in the determination of sex. The following proposition has, in fact, been established: Where the supply of nutriment is deficient, the offspring contains an excessive proportion of boys.