Bidder is in many respects inclined to give his assent to the theories of his predecessors, and states that women who bear their first child between the ages of twenty and twenty-one produce more girls than boys (Düsing). The older the woman is at the time of her first parturition, the greater number of male births. An excess of male births will occur in the case of those who first give birth to children between the ages of thirty and forty (Eckhardt). Ahlfeld insists that this is a universal rule in the case of women who become pregnant in later years. A great number of specialists are of this opinion, and apply the data afforded by statistics to support it in different ways.
The evidence of Stieda, Berner, and Birelli, and especially that of Wilkens, respecting the domesticated mammals, leads, however, to this conclusion, that the theories respecting the relative proportions of male and female births set forth by Sadler and Hofacker must either be given up or their value considerably discounted.
Specialists are also to be found who, in order to explain this theory, have availed themselves of Darwin’s law, and in a certain measure the results admit of this explanation.
The older parent, who evidently under such normal circumstances as might be anticipated has a shorter time to live than the younger individual, his consort, naturally in the struggle for life, makes an effort for the preservation of his sex. Accordingly, the elderly husband of a young wife, or vice versâ, the elderly wife of a young husband, will make an effort to preserve the sex which is first threatened with death, but which may at least be replaced by a majority of births.
In so far as these theories are mere calculations and results which have originated from comparisons of numbers (the numbers themselves being in many cases of no practical value), the conclusions reached may appear to be astonishing, and may be used to support either one view or another, or to contradict them. Only one fact appears to be certainly established, that, on an average, under normal circumstances, the number of male individuals of our population that are born exceeds the number of females. The difference amounts to a small and variable number per cent., but in the case of the new-born, the excess is on the side of the males. (Süssmilch.)
Thus far we have given such data as statistics have furnished. These, it is true, belong principally to past epochs, and no new results of this sort have been used by statisticians. But it would be equally impossible to deduce from new statistics, or from old, or from both together, any law of nature affecting the question before us.
I shall proceed next to examine the further theories on this subject with which I have become acquainted from the perusal of the literature treating of it. With some of the works which I am about to mention I am acquainted at first hand. Others I know only from quotations found in various technical publications. I have not attempted to arrange my materials in any other way.