I am prompted here to quote also the statistical numbers given in Hensen's work, which have been taken from Darwin's 'Descent of Man.' Of pigs, rabbits, and pigeons, more males are born than females. For every 100 mares, 99.4 horses are born. In the case of greyhounds, 110 dogs are born for every 100 bitches. Of horned cattle 94.4 males, of poultry 94.7 males are born for every 100 females. The degree of accuracy and the limits of error which here remain undefined, make fluctuations easily perceptible. The mistakes, also, which may be made in such cases, are not always the same.
Statistics have been in many other ways called in to assist in the discussion of the question before us. In the early decades of the present century a question was raised--what was the effect upon the relative number of births of male or female individuals when the parents were of like or unlike ages?
Hofacker, in the year 1828, and Sadler (an Englishman), in the year 1830, attempted to solve this problem, and found adherents for their theories based upon numerical returns. But the Frenchman, Girou (Paris, 1838), appeared as an opponent of their views, also supporting his opinions by numbers obtained in the same manner, probably, as those of Hofacker and Sadler.
I shall not here reproduce the tables which were constructed for the discussion of this question. Any one who occupies himself with these questions can refer to the respective technical works, and I shall content myself with mentioning some of the results. If the man is older than the woman, more boys will be born. According to Sadler, the statistics showed even 121.4 boys for 100 girls.
If both the parents are of the same age, fewer boys than girls will be born. According to Sadler, in this case for every 100 girls only 94.8 boys are born. But if the woman is older than the man, an excess of girls in the family is the result. According to the two above-mentioned authors, when the mother is older than the father the proportions are: 86.5 boys to 100 girls.
Similar numbers collected by other specialists differ not inconsiderably from those given by Sadler. Regarding the proportions of male and female births as affected by the respective ages of the parents, Sadler's numbers show the widest differences of all. Breslau and Noirot have arrived at numerical results so different, though less than Sadler's, that no thoroughly reliable conclusions can be based upon them. Wall confesses himself an adherent of this law, and lays down the principle that in the intercourse of two quite young parents the male sex tends to predominate. If, on the contrary, the age of the man is distinctly greater than that of the woman, he insists on the excess of females amongst the new-born. The French breeder Girou de Buzareingues is disposed in many respects to support the theory of the influence of difference of age in the parents upon sex of offspring; but, also, on the strength of his own experiences in breeding, is partially opposed to it. According to his theory, he also takes into consideration the character, the food, etc., of the parents, and would have regard to their size and strength. In this way he gave his theory a much wider range. He mentions a great number of facts which he observed in the human subject. He outlines the expenditure of force, mental and physical, entailed on the parents by their occupation, and then sets forth ten very precise particulars from which, in any given case, the sex of the offspring which will result from the wedlock in question may be known. The following cases from Girou may be mentioned. A vigorous man married a corpulent, melancholy, elderly blonde. Seven daughters were the result of the marriage, all of them resembling their father and grandfather. Many similar cases are mentioned by Girou, all of which may be found of interest to the reader, if he be inclined to regard preponderance of temperament, or physical disposition for procreation of the species as important factors. Included in his _repertoire_ of anecdotes are many interesting and piquant details respecting the results of the pairing of dissimilar temperaments which might be quoted, were it not that this would lead us too far from our subject, and also be of no service in the present inquiry.