[Footnote 238-1: Thus, for instance, the sturgeon can, according to _Leuckart_, produce 3,000,000 eggs in a year. According to _Burdach_, the posterity of a pair of rabbits may be over 1,000,000 in four years; and that of a plant-louse, according to _Bonnet_, over a 1,000,000,000 in a few weeks. The prolificacy of a species of animals is wont to be greater in proportion as the structure-material (_Bildungsmaterial_) saved within a given time during the course of individual life, is greater, and as material wants during the embryonic period are limited; also (teleologically), in proportion as to the danger the individual is exposed to. Compare _Leuckart_ in _R. Wagner's_ physiolog. Wörterbuch, Art. Zeugung. Teleologically, _Bastiat_ says: _cette surabondance parait calculée partout en raison inverse de la sensibilité, de l'intelligence et de la force avec laquelle chaque espèce résiste à la déstruction_. (Harmonies, ch. 16.)] [Footnote 238-2: The researches of modern physiology make it probable that an ovum is detached from the ovaries at each period of healthy menstruation. (_Bischoff_, Beweis der von der Begattung unabhängigen periodischen Reifung und Lösung der Eier bei den Säugethieren und Menschen, 1844.) It is hardly possible to ascertain how many of these ova are capable of fecundation. Among the animals, on which the greater number of accurate observations have been made, that is in the case of horses, it has been found that, in the two districts of Prussia most favorably conditioned, of 100 mares that had been lined, 63.3 became pregnant, and 53.5 gave birth to live foals; in the rest of the Prussian monarchy, the births were only 46 per cent. Compare _Schubert_, Staatskunde, VII, 1, 98. In the Belgian _haras_ (places for breeding horses), between 1841 and 1850, about 30 per cent. of the "leaps" proved fruitful, from 2 to 3 per cent. aborted, the rest were either probably or certainly unfruitful. (_Horn._, Statist. Gemälde, 171.) In the human species, also, the great number of first-born generated in the first weeks of marriage, bears witness to a high degree of procreative susceptibility. On the other hand, the healthy male semen ejected during a single act of coition contains innumerable germs, a very few of which are sufficient to produce fecundation. (_Leuckart_, loc. cit, 907.) According to _Oesterlen_, Handbuch der medicischen Statistik, 1865, 196, from 10 to 20 per cent. of all marriages were childless. In the United Kingdom, _Farr_, report on the Census of 1851, estimated that in a population of 27,511,000, there were 1,000,000 childless families, when the term is allowed to embrace widows and widowers as well as married couples.]