2. _Subversion of Sex-relations._—There is nothing more vital to the growth, even to the very existence, of a nation than the sex-relations which it favours by its laws, customs, and preferences. Upon these depend the processes of natural selection by which the number and the power of future generations are decided through inflexible rules. If these relations, as established by the fixed natural laws of species-perpetuation, are traversed by ignorance or wilful disobedience, nothing can prevent the injury to the physical strength and mental ability of the offspring. Such subversions of the sex-relations may be presented under five headings: (_a_) Premature and delayed marriage. (_b_) Abnormal forms of marriage. (_c_) Abstention from marriage through various causes. (_d_) Licentiousness. Divorce. (_e_) Diminution of natality. Infertility. _(a) Premature and Delayed Marriage._—Mr. Galton, in one of his thoughtful works, remarks: “An enormous effect upon the average natural ability of a race may be produced by influences which retard the average age of marriage or hasten it.” He has illustrated this by abundant examples now through his many writings familiar to the public, his general thesis being that the wisest policy for a nation is to retard the age of marriage among the weak and to hasten it among the vigorous classes. This is, of course, to be construed within physiological lines; premature relations of the sexes, too early marriages, are disastrous in every respect. Statistics of European armies show that there is a far higher mortality and much more sickness among the soldiers who have married young than among single men of the same age. Certain Australian and South American tribes force their female children of immature age into marital relations, and to this is due the rapid decrease of their numbers. _(b) Abnormal Forms of Marriage._—Among early Semitic tribes, and to-day in parts of Tibet and India, the custom prevails of “polyandry,” in which one woman is the wife of several husbands. This sometimes arose from female infanticide, sometimes, as in Tibet, where all the brothers of a family have one wife in common, in order to preserve undivided the family property.[2] Footnote 2: [An obvious gap in the manuscript occurs at this point, but one which in no way affects the general argument of the author.—EDITOR.]