Women in warm climates are nubile at eight, nine or ten years of age  : thus childhood and marriage almost always go together. They are old at twenty : reason and beauty therefore never cohabit in them. When beauty wants domination, reason sees that it is denied ; when reason could obtain it, beauty is past. Women must be dependent, for reason cannot provide them in old age with an influence which beauty had not granted them even in youth. It is therefore no surprise that, when religion does not oppose it, a man should leave his wife to take another, and that polygamy should come into being.
In temperate countries, where the charms of women are better preserved, where they become nubile later, and have children at a more advanced age, the aging of their husband in a sense follows their own ; and since they have more reason and knowledge when they marry, if only because they have lived longer, a kind of equality must have naturally come into being in the two sexes, and consequently the law of a single wife.
In cold countries, the almost necessary consumption of strong drinks establishes intemperance among men. Women, who have some natural restraint in this regard because they always have to defend themselves, therefore also have the advantage of reason over them.
Nature, which has distinguished men by strength and reason, has placed no limit on their power other than the limits of that strength and that reason. She has given charms to women, and made things so that their ascendency ends with those charms ; but in warm countries they are present only in the early stages, and never over the full course of their life.
Thus the law that allows only one wife conforms to the physical climate of Europe, and not to the physical climate of Asia. That is why Mohammedanism found it so easy to establish itself in Asia, and so difficult to spread to Europe ; why Christianity has maintained itself in Europe, and been destroyed in Asia ; and finally, why the Mohammedans are making such progress in China and Christians so little.