In the question we are considering the nature of the amorous passions also plays a great part. When they are purely sensual they do not last long as a rule; but when love arises from mental affinities it may be prolonged till old age. Bain remarks that other passions, such as maternal love, hatred, the desire of domination may be extended to many objects, while love has a tendency to concentrate itself on a single one which then takes preëminence over the others and tends to monogamy. We have seen that birds and monkeys generally love only one female. With some conjugal love is so strong that one of the conjoints cannot survive the other; this fact has been observed with certainty, even when the survivor was provided with another mate. Thus, the male of a certain species of monkey (_Hapale jacchus_) after the death of his mate, covers his eyes with his hands, ceases to eat and remains in the same position till he dies. Suicide for love is not rare among certain savage races; a point to which we shall return later. Westermark is certainly right in considering this tendency of love to concentrate itself on a single object as one of the most powerful factors in monogamy. Jealousy is no doubt the reverse of such sentiment, but is the profound despair at seeing the sole object of love desert or become unfaithful. On the other hand, this concentration of love, which may be excellent for isolated families living alone after the manner of wild beasts, is in no way adapted to a society of which all the members are responsible. This is a point we must insist upon. There is certainly a real antinomy which is difficult to reconcile between this dual egoism of exclusive and concentrated love and social solidarity or human altruism. The problem is not insoluble, but we must admit that the solution is not easy. To resume, we first of all observe an evolution from monogamy toward polygamy. The higher apes and the most primitive men are monogamous; among these there are no differences of rank, nor class distinctions, and they live in very small groups. Wealth, civilization, larger communities, agriculture and the domination of castes have gradually given rise to polygamy. Thus, the ancient Hindus were at first monogamous and later on became polygamous. The prerogative of the first wife over the others is only a vestige of monogamy in polygamy. A higher degree of culture then diminishes warfare, shortens the period of nursing, does away with the prejudices against coitus during pregnancy, and improves the social position of women. Ageing less quickly, and adding to her bodily charms those of her mental development woman restores man to monogamy. As the same time wives and children gradually cease to constitute riches, and this diminishes the instinct of procreation. Finally, machinery replaces the female labor of former times. In this way, with a higher degree of human culture, all the factors tend to restore monogamy.