The period of puberty changes widely the direction of the thoughts, and the character frequently undergoes a complete transformation. Children previously studious lose interest in their lessons, while others pursue them with greatly increased devotion. The sexual emotions, which mark the epoch, may absorb the whole being or merely stimulate it to higher efforts. The age at which puberty begins varies, following the general law that the higher the annual temperature the earlier in life does the change set in. This becomes of psychical interest when it is added that the earlier the change the more intense and permeating are the erotic passions; the more do they compel to their sway the other emotions and the intellect. Only two motives, observes Professor Friedrich Müller, can induce the Australian or the typical African to prolonged labour,—hunger and the sex passion. Civilised communities are measurably lifted above the immediate struggle for food, but not in the least above the other impulse. If you could learn the prime motive, says Dr. Van Buren, of the presence of the crowds of men on Broadway, you would find ninety per cent. of them are there through sex feeling. The sentiments of love, of marital and parental affection, of family life, control mankind more completely than any other motives. These are physical, personal feelings, and to that extent narrow and in conflict with many which are broader and more altruistic. Few persons can advance beyond them, and the collective mind is obliged by the laws of its own existence to register them as of the very first importance. The power of a group is, other things being equal, in proportion to the size of the group, and its increase in numbers is in geometrical proportion to its fecundity, provided the food-supply remains sufficient. These are two closely related and essential factors to advance, and have been so felt from man’s earliest infancy. The complicated systems of marriage and relationship in vogue among the Australian and other rude tribes arose from the effort to adjust the birth-rate to the available amount of food. Many of the forms of marriage arose from the same consideration. In polygamous countries most men are monogamous because they cannot keep large families. Legal infanticide, exposure of the new-born, as in China, is another effort in the same direction. Where such measures are not legalised they reappear in other guises. Artificial abortion and intentional limitation of families are frequent in France and the United States. They are outcrops of a sentiment of self-protection which has been familiar to the species from its beginning.