According to this definition, there may be monogamous, polygamous and polyandrous marriages, as well as marriage in groups and limited marriage. It is evident that permanent monogamous unions, such as occur in birds and the higher apes, are, according to this definition, true marriages, of better quality even than those of many men. Among animals which have a definite rutting period, marriage cannot depend solely on the sexual appetite, or egoistic eroticism, without ceasing with the rut. It follows from this that natural selection and the mneme (engraphia) have derived from the sexual appetite certain social or altruistic instincts, with the object of preserving the species by protection of the young. Although not the only means of preserving the species, such instincts are certainly important. The family is thus the root of marriage. This explains the custom, among certain races, of marriage only becoming valid after the birth of a child. In many forms of marriage by purchase, the wife is even bound to return to her husband the sum paid for her if she remains sterile, and among many savages the marriage is only celebrated after the birth of the first child. In Borneo, relations between the sexes are free till pregnancy occurs, and it is this which determines the duties of marriage. In this respect, these savages are more just and wiser than us. In man, a special reason in favor of marriage is the fact that he has no rutting period. In animals the rutting period is generally regulated so that the young are born exactly at the time of year when they will find food most abundant. For example, the muscardin copulates in July and brings forth young in August, at the time when nuts are ripe, while elephants, whales and certain monkeys, who find food at all seasons, do not copulate at any definite period. The anthropoid apes, however, have a rutting period, and something analogous is found among certain human races (Californians, Hindus and certain Australians) in the spring, when sexual orgies are indulged in. In man there is no particular correlation between eroticism and the possibility of easily obtaining food for the children at the time of birth. Nevertheless, a recrudescence of the sexual appetite is generally observed in the spring and beginning of summer, with a corresponding increase in the number of conceptions. This is probably explained by the fact that infants born in the autumn or winter are more robust. Moreover, natural selection has almost entirely ceased in civilized peoples, owing to the artificial means used to rear children, and to the diminution which results from their mortality. We thus see that the institution of marriage in man does not depend on the excitation of the sexual appetite, for this is, on the whole, continuous. ANTIQUITY OF MATRIMONIAL INSTITUTIONS