ring which the union lasts, and the number of individuals who form it,
the chief factor in the determination of these points being the interests
of the offspring. In actual practice, however, sexual unions, not only in
Man but among the higher animals, tend to last beyond the needs of the
offspring of a single season, while the fact that in most species the
numbers of males and females are approximately equal makes it inevitable
that both among animals and in Man the family is produced by a single
sexual couple, that is to say that monogamy is, with however many
exceptions, necessarily the fundamental rule.

It will thus be seen that marriage centres in the child, and has at the
outset no reason for existence apart from the welfare of the offspring.
Among those animals of lowly organization which are able to provide for
themselves from the beginning of existence there is no family and no need
for marriage. Among human races, when sexual unions are not followed by
offspring, there may be other reasons for the continuance of the union
but they are not reasons in which either Nature or society is in the
slightest degree directly concerned. The marriage which grew up among
animals by heredity on the basis of natural selection, and which has been
continued by the lower human races through custom and tradition, by the
more civilized races through the superimposed regulative influence of
legal institutions, has been marriage for the sake of the offspring.[312]
Even in civilized races among whom the proportion of sterile marriages is
large, marriage tends to be so constituted as always to assume the
procreation of children and to involve the permanence required by such
procreation.

Among birds, which from the point of view of erotic development
stand at the head of the animal world, monogamy frequently
prevails (according to some estimates among 90 per cent.), and
unions tend to be permanent; there is an approximation to the
same condition among some of the higher mammals, especially the
anthropoid apes; thus among gorillas and oran-utans permanent
monogamic marriages take place, the young sometimes remaining
with the parents to the age of six, while any approach to loose
behavior on the part of the wife is severely punished by the
husband. The variations that occur are often simply matters of
adaptation to circumstances; thus, according to J.G. Millais
(_Natural History of Brit