an excited condition of the nervous system is a necessary
preparation to pairing.[236] And just here we must recognize the fact
that monogamy is a habit acquired by the race, not because it has
answered more completely to the organic interest of the individual,
but because it has more completely served social needs, particularly
by assuring to the woman and her children the undivided interest
and providence of the man. But in early times the law of natural
selection, not the law of choice, operated to preserve the groups in
which a monogamous or quasi-monogamous tendency showed itself (since
the children in these cases were better trained and nourished), and
in historical times and among ourselves all of the machinery of church
and state has been set in motion in favor of the system. In point
of fact, the members of civilized societies at the present time have
become so refined and have so far accepted ethical standards that
monogamy is the system actually favored on sentimental grounds as well
as on grounds of expediency by a large proportion of any civilized
population. On the other hand, speaking from the biological
standpoint, monogamy does not, as a rule, answer to the conditions of
highest stimulation, since here the problematical and elusive elements
disappear to some extent, and the object of attention has grown so
familiar in consciousnes that the emotional reactions are qualified.
This is the fundamental explanation of the fact that married men and
women frequently become interested in others than their partners in
matrimony. I may also just allude to the fact that the large body of
the literature of intrigue, represented by the tales of Boccaccio and
Margaret of Navarre, is based on the interest in unfamiliar women.

Familiarity with women within the group and unfamiliarity with
women without the group is the explanation of exogamy on the side of
interest; and the system of exogamy is a result of exchanging familiar
women for others. We have seen that capture was not an important
means of securing wives outside the group, and that exogamy was fully
developed before property and media of exchange were developed to any
extent, and consequently before the purchase of women had become a
system. We have seen also that the Australian who wants a woman at
the present time gets her by exchanging another woman for her. Social
groups were necessarily small in the beginning. Before invention and
co-operation have advan