which is greatly disputed and need not be discussed here.
The marriage of a brother and sister would excite a feeling of loathing
among us that seems implanted by nature, but which, further inquiry will
show, has mainly arisen from tradition and custom.

The evidence proves that there is no instinctive repugnance felt
universally by man to marriage within the prohibited degrees, but that
its present strength is mainly due to what I may call immaterial
considerations. It is quite conceivable that a non-eugenic marriage
should hereafter excite no less loathing than that of a brother and
sister would do now.

The dictates of religion in respect to the opposite duties of leading
celibate lives, and of continuing families, have been contradictory. In
many nations it is and has been considered a disgrace to bear no
children, and in other nations celibacy has been raised to the rank of a
virtue of the highest order. During the fifty or so generations that
have elapsed since the establishment of Christianity, the nunneries and
monasteries, and the celibate lives of Catholic priests, have had vast
social effects, how far for good and how far for evil need not be
discussed here. The point I wish to enforce is the potency, not only of
the religious sense in aiding or deterring marriage, but more especially
the influence and authority of ministers of religion in enforcing
celibacy. They have notoriously used it when aid has been invoked by
members of the family on grounds that are not religious at all, but
merely of family expediency. Thus at some times and in some Christian
nations, every girl who did not marry while still young was practically
compelled to enter a nunnery, from which escape was afterwards
impossible.

It is easy to let the imagination run wild on the supposition of a
whole-hearted acceptance of eugenics as a national religion; that is, of
the thorough conviction by a nation that no worthier object exists for
man than the improvement of his own race, and when efforts as great as
those by which nunneries and monasteries were endowed and maintained
should be directed to fulfil an opposite purpose. I will not enter
further into this. Suffice it to say, that the history of conventual
life affords abundant evidence on a very large scale of the power of
religious authority in directing and withstanding the tendencies of
human nature towards freedom in marriage.

Seven different forms of marriage restriction may be