At that time probably only a minority even of healthy children survived, and the slight congenital weakness produced by in-breeding might apparently be fatal to a child's chance of life.

Possibly some dim perception may have been obtained of the different fates of the children of women who restricted their sexual relations to men within the clan and those who resorted to strangers, even though the nature of paternity may not have been understood.

The strength of the feeling and custom of exogamy seems to demand some such recognition for its satisfactory explanation, though, on the other hand, the lateness of the recognition of the father's share in the production of children militates against this view.

The suggestion may be made also that the belief that the new life of a child must be produced by a spirit entering the woman, or other extraneous source, does not necessarily involve an ignorance of the physical fact of paternity; the view that the spirits of ancestors are reborn in children is still firmly held by tribes who have long been wholly familiar with the results of the commerce of the sexes.

The practice of exogamy was no doubt, as shown by Dr.Westermarck, favoured and supported by the influence of novelty in sexual attraction, since according to common observation and experience sexual love or desire is more easily excited between strangers or slight acquaintances than between those who have long lived together in the same household or in familiar intercourse.