This may have been the reason for the introduction of the class system among the Australians and Red Indians, by which all the clans of a certain area were divided into two classes, and the men of any clan of one class could only marry or have intercourse with the women of a clan of the other class.

By such a division the evil results of the mixture of totems in exogamous clans with female descent would be avoided.

The class system was sometimes further strengthened by the rule, in Australia, that different classes should, when they met, encamp on opposite sides of a creek or other natural division [164]; whilst among the Red Indians, the classes camp on opposite sides of the road, or live on different sides of the same house or street.

[165] In Australia, and very occasionally elsewhere, the class system has been developed into four and eight sub-classes.

A man of one sub-class can only marry a woman of one other, and their children belong to one of those different from either the father's or mother's.