he Wathi-Wathi, we find the rule that
a man must marry in his own generation; and this is _prima facie_ the
meaning of the four-class rule. It is true that the origin of the
eight-class rule was not what its _prima facie_ meaning suggests, viz.,
the desire to prevent the marriage of cousins, for we know that it
originated in the distinction between elder and younger sisters. But no
similar theory appears to fit the case of the four-class tribes. No
division within the generation could possibly produce an alternation of
generations.

The Red Indians have in many cases different names for the elder and
younger sister; the Hausa impose on persons standing in these relations
certain prohibitions and avoidances, which are not the same for both
elder and younger; in Australia a man may speak freely to his elder
sisters in blood, but only at a distance to his tribal _ungaraitcha_. To
his younger sisters, blood and tribal, he may not speak save at such a
distance that his features are indistinguishable. In many parts the
elder brother has special rights with regard to the younger, and many
similar customs might be quoted[139].

The question why marriage within the generation--the rule of four-class
and two-phratry tribes alike--should have come into existence is a
complicated one and involves that of the origin of kinship terms. If we
take a crucial case of kinship terminology, we find that a child applies
the same term to its actual mother as to all the women whom its father
might have married, to its potential mothers in fact. If therefore we
have to choose between the gradual extension of the terms from the
single family to the group or their original application to a group,
this instance seems decisive in favour of the latter theory.

Now if marriage was originally not "group" but individual, a question to
be fully discussed in later chapters, we can hardly doubt that
parent-child marriage was forbidden or perhaps instinctively avoided.
But this would be equivalent to prohibiting marriage with one of a
number of men or women embraced under a common kinship term. In the
lower culture generally and especially among the Australians there is a
tendency to follow things out to their logical conclusions. If this were
done in the present case, the result would be to extend the prohibition
to all the persons embraced under the kinship term.

In any case the natural tendency in a small group would be to marry
within the generati