But we must not wander off into questions of origin.
It is enough for our present purpose to have noted the fact that, within the tribe, there are normally other forms of social grouping into which a man is born, as well as the clan.
[Footnote 5: From a Greek word meaning "brotherhood," which was applied to a very similar institution.] Now we come to the tribe.

This may be described as the political unit.
Its constitution tends to be lax and its functions vague.

One way of seizing its nature is to think of it as the social union within which exogamy takes place.

The intermarrying groups naturally hang together, and are thus in their entirety endogamous, in the sense that marriage with pure outsiders is disallowed by custom.

Moreover, by mingling in this way, they are likely to attain to the use of a common dialect, and a common name, speaking of themselves, for instance, as "the men," and lumping the rest of humanity together as "foreigners." To act together, however, as, for instance, in war, in order to repel incursions on the part of the said foreigners, is not easy without some definite organization.