The social organization of all the Indian nations of America is based upon the existence of the _tribe_. The tribe itself is based upon smaller units of individuals which are joined together by a common tie; this tie is either the archaic maternal descent, or the more modern tie of paternal descent, or a combination of both. Among the Indians of North America east of the Rocky mountains, and also among many tribes west of them, the single groups descending from the same male or female ancestor form each a _gens_ provided with a proper name or _totem_ generally recalling the name of an animal.