Taking as the basis of consideration the above-mentioned conditions in Australia, where an approximate regularity in the successive stages of this development is most clearly in evidence, we may distinguish particularly _three_ main forms of exogamy. The first is the simplest. If we designate the two divisions of the tribe between which exogamic relations obtain, by A and B, and the various subgroups of A by _l, m, n, o,_ and of B by _p, q, r, s,_ we have, as this simplest form, _unlimited exogamy_. It corresponds to the following schema:--
A B _l m n o p q r s_ |______________________________|
This means: A man belonging to Class A may take in marriage a woman from any of the subgroups of Class B, and conversely. Marriage is restricted to the extent that a man may not take a wife from his own class; it is unrestricted, however, in so far as he may select her from any of the subgroups of the other class. This form of exogamy does not appear to occur except where the divisions of the tribe are not more than two in number. The marriage classes, A and B, then represent the two divisions of the tribe; the subgroups _l, m, n, o, p,..._ are totem groups--that is to say, according to the view maintained above, cult groups. For the most part, marriage relationships between the specific cult groups meet with no further restrictions. A man of Class A may marry a woman belonging to any of the totem groups _p, q, r, s,_ of Class B--it is only union with a woman belonging to one of the totem groups of Class A that is denied him. Nevertheless, as we shall notice later, we even here occasionally find more restricted relations between particular totem groups, and it is these exceptions that constitute the transitional steps to limited exogamy. Such transitions to the succeeding form of exogamy are to be found, for example, among the Australian Dieri, some of whose totem groups intermarry, only with some one particular group of the other tribal division.
The _second_ form of exogamy occurs when a member of Class A is not allowed to take in marriage any woman he may choose from Class B, but only one from some specific sub-group of B. For example, a man of group _n_ is restricted to a woman of group _r_.