The most essential contribution to our knowledge of the group marriage we owe to the English missionary, Lorimer Fison, who studied this form of the family for years on its classical ground, Australia. He found the lowest stage of development among the Papuans near Mount Gambier in South Australia. Here the whole tribe is divided into two great classes, Kroki and Kumite.[16] Sexual intercourse within each of these classes is strictly prohibited. But every man of one class is by birth the husband of every woman of the other class, and vice versa. Not the individuals are married to one another, but the whole groups, class to class. And mark well, no caution is made anywhere on account of difference of age or special consanguinity, unless it is resulting from the division into two exogamous classes. A Kroki has for his wife every Kumite woman. And as his own daughter, being the daughter of a Kumite woman, is also Kumite according to maternal law, she is therefore the born wife of every Kroki, including her father. At least, the class organization, as we know it, does not exclude this possibility. Hence this organization either arose at a time when, in spite of all dim endeavor to limit inbreeding, sexual intercourse between parents and children was not yet regarded with any particular horror; in this case the class system would be directly evolved from a condition of unrestricted sexual relations. Or the intercourse between parents and children was already proscribed by custom, when the classes were formed; and in this case the present condition points back to the consanguine family and is the first step out of it. The latter case is the more probable. So far as I know, no mention is made of any sexual intercourse between parents and children in Australia. Even the later form of exogamy, the maternal law gens, as a rule silently presupposes that the prohibition of this intercourse was an accomplished fact at the time of its institution.