" A further step for preventing inbreeding is found among the Kamilaroi on the Darling River in New South Wales, where the two original classes are split into four, and every one of these is married as a whole to a certain other class. The first two classes are husbands and wives by birth. According to the place of the mother in the first or second class, the children belong to the third and fourth. The children of these two classes, who are also married to one another, again belong to the first and second class. So that a certain generation belongs to the first and second class, the next to the third and fourth and the following again to the first and second. Hence the children of natural brothers and sisters (on the mother's side) cannot marry one another, but their grandchildren can do so. "

― Friedrich Engels , The Origin of the Family, Private Property and the State