While the first step of organization consisted in excluding parents and children from mutual sexual intercourse, the second was the erection of a barrier between brother and sister. This progress was much more important on account of the greater equality in the ages of the parties concerned, but also far more difficult. It was accomplished gradually, probably beginning with the exclusion of the natural sister (i. e., on the mother's side) from sexual intercourse, first in single cases, then becoming more and more the rule (in Hawaii exceptions were still noted during the nineteenth century), and finally ending with the prohibition of marriage even among collateral brothers and sisters, i. e., what we now term brother's and sister's children, grandchildren, and great-grandchildren. This progress offers, according to Morgan, an excellent illustration how the principle of natural selection works. Without question, the tribes limiting inbreeding by this progress developed faster and more completely than those retaining the marriage between brothers and sisters as a rule and law. And how powerfully the influence of this progress was felt, is shown by the institution of the gens, directly attributable to it and passing far beyond the goal. The gens is the foundation of the social order of most, if not all, barbarian nations, and in Greece and Rome we step immediately from it to civilization. Every primeval family necessarily had to divide after a few generations. The originally communistic and collective household existing far into the middle stage of barbarism, involved a certain maximum size of the family, variable according to conditions, but still limited in a degree. As soon as the conception of the impropriety of sexual intercourse between children of the same mother arose, it naturally became effective on such occasions as the division of old and the foundation of new household communities (which, however, did not necessarily coincide with the family group). One or more series of sisters became the center of one group, their natural brothers that of another. In this or a similar manner that form which Morgan styles the Punaluan family developed from the consanguine family. According to Hawaiian custom, a number of sisters, natural or more remote (i. e., cousins of the first, second and more remote degrees) were the mutual wives of their mutual husbands, their natural brothers excepted. These men now no longer addressed one another as "brother"--which they no longer had to be--but as "Punalua," i. e., intimate companion, associate as it were. Likewise a series of natural or more remote brothers lived in mutual marriage with a number of women, not their natural sisters, and these women referred to each other as "Punalua." This is the classical form of a family, which later admitted of certain variations. Its fundamental characteristic was mutual community of husbands and wives within a given family with the