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