Exogamy, enabling a growing tribe to keep itself compact by constant unions between its spreading clans, enables it to overmatch any number of small intermarrying groups, isolated and helpless. Again and again in the world’s history, savage tribes must have had plainly before their minds the simple practical alternative between marrying-out and being killed out.2

2Tylor, E.B.n/an/an/an/a, "On a Method of Investigating the Development of Institutions; Applied to Laws of Marriage and Descent," , 18: 267.