the closeness of the family circle, where practical as well as religious considerations cause parents to prevent any dangerous connections." Westermarck's theory, he holds, does not account for all the facts; for example, "that to no little extent brothers and sisters, mothers and sons, fathers and daughters, do _not_ live together. This is a result of sexual taboo, and is originally a part of the cause why such marriage is avoided, and not a result of avoidance of incest." In short, it "is the application of sexual taboo to brothers and sisters, who, because they are of opposite sexes, of the same generation, and are in close contact, and for no other reasons, are regarded as potentially marriageable, that is the foundation of exogamy and the marriage system." _Cf._ LANG, _Social Origins_, 10-34, 238-40 note, whose criticism of Westermarck and McLennan follows similar lines; and ATKINSON, _Primal Law_, 209-40, who believes that jealousy may have set up a bar to sexual unions within the "fire-circle" before totems or the idea of incest arose. Exogamy appears, then, to be the result of natural selection, arising "when single families united in small hordes. It could not but grow up if the idea of union between persons intimately associated with one another was an object of innate repugnance." Conversely, the law of similarity enables us to understand the coexistence of clan-exogamy and tribal endogamy. The one springs from a horror of sexual union between persons who are too near; the other arises in a dislike of connection between those who are too remote. Among primitive men, and sometimes even among those well advanced in civilization, there exists a shrinking from physical contact with strange races only less violent than the aversion which the dread of incest excites. But this prejudice yields to the sympathy produced by the growing similarity of interests, ideas, sentiments, and general culture among men. Sympathy, upon which affection mainly depends, has widened the sphere of sexual selection.[398] [398] Consult the very interesting chapter of WESTERMARCK on "Selection as Influenced by Affection and Sympathy, and by Calculation," _op. cit._, 356 ff. "Affection depends in a very high degree upon sympathy. Though distinct aptitudes, these two classes of emotions are most intimately connected: affection is strengthened by sympathy, and sympathy is strengthened by affection.... If love is excited by contrast, it is so only within certain limits. The contrast must not be so great as to exclude sympathy."--_Ibid._, 362. "Civilization," he adds, "has narrowed the inner limit, within which a man or woman must not marry;" while "it has widened the outer limit within which a man or woman _may_ marry and generally marries. The latter of these processes has been one of vast importance in man's history."--_Ibid._, 376. IV. THE PROBLEM OF THE SUCCESSIVE FORMS OF THE FAMILY