3. EXOGAMY is, or has been, as widely spread as the opposed rule of endogamy just described. It is the duty enforced by custom, religion, and law, of marrying _outside_ one’s own clan, and is usually in force amongst small and barbarous communities. Its former distribution is attested by the survival in nearly all countries, of ceremonies based on “marriage by capture.” The remarkable monograph on this subject by the late Mr. McLennan is of peculiar interest. It was one of the earliest, and perhaps the most successful, of all attempts to decipher prehistoric customs by means of those now existing among barbarians, and by the marks they have left on the traditional practices of civilised nations, including ourselves. Before his time those customs were regarded as foolish, and fitted only for antiquarian trifling. In small fighting communities of barbarians, daughters are a burden; they are usually killed while infants, so few women are found in a tribe who were born in it. It may sometimes happen that the community has been recently formed by warriors who brought no women, and who, like the Romans in the old story, could only supply themselves by capturing those of neighbouring tribes. The custom of capture grows; it becomes glorified because each wife is a living trophy of the captor’s heroism, and marriage within the tribe soon comes to be considered an unmanly, and at last a shameful act. The modern instances of this among barbarians are very numerous. 4. AUSTRALIAN MARRIAGES. The following is a brief clue, and apparently a true one, to the complicated marriage restrictions among Australian bushmen, which are enforced by the penalty of death, and which seem to be partly endogamous in origin and partly otherwise. The example is typical of those of many other tribes that differ in detail. A and B are two tribal classes; 1 and 2 are two other and _independent_ divisions of the tribe (probably by totems). Any person, taken at random, is equally likely to have either letter or either numeral by birthright, and his or her numeral and letter are well known to all the community. Hence the members of the tribe are sub-classed into four sub-divisions, A1, A2, B1, B2. The rule is that a man may marry those women only, whose letter and numeral are both different to his own. Thus A1 can marry only B2, the other three sub-divisions A1, A2, and B1 being absolutely barred to him. As to the children, there is a difference of practice in different parts: in the cases most often described, the child takes its father’s letter and its mother’s numeral, which determines class by paternal descent. In other cases the arrangement runs in the contrary way, that is by maternal descent. The cogency of this rule is due to custom, religion and law, and is so strong that nearly all Australians would be horrified at the idea of breaking it. If anyone dared to do so, he would probably be clubbed to death.