It is doubtless thus, after the manner of the great monkeys, that primitive human societies have been formed. With the chimpanzees these hordes can never become very large, for the male progenitor will not endure rivals, and drives away the young males as long as he is the strongest. The first men were surely more sociable, because of their human nature. The young males of the human horde were able to remain, in greater or less number, within the association, but the jealousy of the progenitor-in-chief, the father of the family, must often have obliged them to procure one or several females by capturing them from neighbouring or rival hordes; they thus became more or less exogamous; and, in their embryo societies, marriage, or rather sexual union, ended by being prohibited between brothers and sisters, not because there was the least moral scruple about incest, but because, within the limit of the horde, the young women were claimed by the most robust males, who would not yield them up. We know that this is still the case in the Australian tribes.[874] In this gross social state it is necessarily the mother who is the centre of the family, just as she is in the families of mammifers; it is, therefore, quite natural that the children should bear her name and not that of their father, which, for that matter, is not always easy to designate. When once the custom of exogamy was well established, what was at first a necessity ended by becoming an obligation, and men were forbidden to unite themselves with women of the group to which they belonged, and which bore the same name as their own. Such is still the general rule in Australia.[875] But in Australia this group is often only a sub-tribe, a _gens_ or clan; for the hordes, becoming too numerous, are subdivided into factions or large families, who unite together for common defence or vengeance. The children of each group belong sometimes to the clan of the mother, and there is then no legal parenthood between them and their father;[876] also, in case of war, the son must join the maternal tribe.[877] But this is not a universal rule, and in many tribes the children now belong to the paternal clan.[878] These are general cases, common to the greater part of the Australian tribes, but not to all. There are some who have organised their marriage and their family into classes, thus regulating, in a certain measure, the primitive confusion, and establishing by this very regulation a sort of limited promiscuity. The word “classes,” employed by travellers who have made us acquainted with these curious customs, is improper, for neither social classes nor castes exist in Australia. These so-called classes are simply sub-tribes or clans, analogous to the Roman _gens_.