Each member of a certain male marriage class may only marry a member of a marriage class of the other phratry, placed opposite in the table. Thus, for instance, an Adshumbitch man marries an Alpungerti woman, an Apungerti man an Aldshambitch woman, etc. The children always belong to the phratry of the men, but to another marriage group of theirs. Thus, for instance, the boys born from the union of an Adshumbitch man with an Apungerti woman belong to the Apularan class, and the girls born of this marriage belong to the Alpularan class. Further complications arise in consequence of the totem system, which exists among most of the Australian tribes. As the local groups of a tribe are numerically weak and consist of members of all marriage classes, the choice of mates is restricted to quite a small number of persons, being further limited to a great extent by the marriage of girls in childhood. But even when adults marry, they can rarely decide according to their own will, but are dependent on the circumstances of relationship. On the northern coast of Australia the marriage class system does not exist, but exogamy exists there, the members of certain local groups not being allowed to marry each other. The now extinct tribes in the south-east of the continent also had no marriage class system.
But it still remains a mystery how it was found out that marriages of blood relations were harmful. One objection is, that some of the Australians are ignorant of the process of generation; they do not even know that pregnancy is the result of cohabitation. It is also doubtful whether the Australian natives can in any case be considered as typical representatives of primitive man. If this were so, all mankind would still be in a very low state of civilisation, for the Australians appear incapable of progressive development. And further, if exogamous classes were purposely instituted in order to prevent cohabitation between blood relations, how is it that other people also are excluded from sexual intercourse who are not blood relations? Frazer's comparison with a watch is also badly chosen. We must take into consideration the intellectual stage of development of mankind at the time when exogamy arose, and when the watch was invented. Even if we do not admit that exogamy was instituted with a conscious purpose, this does not by any means, as Frazer says, do away altogether with will and purpose from the history of human institutions. There is no need to doubt that the Australian system of exogamy became more and more complicated through the deliberate action of man.
Frazer himself assumes that the Australians had an aversion to cohabitation between brothers and sisters even before it was definitely fixed by binding rules. Sexual aversion between parents and children, according to him, is universal among them, whether there be in vogue the two-, four- or eight-classes system, i.e., whether incest between parents and children is expressly forbidden or not. "In democratic societies like those of the Australian natives, the law sanctions only thoughts that have already been long the mental possession of the majority of people." Hence the agreement of the marriage class system with the feelings of the people becomes explainable.
Since the aversion to sexual intercourse within certain classes was already in existence before the formation of marriage classes, the classificatory system being merely the formal expression of it, we have to find some explanation for it. For the appearance of this aversion marks the real beginning of exogamy, which cannot be explained by the complicated system of the Australians. It is possible that the sexual aversion towards blood relations is already a characteristic trait of the human race before its truly human development, and that it may have to be looked upon as an instinct. This is the opinion of F. Hellwald, which has also been upheld of late by A. E. Crawley. It is assumed that among brothers and sisters, as among boys and girls who have lived together from childhood, the pairing instinct generally remains in abeyance, because the conditions are wanting that are likely to awaken this instinct. Courting the favour of a person of the other sex is the process that gradually brings about the sexual excitement necessary for union. The possibility of sexual excitation between people who have lived together from childhood is decidedly lessened through habituation, if not completely inhibited. In this respect brothers and sisters reach already at puberty that state towards each other to which people married for a long time approach gradually, through the constant living together and the exhaustion of youthful passion. If brother and sister sometimes show passion for each other, it is generally the result of the same circumstances that are necessary to arouse it under normal conditions, e.g., a long separation. As the absence of sexual attraction between brother and sister who have grown up together is a natural thing, it is strange that cohabitation between them should have to be specially prohibited and enforced by strict measures among primitive peoples. The explanation, according to Crawley, is simple. "In many departments of primitive life we find a naïve desire to, as it were, assist Nature, to affirm what is normal and later to confirm it by the categorical imperative of custom and law. This tendency still flourishes in our civilised communities, and, as the worship of the normal, is often a deadly foe to the abnormal and eccentric, and too often paralyses originality. Laws thus made, and with this object, have some justification, and their existence may be due, in some small measure, to the fact that abnormality increases pari passu with culture. But it is a grave error to ascribe a prevalence of incest to the period preceding the law against it." All the facts tend to show that the most primitive people procured their wives by friendly arrangements. From this standpoint it would be most practical if each tribe were divided into two groups, the men of each group marrying wives from the other group. This state of affairs is actually to be found among many uncivilised peoples that are divided into two exogamous groups or phratries. It has still to be discovered how this bipartition arose. It is unthinkable that a division into two groups was intentionally brought about by the members of the groups for the purpose of preventing marriages between blood relations of a certain grade. No tribe has ever been divided in such a manner; the division must therefore be explainable in another way. The phratries are large families (in the broad sense of the word); they descend from families (in the narrower sense of the word), reciprocally supplying each other with wives. The names of the phratries are generally unintelligible, in contradistinction to the names of the totem groups, and therefore most probably older. The totem groups, of which a phratry consists, are to be considered as younger branches of the original double family, which have arisen through wives being taken from other groups whose children again received the name of their mothers. If it should be asked why the members of two phratries should constantly intermarry, it should be pointed out that among communities in the lowest stage of civilisation women are not easily procurable, and the force of external circumstances would favour the unions just mentioned (Crawley, pp. 54 et seq.).
A biological explanation of the origin of exogamy is given by Herbert Risley. Without basing it on the assumption that primitive people have a knowledge of the harmfulness of incest, he gives the following exposition: "Exogamy can be brought under the law of natural selection without extending it too far. We know that among individuals or groups of individuals there exists a tendency to vary in their instincts, and that useful variations (such as are suitable to the conditions of life) tend to be preserved and transmitted by inheritance. Let us assume now that in a primitive community the men varied in the direction towards choosing wives from another community, and that this infusion of fresh blood was advantageous. The original instinct would then be strengthened by inheritance, and sexual selection would be added in the course of time. For an exogamous group would have a greater choice of women than an endogamous one, ... and in the competition for women the best would fall to the strongest and most warlike men. In this way the strengthened exogamous groups would in time exterminate the endogamous neighbours, or at least take away their best marriageable maidens. Exogamy would spread partly through imitation, partly through the extermination of endogamous groups. The fact that we cannot explain how it came about that the people varied in the aforesaid direction is not fatal to this hypothesis. We do not doubt natural selection in the case of animals because we cannot give the exact cause of a favourable variation."
E. Westermarck holds a similar theory about the cessation of incest. He thinks that "among the ancestors of man, as among other animals, there was, no doubt, a time when blood relationship was no bar to sexual intercourse. But variations here, as elsewhere, would naturally present themselves; and those of our ancestors who avoided in-and-in breeding would survive, while the others would gradually decay and ultimately perish. Thus an instinct would be developed which would be powerful enough, as a rule, to prevent injurious unions. Of course it would display itself simply as an aversion on the part of individuals to union with others with whom they lived; but these, as a matter of fact, would be blood relations, so that the result would be the survival of the fittest. Whether man inherited the feeling from the predecessors from whom he sprang, or whether it was developed after the evolution of distinctly human qualities, we do not know. It must necessarily have arisen at a stage when family ties became comparatively strong, and children remained with their parents until the age of puberty or even longer."
It may be surmised that the impulse towards the appearance of the exogamous tendency arose through economic progress, which led to an increase of the means of existence, and this in its turn produced a more friendly relationship between neighbouring groups that previously had quarrelled about food. The men thus came into contact with strange women, and this awakened a heightened sexual feeling, in other words the instinct which is said to have led to the avoidance of incest. Thus among the peoples on a very low economic level (e.g., the Pigmies) no laws for the prevention of incest are to be found, a fact that may be held to confirm this idea. Primitive people could in any case not understand the harmfulness of incest, while it is certain that strange members of the opposite sex could exert a stronger attraction, and thus render the sexual impulse permanent, which previously was periodical, as among the animals.