I will give another example. Primitive tribes are very often divided into a definite number of subdivisions. There is little doubt that this form of social organization has arisen independently over and over again. The conclusion is justified that the psychical conditions of man favor the existence of such an organization of society, but it does not follow that it has developed everywhere in the same manner. Dr. Washington Matthews has shown that the groups of the Navaho have arisen by the association of independent elements. Captain Bourke has pointed out that similar occurrences have given rise to the Apache groups, and Dr. Fewkes has reached the same conclusion in regard to some of the Pueblo tribes. On the other hand, we have proof that such groups may originate by division. Such events have taken place among the Indians of the North Pacific coast (Boas). Other divisions of tribes seem to have had an entirely different origin; as, for instance, the frequent twofold exogamic division of tribes, which may, perhaps, be adequately explained by the application of the laws of exogamy in a small community. Thus it would seem that a variety of causes has led to results which appear identical to all intents and purposes. The principal obstacle in the way of progress on these lines seems to my mind to be founded on the lack of comparability of the data with which we are dealing. Attention has been directed essentially to the similarity of ethnic phenomena, while the individual variations were disregarded. As soon as we turn our attention in this direction, we notice that the sameness of ethnic phenomena is more superficial than complete, more apparent than real. The unexpected similarities have attracted our attention to such an extent that we have disregarded differences; while in the study of the physical traits of distinct social groups, the reverse mental attitude manifests itself. The similarity of the main features of the human form being self-evident, our attention is directed to the minute differences of structure.