It seems desirable to understand more clearly what this theory of parallelism of cultural development implies. It seems to mean that different groups of mankind started at a very early time from a general condition of lack of culture; and, owing to the unity of the human mind and the consequent similar response to outer and inner stimuli, they have developed everywhere approximately along the same lines, making similar inventions and developing similar customs and beliefs. It also seems to involve a certain correlation between industrial development and social development, and therefore a definite sequence of inventions as well as of forms of organization and of belief. In the absence of historical data in regard to the earliest history of primitive man the world over, we have only three sources of historical proof of this assumption,—the evidence contained in the earliest history of the civilized people of the Old World, survivals in modern civilization, and archæology. The last-named is the only method by means of which we can approach the problem in regard to people that have no history. While it is certainly true that analogues can be found between the types of culture represented by primitive people and those conditions which prevailed among the ancestors of the present civilized peoples at the dawn of history, and that these analogues are supported by the evidence furnished by survivals, the evidence of archæology does not support the complete generalization. The theory of parallel development, if it is to have any significance, would require that among all branches of mankind the steps of invention should have followed, at least approximately, in the same order, and that no important gaps should be found. The facts, so far as known at the present time, are entirely contrary to this view. We find, for instance, large areas of the world inhabited by people well advanced in the arts of life, but who have never made the discovery of pottery, one of the essential steps in the advance of civilization. Pottery is not found in the extreme southern parts of Africa, in Australia, in northeastern Siberia,[6] in the whole northwestern part of North America, and in the extreme south of South America. According to what has been said before (p. 169), it would seem as if Old-World pottery covers about the same territory as the other characteristic traits referred to before, while in America its centre lies in the area of more advanced culture in the middle part of the continent. Thus it happens that the well-advanced tribes of Northwest America have no pottery, and its presence or absence seems to be due more to geographical location than to general cultural causes.