There remains, however, one point of view to be considered, which might furnish a basis for investigation. The variety of forms in which the fundamental ideas occur were early correlated with general impressions regarding the degrees of civilization, and attention was directed to the recurrence of similar forms the world over, which appear to present an increasing degree of complexity of culture. This led anthropologists to the conclusion that the types of human culture represent an evolutionary series; that the primitive tribes of our times represent an older stage of cultural development, through which the more advanced types passed in earlier periods. If this is true, and if, furthermore, it could be shown that the single tribes develop independently, we might well say that those races must be less favorably developed in which earlier types of culture are found with great frequency, later developments rarely. I have referred to this possibility at another place (p. 125). For this reason the theory of a uniform development of human civilization must be considered in our investigation of the relation between racial types and cultural progress. The investigations of Tylor and Bachofen, Morgan and Spencer, fixed the attention upon the data of anthropology as illustrating the gradual development and rise of civilization. The development of this side of anthropology was stimulated by the work of Darwin and his successors, and its fundamental ideas can be understood only as an application of the theory of biological evolution to mental phenomena. The conception that the manifestations of ethnic life represent a series, which from simple beginnings has progressed to the complex type of modern civilization, has been the underlying thought of this aspect of anthropological science. The arguments in support of the theory that the development of civilization has followed a similar course everywhere, and that among primitive tribes we may still recognize the stages through which our own civilization has passed, are largely based on the similarities of types of culture found in distinct races the world over, but also on the occurrence of peculiar customs in our own civilization, which can be understood only as survivals (Tylor) of older customs, that had a deeper significance at an earlier time, and which are still found in full vigor among primitive people. It is necessary to point out at least a few of the aspects of this general problem, in order to make clear the significance of the evolutionary theory of human civilization.