The immense advance made by the science of man in the second half of the nineteenth century cut the ground from under the older anthropology and the dualistic system of Kant. A number of newly founded branches of science co-operated in the work. Comparative anatomy showed that our whole complicated frame resembles that of the other mammals, and in particular differs only by slight stages of growth, and therefore in the details of the organs, from that of the anthropoid apes. The comparative histology of the brain especially showed that this is also true of the brain, the real organ of mind. From comparative embryology we learned that man develops from a simple ovum just like the anthropoid ape; in fact, that it is almost impossible to distinguish between the ape and the human embryo even at a late stage of development. Comparative animal chemistry explained that the chemical compounds which build up our organs, and the conversions of energy which accompany its metabolism, resemble those in the other vertebrates. Comparative physiology taught us that all man's vital functions--nutrition and reproduction, movement and sensation--can be traced to the same physical laws in man as in all the other vertebrates. Above all, the comparative and experimental study of the sense-organs and the various parts of the brain showed that these organs of the mind work in the same way in man as in the other primates. Modern paleontology taught that man is, it is true, more than a hundred thousand years old, but only appeared on earth towards the close of the Tertiary Period. Prehistoric research and comparative ethnology have shown that civilized nations were preceded by older and lower races, and these by savages, which have a close bodily and mental affinity to the apes. Finally, the reformed theory of descent (1859) enabled us to unite the chief results of the various branches of anthropological study, and explain them phylogenetically by the development of man from other primates (anthropoid apes, cynocephali, lemures, etc.). By this means a new and monistic basis was provided for modern anthropology; the position assigned to man in nature by dualistic metaphysics was shown to be utterly untenable. I have attempted in the last edition of my _Anthropogeny_ (of which an English edition is in preparation) to combine all these results of empirical investigation in a sketch of the natural evolution of man, paying special regard to embryology. I pointed out in chapters ii.-vi. of the _Riddle_ how important a part of our monistic philosophy this phylogenetic anthropology is.