It has long been recognised that Nature must have been the principal factor in producing, that is to say, in differentiating, the various races of mankind as we find them differentiated when our records begin. How this happened is one of the darkest problems that history presents. By what steps and through what causes did the races of man acquire these diversities of physical and intellectual character which are now so marked and seem so persistent? It has been suggested that some of these diversities may date back to a time when man, as what is called a distinct species, had scarcely begun to exist. Assuming the Darwinian hypothesis of the development of man out of some pithecoid form to be correct--and those who are not themselves scientific naturalists can of course do no more than provisionally accept the conclusions at which the vast majority of scientific naturalists have arrived--it is conceivable that there may have been unconnected developments of creatures from intermediate forms into definitely human forms in different regions, and that some of the most marked types of humanity may therefore have had their first rudimentary and germinal beginning before any specifically human type had made its appearance. This, however, is not the view of the great majority of naturalists. They appear to hold that the passage either from some anthropoid apes, or from some long since extinct common ancestor of man and the existing anthropoid apes--this latter alternative representing what is now the dominant view--did not take place through several channels (so to speak), but through one only, and that there was a single specifically human type which subsequently diverged into the varieties we now see. [Illustration: TREE DWELLERS IN THE TWENTIETH CENTURY We must remember that such terms as “The Stone Age,” “The Bronze Age,” and so forth, are only loosely applied. The ages so called did not close at certain periods. There are races now living in all the conditions of these past ages. This photograph, for example, shows the actual tree dwellings of the Papuans in New Guinea to-day--one of the most primitive forms of human habitation. ]