It seems probable that the differences between the groups have arisen incidentally in the evolution of mankind, possibly as the result of the parallel descent of two or more original stocks from different sources, which afterwards converged and mingled, with the production of serological hybrids. In view of this it is of interest to find that some investigation of the racial incidence of blood groups has already been carried out. On the Macedonian front during the war a large number of men of many different races were gathered together, and scientific advantage of this opportunity was taken by L. and H. Hirschfeld. The blood groups were determined in approximately 8,000 individuals, including French, English, Italians, Germans, Austrians, Serbs, Greeks, Bulgarians, Arabs, Turks, Russians, Jews, Malagasies, Senegal Negroes, Annamese, and Indians. According to the results obtained by the Hirschfelds, the groups designated II and III show a definite variation in their distribution among different races. As will be seen hereafter, Group I is compounded of the two factors producing Groups II and III, while Group IV results from their absence. It is therefore necessary only to consider the incidence of Groups II and III in calculating the racial differences. For the statistical tables and diagrams the reader must be referred to the original paper published in 1919, but the results may be roughly summarized as follows. It was found that the factor producing Group II is prevalent among European peoples, whereas the factor producing Group III is characteristic of men from Asia and Africa. Thus the Group II factor was found in not less than 45 per cent. among most European peoples. It gradually diminishes in the countries lying between Asia and Central Europe, being present in Arabs 37 per cent., in Russians 37 per cent., in Jews 38 per cent. In Asiatics and Africans it falls considerably, being in Malagasies 30 per cent., in Negroes 27 per cent., in Annamese 29 per cent., in Indians 27 per cent. On the other hand, the factor producing Group III shows exactly the opposite variation. Among the English, the most Western people of Europe, it is rare, being found by these observers to be present in only 10 per cent.; it rises to 14 per cent. in French and Italians, to 18 per cent. in German Austrians, and to 20 per cent. in the Balkan peoples. In Africa and Asia the Group III factor rises considerably, being present in Malagasies 28 per cent., in Negroes 34 per cent., in Annamese 35 per cent., and in Indians 49 per cent. We may still be far from elucidating the anthropological meaning of these facts, for the mingling of the hypothetical stocks of which mankind is made no doubt began in a remote antiquity, and it is possible that a serologically pure race does not exist. The investigation, however, of the more isolated peoples might throw much light on the problems of anthropology.