Sometimes we find the statement made that cancer is more frequent among the well to do than among the poor. It is doubtful whether this statement is correct. Some recent statistics point rather to the reverse. Again there seems to be in this respect some difference in the various kinds of cancer. Cancer of the cervix of the uterus appears to be more frequent among the poor, while mammary cancer is perhaps more frequent among the well to do.
There exists no marked difference in the incidence of cancer in country and city. On the whole, we are apt to find a somewhat greater incidence of cancer in the cities, especially in parts of the world where the number of physicians per unit of population is considerably smaller in the country and where therefore in all probability the number of unrecognized cases of cancer is greater in the country. The difference is therefore probably rather apparent than real.
There have been noted in various countries certain areas where cancer appeared to be more frequent than in others. Some observers believed especially damp-wooded country to be favorable to the development of cancer. Some physicians recognized certain houses or streets in villages or small towns, where the incidence of cancer was especially great. Behla for instance reported such an occurrence in the small Pommeranian town Luckau. These observers declared cancer to be "endemic" in certain localities and were often inclined to attribute this localized increased frequency to unknown infectious agencies. Although it is very difficult to interpret these observations, it does at present not appear probable that they ought to be referred to infection. Such cancer houses have been found only rarely: the number of cancers that occurred in these houses were relatively few, and furthermore cancer occurred also in the neighboring districts. Neither can we exclude the possibility that hereditary factors played a part in some of these cases. There is, however, no doubt that in certain parts of various countries cancer is more frequent than in others.
So far we have considered the incidence of cancer mainly among the white population of Europe and America. The incidence of cancer among certain races living under more primitive conditions is quite different. Among the negroes of Africa cancer is much rarer than among the whites of Europe and America. Although according to von Hansemann the same varieties of cancer occur there as in civilized countries, the proportion of the various kinds of cancers differs markedly. We find among the African negroes relatively frequently benign tumors of the connective tissue group as for instance lipomata (fat tumors). Sarcomata are correspondingly much more frequent than among the Europeans and it is furthermore very probable that the relative frequency of the various kinds of carcinomata is different. Also among the Arabs of Algiers sarcoma is more common than car-
