214 THE SCIENTIFIC MONTHLY
occurred in about 70 per cent, of all females, in others it occurred in only 2 to 3 per cent. If we crossed two mice belonging to strains with different cancer rates, we found that in a considerable number of cases the offspring followed approximately the cancer rate of the parent with the higher cancer percentage; in other cases, however, the ofbpring followed the parent with the lower cancer percentage. There is there- fore not the least dovbt thai ca/ncer in animais is hereditary. And not only is the cancer frequency hereditary, but also the age at which can- cer most frequently appears in each family. The hereditary transmis- sion of the cancer age is independent of the frequency. Thus it may occur that if we compare two families we may find that cancer is about equally frequent in both, but that it appears in one most frequently in middle life, while in the other it usually develops in later life. And these conditions are constant in succeeding generations.
Does this conclusion apply equally to cancer in man? The com* plicated conditions in human sociely make it very difficult to interpret statistical data with the same definiteness as in the case of animals. There are, however, indications which make it very probable that in man also heredity is a definite factor. We know that some races are almost immune to cancer, as, for instance, the American Indian, the Negro in Africa, and some aborigines of Australia and the South Sea Islands. It is very probable that this immunity to cancer is not due to the differences in food or to climatic conditions. Europeans remov- ing to these regions are not immune. Furthermore, we note that the negro in America has a much higher cancer rate than the negro in Africa. This may possibly be referable to the interbreeding which has taken place among the negroes and whites in this country. Definite data for man which could answer this question are, however, lacking, and until more exact investigations in this direction can be undertaken we must abstain from making final decisions as to this point. The ob- servation that among white people the cancer rate differs relatively little in various civilized countries does not speak against the existence of a heredity factor. It might merely indicate that the hereditary disposi- tion to cancer is about equal in different families or strains. Further- more, in human society intermarriages have been so constant and so frequent that differences between various families, which might have existed to a greater degree at earlier stages of development may gradu- ally have been eliminated or obscured. It is, however, well known that in certain human families cancer has been very frequent. Only re- cently a very interesting observation was published concerning two sisters, who died within a very short time of one another, and were boHi found to have been affected by the same very rare kind of cancer. While it is perhaps possible to explain all such occurrences among htmian beings as referable to the laws of chance, the facts established
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