Tyzzer and Little suggest in fact that 12 to 14 independently inherited factors are involved. Larger numbers of tests will be necessary before it is possible to state how many factors are needed. A curious feature of the case should not pass unnoticed. Many or all of the factors for _susceptibility_ must be assumed to be dominant. It is not generally known, but there is some evidence that the so-called Japanese waltzer originated from Asiatic house mice, which according to some writers belong to a distinct species or at least a distinct variety. The results suggest that we may be dealing here with species or varietal differences, hence the large number of factor differences involved. It may be necessary to work with a simpler situation where fewer factors are involved; possibly such a case as that of the Jensen tumor will furnish proper material, but it will be necessary to work with pedigreed material rather than with “Danish,” “French,” “German,” or even English breeds of mice. [Illustration: FIG. 13. Diagram showing inheritance of immunity to cancer. (From Tyzzer and Little.)] In plants also the inheritance of immunity of wheat to rust has been studied. Biffen’s results with wheat are those best known. An immune race crossed to a susceptible race gave first generation plants that were attacked. This means that immunity is a recessive character. In the next generation there were 64 immune and 194 affected plants (a 1:3 ratio). If the immune plants are self-fertilized, they yield only immune plants in later generations. Nilsson-Ehle and Vavilov think that such simple relations are rather the exception than the rule. Vavilov found that Persian wheat, immune to mildew, crossed to different susceptible species produced offspring that were immune in 13 combinations. In these cases immunity is dominant. In the next generation several degrees of resistance were noted—and a few plants were even more susceptible than their grandparents. It is interesting again to note that susceptibility and immunity are species and variety characters in these cases, but this does not mean that the differences are not Mendelian. It suggests however the possibility that several or many factor differences are often involved. There is no more interesting field in which genetics and pathology meet than that of cancer. I realize how careful we on our side must be in discussing this question with you who are experts, nevertheless there are certain aspects of the problems of cancer from the genetic side that I may be allowed briefly to mention—not, however, without some misgivings. Suppose all men over seventy-five died of arteriosclerosis. Could one say that hardening of the arteries is inherited? I think that it would be proper to use the word heredity to include such a case, but we would not know how it was inherited unless there existed another race of men who never died of the malady, and suitable matings were made between the two races.