Feeblemindedness, insanity, and even some types of criminality have been _said_ to be inherited according to a simple one-factor Mendelian difference. Owing to the difficulty of diagnosis it is obvious that the student of genetics would be expected to approach these problems with the utmost caution. The data, on which some rather sweeping conclusions have been based, sometimes show, on closer scrutiny, obvious contradictions. Take, for example, the case of feeblemindedness which has been represented as though it differed from the normal (whatever that may be) by a single Mendelian factor difference. The evidence for this is far from convincing, and all that can be safely said, I think, is that there are types of imbecility that may possibly be due to multiple factors, but until the relation of imbecility to various disorders of the glands and to syphilis has been thoroughly studied, even my cautious statement may seem to go too far. Curiously enough no one has as yet had the temerity to suggest that some of the high-grade imbecile types—the moron, for example—might represent an ancestral stage of the human race. If this were true, intelligence would then be looked upon as an innovation in the race, that has not yet spread to all of its members. I am aware that a similar suggestion has been made with respect to the criminal. Lombroso’s “criminal type” is notorious. The criminal has been painted as the ancestral brute from which the more docile human animal has arisen through loss of “wild-type” genes. I need not state, perhaps, that no one takes such speculations seriously today from a genetic standpoint. Immunity and resistance to disease are subjects of great interest to geneticists as well as to pathologists. Setting aside, of course, cases where the immunity is due to some _temporary_ physiological state (little understood at present, I believe), and also setting aside immunity acquired by recovery from attack or inoculation, there still remain races that have, as we say, a constitutional resistance. The best ascertained cases in this field are those worked out by Tyzzer and Tyzzer and Little. A carcinoma that originated in Japanese waltzing mice grew in practically every individual of the race when implanted. It failed to grow in “common” mice. The hybrid mice from these two races were also susceptible in nearly every case. When the F₁’s were back-crossed to “common mice” the offspring were not susceptible. When the F₁’s were back-crossed to the Japanese waltzer all were susceptible. When the F₁’s were inbred only about 2.5 per cent. of the offspring were susceptible, Fig. 13. These results show at least that there must be more than one, two or three factor differences between the two races that are concerned with tumor susceptibility.