Suppose, as a theoretical possibility, that spontaneous cancer is due to a recurrent somatic mutation of a specific gene to a dominant one that leads to cancer. Then the proportion of individuals that develop spontaneous cancer in such a strain will depend on the frequency of mutation of this specific gene. Consequently, if such a strain is out-crossed to another race (that introduces the allelomorph of the postulated gene), the number of F₁ offspring that develop the specific cancer would be half as numerous as in the original cancer strain (since the gene in question occurs only half as many times as in the original complex). In the F₂ generation the frequency for the extracted double dominant will be that of the original strain, that of the F₂ heterozygotes will be the same as that of the F₁, and the extracted double recessive class will not develop cancer at all. Now, if it is not possible to distinguish between these different F₂ classes by inspection, the difficulty of finding out how cancer is “inherited” would be very great. In such an imaginary situation, the ratio of cancer-developing mice may not appear to correspond to any of the known Mendelian ratios, because superimposed on the genetic situation there would be added results depending on the frequency of mutation when a specific gene is present. Other complicating conditions will also suggest themselves to any one familiar with genetic and mutation processes; for, the possibility that the mutation itself is more or less likely to occur in one or another genetic complex must be reckoned with, as well as the likelihood of the mutation showing itself or developing in any tissue or only in cells of specific tissues, etc. I am far from wishing to suggest that spontaneous cancer is a mutational process, despite certain rather obvious resemblances to mutational effects in plants and animals, but I should like to insist that the appearance of spontaneous cancer is in its nature so peculiar that one can not afford to ignore such a possibility in any discussion as to whether spontaneous cancer is or is not “inherited.” There are several cases of inheritance of tumors in our Drosophila material. Here I am on safer ground. One of them, discovered by Dr. Bridges, worked out by Dr. Stark, I should like to speak about, because it shows how linkage of characters can be used in the study of heredity of a character and conversely in its elimination. In a certain culture one fourth of the maggots develop one or more black masses of pigment in the body; such maggots always die. They are always males. Consequently there are twice as many daughters as sons in such a strain. The gene is carried by the X-chromosome and its inheritance is like that of all sex-linked characters as shown in Fig. 14.