474 THE AMERICAN NATURALIST [Vol. LI 



epithelial tumor which originated in the Japanese waltz- 

 ing race. The parent animals all of them came from 

 races which, because of long continued inbreeding, may 

 be considered to have reached a degree of great genetic 

 homogeneity. It will be seen from the table (loc. cit., p. 

 403) that all the Japanese waltzing mice inoculated, 58 

 in number, grew the implanted tumor, while none of the 

 common mice showed continuous growth of the tumor. 

 This absolute difference between the parent races is 

 extremely interesting and offers ideal material for the 

 formation of intermediate conditions of susceptibility 

 in hybrid generations. They further offer a test of 

 the relative value of the hypotheses of multiple factor 

 and of blending inheritance. Sixty-two hybrids 

 obtained by crossing the Japanese with tame races were 

 inoculated. Of these, sixty-one grew tumors. The tumors 

 grew in most cases fully as rapidly if not more rapidly 

 than in the susceptible Japanese parent races. It is prob- 

 able that the one animal in this generation which failed to 

 grow the tumor is not a true exception, but that its be- 

 havior may be due to poor technique. The second genera- 

 tion hybrids have given an extremely interesting result. 

 Of the 183 inoculated, only three have shown continuous 

 growth of the tumor. This result is surprising in view 

 of the fact that susceptibility appeared to be a dominant 

 character in the Fj generation. Only thirty-eight animals 

 of the Fg generation were inoculated; none of them grew 

 the tumor. The striking difference between the F, and 

 the Fg generations suggests at once, alternative rather 

 than blending inheritance. As we have suggested in our 

 previous paper, the most logical interpretation appears 

 to be that a certain physiological condition on which the 

 growth of the tumor depends, is produced in the animals 

 of the Japanese waltzing race. This condition is not 

 found in the tame mice used. The differences between the 

 races are hereditary, since succeeding generations of the 

 Japanese and tame mice behave like their parents. The 

 fact that susceptibility of the tumor occurs in both Fj and 



