478 



THE AMERICAN NATURALIST [Vol. LI 



intermediate, tliough on the basis of blending inheritance 

 ive should expect the percentage of positive animals to be 

 much closer to that observed in the generation than it 

 actuallif is. 



On an hypothesis of multiple factors underlying sus- 

 ceptibility or immunity to the inoculated tumor the ex- 

 perimental results may well be explained. The F3 gen- 

 erations of Loeb and Fleisher's work give a result further 

 indicating the possible presence of multiple factors. If 

 a large number of F2 animals mated inter se at random 

 are used to produce the F;j generation the percentage of 

 susceptible animals in F2 should be roughly approached 

 in the F3 generation. 122 animals comprising the F3 

 generation show twenty-four per cent, susceptibility as 

 compared with thirty per cent, in the Fg generation of the 

 same cross. In the F3 generation of the American times 

 European II cross, sixty-six animals have been inoculated 

 and have given only two per cent, of susceptibility. This 

 difference is possibly due to the fact that Fo animals 

 forming gametes each closely resembling those of the 

 grandparent European race were unconsciously chosen 

 as parents for this generation. 



The interesting part of Loeb's work is the fact that the 

 relative homogeneity of the races of mice which he used 

 approximate closely the conditions in respect to sucepti- 

 bility and non-susceptibility which one ordinarily is deal- 

 ing with in size crosses as, for example, Castle's work 

 and also MacDowell's work with rabbits. In none of 

 these cases has there been excessively close inbreeding in 

 either parent race before crosses were made. There is, 

 therefore, no definite complex of factors characterizing 

 the race. As a result xtie percentage of susceptible 

 animals varies and depends on the character of the par- 

 ticular animals used for breeding. The whole effect 

 produced is to obscure the true nature of the processes 

 involved. In res])e('t to liomogeneity the material at the 

 disposal of Txzz.t and tlir writer possessed a great ad- 

 vantage which bccaiiH" apparent in the definite results 

 produced. 



