368 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



worked with pure lines. Levin and Sittenfield soon afterwards noted that the 

 offspring of non-susceptible rats were less susceptible to the growth of a 

 sarcoma than the offspring of susceptible rats. 



In experiments with mouse carcinoma IX it was shown by Fleisher and 

 the writer (1912) that this tumor could be successfully transplanted into a 

 strain of American mice in 80 per cent of the cases, into a first strain of Euro- 

 pean mice in 23 per cent, and into a second strain of European mice in 3 per 

 cent of the animals. However, in the early period following transplantation 

 the tumors grew as well in European as in American mice, namely, in 85 to 

 95 per cent in both ; but after twelve days a large proportion of the tumors 

 became stationary and retrogressed in the European mice, while in the Ameri- 

 can mice the proportion of retrogressing tumors was small. In the ¥ 1 hybrids 

 between American and European mice the tumor grew as well or almost as 

 well as in the American mice ; but in the F 2 and F 3 generations there was a 

 sharp fall in takes, which was followed again by a rise in the F 4 and F 5 

 hybrids. Except for the partial recovery in F 4 and F 5 and the results in back- 

 crossing, our results and Tyzzer's were therefore similar. We concluded that 

 these findings were compatible with Mendelian principles if we assumed that 

 the susceptibility for growth in Tyzzer's, as well as in our experiments, de- 

 pended on multiple factors. In this connection we applied the term multiple 

 factors in the usual sense of the Mendelian theory, and in the same way in 

 which we applied this term in our transplantations of normal tissues and in 

 our analysis of the origin of tumors. In this sense we also explained subse- 

 quently the difference in the results obtained between transplantations of nor- 

 mal tissues from children to parents and in reciprocal transplantations from 

 parents to children. In the former case, the number of genes which are present 

 in the transplant but not in the host, should, on the average, be greater, and, on 

 the average, the reaction should accordingly be more severe than in the 

 reciprocal transplantations.* 



In continuation of our experiments (1916), we extended our study to a 

 number of other strains of mice. Again we found that American and various 

 imported strains did not differ in respect to the number of original takes, but 

 that they differed greatly in regard to the number of subsequent retrogres- 

 sions. No marked individual differences in the growth energy could be estab- 

 lished by the standards used at that time, either in that group of mice in which 

 the tumor grew definitely, or in the other group in which it retrogressed. 

 However, the marked differences which we had observed formerly in the 

 number of takes or growth energy between different hybrid generations were 

 no longer found ; the growth throughout was about intermediate between that 

 observed in the American and in the imported strains. Likewise, in hybrids 



* In a paper by the writer on "The individuality differential and its mode of inherit- 

 ance," in the American Naturalist, Vol. IV, Jan.-Feb. 1920, there occurs in the last 

 paragraph of page 58, the sentence : "In the case of transplantation from child to mother, 

 on the other hand, the graft would lack one-half the chromosomes — ." It is quite evident 

 that this is a misprint and that instead of "graft" it should read "host." This correction 

 is made here, because this misprint has led to an erroneous interpretation of the views 

 of the writer. 



