EXPERIMENTAL EVOLUTION. 



133 



factors whose number was estimated at from 12 to 14. Simultaneous presence 

 of these factors, themselves introduced by the Japanese waltzing race, was 

 considered necessary for progressive growth of the tumor. 



"Later, while working with a transplantable sarcoma (J. w. B.) of the 

 Japanese waltzing mouse, results were obtained which showed what seemed 

 to be a somewhat simpler quantitative condition of the same process. In 

 this case the parent races and Fj hybrids behaved as before, but the F2 hybrids 

 gave a total of 23 susceptible to 66 non-susceptible animals. It was previously 

 estimated that from to 7 factors were involved. In order to determine more 

 closely the number of factors, new experiments were devised, as follows: 

 Fi hybrid mice, themselves susceptible, were crossed back with the non- 

 susceptible parent race. This has during the past year given a back-cross 

 generation whose susceptibihty would depend upon the factors introduced 

 through the gametes received from their Fj parent. If one factor was involved, 

 the ratio of gametes containing it formed by the Fi animal to those lacking it 

 would be 1:1; if two factors, 1:3; if three factors, 1:7; if four factors, 

 1:15; if five factors, 1:31; if six factors, 1:63; and if seven factors, 1:127. 

 Susceptible to non-susceptible individuals would occur in the back-cross 

 generation in similar proportions. 



"The actual numbers obtained were 21 susceptible to 208 non-susceptible. 

 This result may be compared vdih expectations on three, four, five, and 

 seven factor hypotheses, as shown in table 9. 



Table 9. — Comparison of observed and varioiis theoretical ratios of susceplibilily and 

 non-stcsceptibility in back-cross. 



Susceptible. 



Non-susceptible. 



Ratio. 



Expected 3-factor 



Observed 



Expected 4-factor 

 Expected 5-factor 

 Expected 7-factor 



28 

 21 

 14 



7 



1.8 



201 

 208 

 215 

 222 

 227.2 



1:7 



1:99 



1:15 



1:31 



1:127 



"The observed figures fall between the three and four factor hypothesis. 

 The numbers are not large enough to give a definite test, but the F2 generation 

 already mentioned is interesting as a supporting line of evidence. If we 

 compare this with the expectation, we find that the observed figures lie 

 between the four and five factor hypotheses (table 10) . 



Table 10. — Comparison of observed and various theoretical ratios in Fi. 



Susceptible. 



Non-susceptible. 



Ratio. 



Expected 3-factor 

 Expected 4-factor 



Observed 



Expected 5-factor 



39 

 29 

 23 

 21 



50 

 60 

 66 



1:1.3 

 1:2.1 

 1:2.8 

 1:3.2 



" In both cases the four-factor hypothesis figures are close and the three 

 and five factor hypotheses are to be still considered as possibilities, though 

 not probabilities. The six and seven factor hypotheses appear to be defi- 

 nitely eliminated. 



