184 THE AMERICAN NATURALIST [Vol. LYl 



It is at once apparent that the mass re-tests on line 101 

 gave extremely satisfactory results as to constancy of 

 duration of life in the line, after intervals of approxi- 

 mately 5 and 11 months. The mean value for either the 

 A or the B test does not significantly differ, having regard 

 to its probable error, from the mean shown on the original 

 test at the start of the line. The mean of the A mass re- 

 test almost exactly agrees with that of the second inbred 

 test of the same line, as given in Table VI. 



In the case of line 100, the mass re-test after 5| months 

 approximately does not give such close agreement. The 

 mean is significantly low^er, the difference being 6.6 times 

 its probable error. No explanation of this result is, as yet, 

 forthcoming, but it probably means no more than lack of 

 genetic purity in the line. It is, however, interesting to 

 note that the sense of the change is in the same direction 

 as that in which line 100 in general differs from line 101, 

 which we regard as our most typical wild-type line in re- 

 spect of duration of life. That is, line 100 is, as com- 

 pared with 101, a shorter-lived line. Its mass culture 

 re-test is still shorter lived. 



The variability in respect of duration of \\\\\ whrtlici- 

 measured in absolute or relative terms, is miironiiK- 

 higher and in two cases out of the three hy ;i >iu]ii llcnir 

 amount in the mass culture tlinii in the oi-iuiiiMl iiihrcd 

 tests. This is, of courv. . cxaetlv ^^\r,\\ would h,. ..\]),'ct d 



as has been shown by Pearl {'M)), Jennings (.'Jl) and 

 others, reduces the heterozygosis in the strain by only 7yi) 

 per cent. It is interesting to note, in connection with t!ie 

 explanation suggested above for the difference in tlio 

 means in the case of line 100, that the variability in t'l > 

 mass r(-test on that line is very nuich hi-her than in th > 

 original inbred test. 



A mass re-test was carried out on two of the lines from 

 the second brother X sister matings. The results from 

 these experiments are presented in Table IX. 



