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THE AMERICAN NATURALIST [Vol. LVI 



The substantial constancy of line 101, in both mass and 

 inbred tests, is evident. In respect of variability the line 

 behaved somewhat like 303 discussed below. 



In line 303 again the constancy of the line in respect of 

 mean duration of life is as definite as could be expected. 

 Over periods of approximately 7 and 13 months, the mean 

 duration of life has not sensibly changed, having regard 

 to the probable error involved. The results respecting 

 variability are somewhat anomalous. Both the second 

 inbred and the mass re-test show variability of a dis- 

 tinctly lower order than was exhibited by the progeny of 

 the original brother X sister mating. It seems probable 

 that the original test by accident gave a variability re- 

 sult higher than was really characteristic of the line. But 

 the mass culture re-test exhibits a lower variability, not 

 certainly significant, to be sure, than the first test on line 

 309. Of course it is to be expected that with continued 

 brother X sister mating the variability of mass cultures 

 from the line would come nearer and nearer to that of a 

 further inbred lot of progeny from the same line. Prob- 

 ably the results of Table IX are an expression of the 

 realization of such expectation, obscured by the fact that 

 the numbers are small and the errors of sampling conse- 

 quently relatively large. 



Discussion and Summary 

 The data presented in this paper appear to demon- 

 strate, with comprehensiveness and accuracy, three broad 

 facts. 



A. That there exist in a general population of Droso- 

 phila melmiogaster (or its mutants) genetic differences 

 in respect of duration of life. 



B. That these genetic differences are capable of isola- 

 tion, by appropriate selection and inbreeding. 



C. That within an even moderately inbred line, the gen- 

 etic differences in duration of life remain constant over 

 periods of at least 10 to 25 or more generations. 



