STUDIES ON THE DURATION OF LIFE 201 



proved by the most careful and critical of tests to have 

 remained entirely free of bacterial contamination through- 

 out their lives, exhibited, when grown at a constant tem- 

 perature of 25° C. an average duration of life of 28.5 

 days. In our experiments 2,620 male flies, of all strains 

 of Drosophila in our cultures taken together, thus giv- 

 ing a fair random sample of genetically the whole Droso- 

 phila population, gave an average duration of life at the 

 same constant temperature of 25° C. of 31.3 ±.3 days, 

 and 3,216 females under the same temperature lived an 

 average of 33.0 ±.2 days These were all non-sterile 

 flies, subject to all the bacterial contamination incident 

 to their normal laboratory environment, which we have 

 seen to be a decaying germ-laden mass of banana pulp and 

 agar. It is thought to be fairer to compare a sample of 

 a general population with the Loeb and Northrop figures 

 rather than a pure strain because probably their Droso- 

 phila material was far from homozygous in respect of 

 the genes for duration of life. 



The detailed comparisons are shown, in Table 23. 



TABLE 23 

 Average duration oj life of Drosophila in the imago stage at S5° C. 



We reach the conclusion that bacteria-free Drosophila 

 live no longer on the average, and indeed perhaps even 

 a little less long, under otherwise the same constant 



