EXPERIMENTS ON LONGEVITY 



397 



up to say 500 flies, and then determines the 

 time of death of each fly in each bottle, 

 it is found that the average duration of 

 life follows, with increasing density, the 

 course shown in figure 4. 



Between initial densities of z and 15 

 flies per ounce bottle the mean duration 



upon average duration of life. After a 

 density of 55 flies per bottle is passed 

 longevity declines steadily with advancing 

 density. 



The form of the upper limb of the curve 

 of decreasing average longevity with in- 

 creasing density of population suggests 



/ 7 13 19 25 31 37 43 49 SS 61 67 73 79 85 91 



Age in Days 



Fig. 5. Survivorship Distributions for Initial Densities of 35, 65, 95, 115, and 2.00 Wild Type 



Flies per Bottle 



of life increases rapidly with increased 

 density. Between densities of 15 and 55 

 flies per bottle there is a slow and gradual 

 increase in average longevity. In fact it 

 is not certain that this region of the curve 

 does not really represent a plateau of 

 optimal density, in which region small 

 differences in density have no great effect 



that there is a tendency to approach a 

 constant level at extremely high densities. 

 There is only a very gradual decline of 

 mean duration of life with increasing 

 densities of population after a density of 

 zoo flies per bottle has been passed. 

 Beyond this density little further effect on 

 longevity is produced by greater crowding. 



