645] THE DURATION OF LIFE 



investigation than it has had. The indications all are 

 that it is the most important and significant element in 

 the biological, as distinguished from the physical, en- 

 vironment of organisms. ' ' In pursuance of this idea we 

 desire to present in this paper our accumulated statisti- 

 cal data on the influence of density of population upon 

 duration of life in Drosophila. This material is to he 

 regarded as preliminary rather than final. For reasons 

 which will appear as we proceed, we are inclined to with- 

 hold final conclusions as to the exact form of the regres- 

 sion of duration of life upon density until we have com- 

 pleted an extensive ad hoc experimental investigation of 

 the problem. This experimental work is now in progress 

 and we hope to be able to report upon it in full in the 

 course of the next year. In the meantime we have an 

 impressive body of statistical data gathered from the 

 control groups of other experiments which it seems de- 

 sirable to discuss now in a preliminary way. 



II 



The data of this study are derived from the normal 

 control groups of various experiments on duration of 

 life which we have carried out with Drosophila, accord- 

 ing to the technique described by Pearl and Parker (27). 

 All of the determinations of duration of life recorded in 

 the tables of this paper were made under constant condi- 

 tions of temperature (25° C), food, etc., as described in 

 the paper referred to. We have divided the material 

 for the purposes of the present study into three groups 

 by stocks (cf. Pearl and Parker . (27) ), viz.: (a) wild 

 type flies, including our Old Falmouth, New Falmouth, 

 and Eagle Point stocks, (b) Sepia, and (c) Quintuple. 



Throughout this paper density of population is taken 

 as the initial density/ (number of flies per bottle) in the 

 small bottles used in testing duration of life. Thus a 

 density of 22 means that 22 flies started in this particular 

 bottle. As time went on the number was diminished by 

 deaths until finally none was left. One of course might 

 use as the variable mean density over the whole life of a 



