210 BIOLOGY OF DEATH 



three developmental stages and perhaps live a long time 

 because they live slowly. At high temperatures, on the 

 other hand, the fly is very active and lives its life through 

 quickly at the pace that kiUs. ' ' These results are exactly 

 comparable to the effect of a regular increase of tempera- 

 ture upon a chemical reaction. Indeed, Loeb and North- 

 rop consider that their results prove that 



With a supply of proper and adequate food the duration of the larval 

 stage is an unequivocal function of the temperature at which the larvae are 

 raised, and the temperature coefficient is of the order of magnitude of that 

 of a chemical reaction, i. e., about 2 or more for a difference of 10° C. It 

 increases at the lower and is less at the higher temperatures. The duration 

 of the pupal stage of the fly is also an unequivocal function of the tempera- 

 ture and the temperature coefficient is for each temperature practically 

 identical with that for the larval stage. The duration of life of the imago 

 is, with proper food, also an unequivocal function of the temperature and 

 the temperature coefficient for the duration of life is, within the normal 

 temperature limits, approximately identical with that for the duration of 

 life of the larva and pupa. 



How are these results to be reconciled with the pre- 

 vious finding that heredity is a primary factor in the 

 determination of duration of life of Drosophila? We 

 have here, on first impression at least, an excellent exam- 

 ple of what one always encounters in critical genetic 

 investigations: the complementary relations of heredity 

 and environment. In our experiments a general mixed 

 population of Drosophila kept under constant environ- 

 ment was shown to be separable by selection into a num- 

 ber of very diverse strains in respect of duration of life. 

 In Loeb and Northrop 's experiments, a general mixed 

 population of Drosophila, but of presumably constant 

 genetic constitution, at least approximately such, through- 

 out the experiment, was shown to exhibit changes of 

 duration of life with changing environments. It is the 

 old familiar deadlock. Heredity constant plus changing 



