398 



THE QUARTERLY REVIEW OF BIOLOGY 



If we examine the curves of survivor- 

 ship at different densities of population 

 certain further points of importance are 

 apparent. In figure 5 are shown survivor- 

 ship curves at the optimal density of 35 

 flies per bottle, and at five higher densities. 



From figure 5 it is seen that at density 

 loo, not only has the mean duration of 

 life of the normal wild type flies been 



iooo 



that differences in life duration and in the 

 form of the life curve have their foundation 

 in the genetic constitutions of the in- 

 dividuals — in heredity, in short — whereas 

 another body of equally careful and cogent 

 evidence indicates that by appropriate 

 modification of the environment (density 

 of population) it is possible to alter the 

 mean duration of life and the form of the 



I 









-- 





-— 



■-- 



-., 



-.:.- 





































"*• 



■ ^ 







"****. 































■»•* 







\ 





























' 



--,- 





\^Starved Wild Type ££ 

































% 



> 



























fie 



J I 



Wft 



'& 



>e 



v?. 



\ 



\ 









































V 



ft 













































\\ 











































V 



\ 













































\ 



A 













































1 















































\ 



\ \ 















































\i 















































\ 

 \ 



















































n 



































































































»t 

















































il 







































































































































































































O ■f 8 12 IS 20 24 28 32 36 40 44 48 S2 S6 60 64 68 72 76 80 84 88 32 36 /OO 

 Centifes of Eauinafsnt Life. Span 



Fig. 6. Comparing the Survivorship Curves in a Centile Age Basis of (a) Starved Wild Ttpe Males 

 (Solid Line) and (fi) Fed Wild Type Males (Broken Line) 



greatly reduced, but also the survivorship 

 curve for such flies has been transformed 

 into something practically identical with 

 the straight line diagonal type of life 

 curve shown in figure i to be characteristic 

 of the mutant fly vestigial. 



So then we find ourselves in a somewhat 

 paradoxical situation. One body of ex- 

 perimental evidence seems to show clearly 



life curve of long-lived wild type 

 Droso-phila to a point where these charac- 

 teristics become substantially identical 

 with those normally found in the short- 

 lived mutant form vestigial. 



Plainly further, and more penetrating, 

 analysis of the situation is demanded. 

 In order to accomplish this attention may 

 next be turned to a different line of experi- 



