490 



THE AMERICAN NATURALIST [Vol. LV 



been made to keep the flies in aseptic culture as had been 

 done by Loeb and Northrop {eve. cit.) and other workers. 

 Our choice in the matter has not been dictated by tech- 

 nical difficulties, which are not great, but has been deliber- 

 ate. Aseptic life is by no means normal life for Droso- 

 phila. Normally it is as loaded with a bacterial flora as 

 we are. It was folt tliat in the beginning it would be well 

 to establish iim-ins of duration of life for normal life 

 conditions. Latci- we expect to make a special study of 

 duration of life iiiidcr as("i)tic conditions. 



Duration of life in this work with Drosophila is always 

 measured in days, and all of our records relate to dura- 

 tion of adult or imaginal life. No account is taken in any 

 figures of the larval or pupal stages. The reason for this 

 convention is first accuracy and second convenience. It 

 is far more difficult to measure accurately either larval 

 or pupal duration of life than it is imaginal. And from 

 the point of view of these studies nothing significant in 

 principle is lost by dropping these early stages, so far 

 as we have been able to discover, either from the lit- 

 erature or experience with the flies. 



Tiie most cxacl and coinpi-chcnsivc manin'r in which 

 the facts aliont the dni-ation of life in any oi-u'anism can 

 be ])i-o^.'nto(l !•> nu'an- of life taolo, of t lie t\ ]»e usedfor 

 nuuiy \cars by actuaries in their work. The biologically 

 essential features of a life tabh' nnay be nu'utioned here 

 briefly for the benefit of biologists not iiinnediately famil- 

 iar with the development of actuaiial seiciice. A com- 

 plete life table includes, inter alia, the following items: 



1. The number of individuals surviving up to each of 

 the ages j\„ etc., out of a given number (1,000 or 10,000 

 or whatever number one chooses) assumed to have started 

 life together at exactly the sann^ instant of time. These 



technically kiiown as the /, line of a life table. 



2. The number of individuals dying within any short 



