56 THE AMERICAN NATURALIST [Vol. LV 



The connection between observed character-change and in- 

 viability is even more indirect than suggested above. In the 

 Drosophila work it is not the comparative viahility of adults 

 possessing given character differences that is of the most im- 

 portance. Even though many of the characters are of such a 

 nature that their possessors would be under a serious handicap 

 in competition, in relatively few cases does this fact lead to 

 alterations in the observed ratios, since the classifications are 

 made usually soon after the flics hatch, i.e., every 24 to 48 hours. 

 It is true that certain mutant forms such as "divergent" and 

 "gull" and "'bifid" wings, also "dachs" and "reduplicated" 

 legs tend to become entangled in the culture media and drowned 

 immediately after emergence, so that in these cases the observed 

 ratios are somewhat different from the hatching ratios. There 

 are also a few mutantsi — mostly semi-lethals — in which the adult 

 is unable to live very long even under the most favorable condi- 

 tions. Among these may be mentioned "lemon," "apterous," 

 and especially "decrepit." The "decrepit" flies die a few 

 hours after hatching in spite of all care in helping them emerge 

 from the pupa case, in keeping them in quarters not too dry 

 or wet, and in suplying them with suitable food. It would 

 seem that the death of such flies as are obviously weak on hatch- 

 ing is to be referred to difficulties encountered in the pupa stage. 



Even inviability arising in the pupal stage, like that in the 

 adult stage, is less general and significant than that in the larval 

 stage, ilost of the inviability that affects the ratios of adults 

 is to be referred to differences acting in the larval stage, as is 

 evident from comparative studies of the results of pair and mass 

 cultures and of changes in culture methods that affect only the 

 larval period. The difference between mass and pair cultures 

 is essentially a difference in the number of larvae that are in 

 competition, the food conditions and the character of the larvae 

 being at first identical in the compared cultures. It is found 

 that the distortion to the ratios among the adults is roughly 

 proportional to the number of larvae in competition. How ex- 

 treme such competition may be is evident from the fact that a 

 point is soon reached after which further increase in the number 

 of mothers brings no inerea.se in the number of progeny and 

 may even result in a decrease. So predominant is the larval 

 stage in its influence upon viability that the chief field of 



