The study of the genetic determinism of longevity in Drosophila melano
gaster has made use of the technique of late-age reproduction. At low
larval density, that indirect selection showed no effect. At high larv
al density, however, increased mean life-span in lines reproduced at l
ate age was observed. When these last data are examined as a function
of the number of days after the beginning of the experiment, instead o
f as a function of generations, the difference in life-span between ea
rly and late lines at high larval density disappears. The erratic evol
utions of mean longevities in all the experiments here described may b
e attributed to unexplained variations in life-span previously observe
d in a 4-year experiment. Considering the time lag between the measure
ment of a given generation in early and late lines, the experiments of
reproduction at late age cannot demonstrate the genetic determinism o
f longevity.