244 ANIMAL AGGREGATIONS 



ly shorter duration of life than do their companions in the same 

 bottles who spent the first 1 5 days of their lives in bottles of initial 

 density 35. The difference is 22.83 + 0.19 minus 19. 71 + 0. 17, which 

 is 3.12 + 0.25 days. This difference is more than 12 times the prob- 

 able error. It may be taken as probable to the point of practical 

 certainty that excessive crowding in early life deleteriously affects 

 the survivors of 16 days of age so that they are significantly less 

 resistant to the effects of heavy crowding again at that time than 

 are flies which lived at optimal densities in early life." 



These Drosophila data indicate that the harmful effect of supra- 

 optimal densities of population is most marked near the beginning 

 of adult life. This original sensitivity decreases with age, so that 

 older flies are less affected either by the original or the average den- 

 sity of population. Even so, excessive crowding has been shown to 

 increase the death-rate at later ages of these flies and to produce this 

 effect almost immediately upon the increase in density. "Further- 

 more," as Pearl and his collaborators state, *'it appears that the 

 amount of shortening of life produced by crowding at any age is 

 influenced by the previous history of the flies relative to density of 

 the population. This suggests that there is a deleterious effect of 

 supra-optimal crowding in early life even upon those flies that do 

 not immediately die as a result of it, and that this effect endures for 

 at least the first 15 days of life." Unfortunately for us. Pearl's 

 experiments have not been concentrated at the part of the survival 

 curve which would show whether suboptimal crowding will have the 

 same effects that supra-optimal densities have been demonstrated 

 to have. 



These experiments on Drosophila, like those of Chapman on the 

 confused flour beetle, are distinct from the great majority of crowd- 

 ing experiments in that they are carried on in a non-liquid environ- 

 ment, whereby the temptation to postulate the production of some 

 X-substance responsible for the phenomena observed is much re- 

 duced. Pearl suggests that the biological explanation for his ob- 

 servations lies in an entirely different direction, but the matter has 

 not been discussed publicly, even in general terms, by members of 

 his laboratory. We know that in the case of effect of crowding on 



