128 ANIMAL LIFE AND SOCIAL GROWTH 



were found to reproduce at a higher rate per 

 female during the first 11 days of the culture than 

 did the female of a single pair isolated into the 

 same size of environment. The rate of reproduc- 

 tion in such populations is shown in Fig. 10. 



2 4 8 16 32 64 



Initial population per ^2 %ms. of flour 



Fig. 10. Tribolium beetle populations increase more rapidly at 

 first if more than one pair is present in 32 grams of flour. "Rate per 

 female day," shown on the vertical axis, really means rate of popula- 

 tion increase per female beetle per day. (From Allee, Animal 

 Aggregations.) 



Later, with increasing age of the culture, the rate 

 of population increase decreases with the higher 

 concentrations of beetles until finally the popula- 

 tions in the different cultures become approxi- 

 mately stationary, as was shown in the preceding 



