UNBALANCE IN NATURE 87 



can be screened out of the flour from time to 

 time; even the eggs and grub-like larvae can be 

 recovered uninjured from the screenings and be 

 transferred to fresh flour. They are thus supplied 

 with fresh food in an environment that is un- 

 contaminated by the waste products of the popu- 

 lation of beetles which have been occupying the 

 flour. The course of the growth of such a com- 

 munity under laboratory conditions is shown in 

 Fig. 9 which records the increase of different initial 

 populations to an approximate balance at the 

 same number of beetles per gram of flour regard- 

 less either of the initial population or of the 

 number of grams of flour used. 



When such beetles were reared in whole wheat 

 flour at twenty-seven degrees Centigrade with an 

 initial "seeding" of one pair of beetles for each 

 four grams of flour, the population became prac- 

 tically constant at about forty-four individuals 

 per gram of flour after one hundred days and 

 remained so for the next fifty days. With differ- 

 ent sorts of flour, the beetles come to have a 

 constant number per gram of flour which varied 

 apparently according to the food value of the 

 flour used. The exact series of reactions which 

 lead to the establishment of this population bal- 

 ance have not yet been analyzed but there are 

 many indications that there is a certain amount 



