138 ANIMAL AGGREGATIONS 



more rapidly than do the other seedings or acts in both ways at the 

 same time. 



The point of especial interest to us in these observations is the 

 fact that with certain initial densities of populations, even in an un- 

 changed medium, the maximum reached is practically identical and 

 independent of the numbers originally introduced. 



The effect of the exhaustion of food supply has been eliminated by 

 Chapman (1928) in his work with the confused flour beetle, Triboli- 

 um confusum. Chapman introduced varying numbers of these beetles 

 into a definite amount of whole-wheat flour and found for this insect, 

 as Robertson, Cutler and Crump, Myers, and others had previously 

 found with various Protozoa, that there is a definite limit to the 

 number of organisms that will develop in a unit volume of culture 

 medium. 



Chapman's work does, however, introduce one new fact into the 

 situation. His choice of experimental material is particularly for- 

 tunate in that the beetles can be screened out of their floury environ- 

 ment and the eggs, larvae, and pupae, as well as imagoes, can be 

 counted and the flour renewed at each observation. Hence, in place 

 of the usual more or less symmetrical population curve found by 

 other workers dealing with a population composed of all age groups, 

 in which the population, after rising to the maximum determined by 

 the nature of the culture medium and the amount of space available, 

 falls away to approximate or total extinction on account of the ex- 

 haustion of food or the addition of excretory products. Chapman is 

 able, by periodically renewing the environment, to carry his beetle 

 population along for extended periods, perhaps indefinitely, with 

 approximately the same number of individuals present per gram of 

 flour. In his terminology, "a condition of equilibrium is attained in 

 which the biotic potential is equalled by the environmental resist- 

 ance and the population remains relatively constant." 



Appropriate tests showed that the stationary character of the 

 population when in equilibrium was not due to absence of eggs or to 

 their lack of fertility. Rather, the lack of increase in population be- 

 yond a certain point was due to the eggs, pupae, and, to some extent, 

 the larvae, being eaten by the adult beetles. When eggs were placed 



