126 ANIMAL LIFE AND SOCIAL GROWTH 



to control these "weeds" without exhausting the 

 food supply or without unduly increasing the 

 concentrations of excretions to a degree that would 

 be harmful. As the population increases beyond 

 the optimum, both these latter factors act to 

 retard growth. 



With these same fruit-flies, other investigators 

 have found that adult flies will live longer in 

 one-ounce bottles with a standard amount of food, 

 if from 35 to 55 are present than if either more or 

 fewer than that number are used. This again 

 shows that there is an optimum population for 

 survival which is well above the minimum popula- 

 tion and well below the maximum population 

 possible. Again despite the well known harmful 

 effects of crowding on the rate of reproduction, 

 with the one-celled protozoans and with certain 

 larger animals as well, we now know that the rate 

 of reproduction is greater per individual under 

 certain conditions if more than a single animal 

 is present in the case of the asexually reproducing 

 protozoans or if more than one pair is present 

 with certain sexually reproducing beetles. In 

 both cases the numbers of animals present must 

 stand in the proper relation to the size of the 

 environment if the rate of reproduction is to be 

 stimulated by optimal crowding. 



The common Paramecium (Fig. 6) is a protozoan 



