CHAPTER VI 

 UNBALANCE IN NATURE 



U: 



NDER constant labora- 

 tory conditions the number of constituents in a 

 simple laboratory community should come to a 

 constant level. Ordinarily such a combination of 

 conditions is difficult to realize even in modern 

 laboratories with their various devices for environ- 

 mental control. In the case of protozoan infu- 

 sions, the population of any given species or of 

 all species taken together begins with only a few 

 representatives, rises to a climax and falls off to a 

 few active forms or to none at all. The decrease 

 in population in such infusions is apparently due 

 to an exhaustion of the food supply, or to an 

 increase in the amounts of excretory products 

 present, or to both acting together. 



When these two causes of decline are eliminated, 

 the population should come to the maximum 

 level at which it can be maintained and should 

 continue indefinitely at that level. This condi- 

 tion has been reached in laboratory populations 

 of the flour beetle (Tribolium) which thrives in 

 cultures of common wheat flour. The beetles 



85 



