66 ANIMAL LIFE AND SOCIAL GROWTH 



so. For example there are the changes which 

 take place in a protozoan infusion that runs its 

 course rapidly enough so that the entire progress 

 up to the climax stage can be followed within a 

 few months. 



It is a common laboratory practice to set up such 

 protozoan cultures in the brownish "hay tea" 

 made by boiling common hay in water. The hay 

 may then be filtered out and the "tea" allowed 

 to cool. If such an infusion is allowed to stand 

 open to the air, first bacteria will collect and grow 

 and later air-borne cysts of Protozoa will fall 

 into the liquid and populate the medium. In 

 such a non-seeded culture, the development or 

 evolution proceeds slowly. Its speed can be 

 quickened by seeding the newly cooled infusion 

 with a variety of organisms including bacteria 

 and a number of different sorts of Protozoa. 

 Tests have shown that the numbers of kinds seeded 

 have little if any effect upon the order of their 

 development in the ageing culture. 



The liquid of the newly made infusions is trans- 

 parent though colored, but within forty-eight 

 hours it becomes plainly cloudy due to the de- 

 velopment of countless bacteria which at this 

 time are equally distributed through the medium. 

 During this period the acidity of the liquid rises 

 rapidly due to the fact that the bacteria cause 



