74 ANIMAL LIFE AND SOCIAL GROWTH 



celled animals do not usually enter the cultures 

 during the early period which we have been 

 following but when they do come in, they persist 

 for months. If a small amount of bread or a 

 few grains of cracked wheat are added to one of 

 these old dormant cultures, the fresh food will 

 cause the whole cycle to start up again and run 

 through approximately the same course which it 

 originally followed. 



There is evidence that the cycle is caused in 

 part by the excretions which the protozoans them- 

 selves give off as well as by the food which they 

 consume. Paramecia, for example, normally fol- 

 low the hypotrichs in the succession series; they 

 reproduce fully as rapidly in media contaminated 

 with hypotrich excreta as they do in uncontami- 

 nated media but they reproduce more slowly in 

 the presence of their own excretions. On the 

 other hand, hypotrichs are retarded in their rate 

 of reproduction by their own and by Paramecium 

 contamination. 



With protozoan cultures it appears to be true 

 that these animals by living in their environment 

 tend to make it unsuited for their prolonged 

 existence; however, they make changes which 

 prepare the way for the coming of other animal 

 life. The evolution of an animal community 

 through different stages in the same habitat is 



