BEGINNINGS OF CO-OPERATION 77 



timeter more or less, she got an increase in division 

 rate with the presence of a second individual, as 

 Robertson had found it in the Australian form he 

 had studied. 



Still the critics were not convinced. Accordingly 

 Dr. Johnson, now of Stanford University, repeated 

 this whole study using a different protozoan, one of 

 the Oxytricha. (68) When sister cells from pure-line 

 cultures were used there was no difference at the 

 end of the first day, whether the Oxytricha were in- 

 troduced singly or in pairs into one or two drops of 

 good medium. Later, the cultures started with one 

 organism always were ahead. With larger volumes, 

 two organisms showed a higher rate of reproduction 

 per original animal at the end of the first day than 

 if started with a single protozoan. 



Again for larger volumes Robertson's results were 

 confirmed, and those of his critics for smaller vol- 

 umes. But Johnson had only started. He knew from 

 the work of others that if a protozoan is washed 

 through several baths of sterile water the associated 

 bacteria are rinsed off. Then if the washed protozoan 

 is put into a weak solution of the proper salts, into 

 which has been introduced known numbers of the 

 bacteria on which they normally feed, the problem 

 can be studied with a controlled food supply, both 

 as to kind and amount. 



