82 THE SOCIAL LIFE OF ANIMALS 



problem, and has concluded that the results which 

 he has observed can be explained as due to bacterial 

 crowding; that the larger number of protozoans in- 

 troduced into dense cultures thrive best because they 

 are able to reduce the bacteria to density optimal 

 to the protozoa faster than their isolated sister cells 

 can; and therefore they show a higher rate of re- 

 production. 



This does not seem to be the whole story; for from 

 points as distant as Baltimore (79) and Jerusalem, 

 (101) I have reports from trustworthy men that with 

 still simpler protozoans they are getting results which 

 suggest that some modification of Robertson's hy- 

 pothesis may be correct after all. These organisms 

 stimulate each other to more rapid growth merely 

 by their presence in the same small space. 



With fine courtesy, Professor Mast of Johns Hop- 

 kins has placed a report of his experiments in my 

 hands in advance of publication and has permitted 

 me to summarize his results. He finds that popula- 

 tions of a flagellate protozoan grow more rapidly in 

 a sterile medium of relatively simple salts when 

 larger numbers are introduced than if the cultures 

 are started with only a few organisms. 



I must not put too much stress on these reports, 

 pending the appearance of yet more data, but I 

 should expect to find here, as elsewhere, that com- 



