THE STORY OF THE BACTERIA. 5! 



has been found that sometimes two species can 

 be isolated and grow together, but if the at- 

 tempt be made to separate them, one of them 

 always dies. The exact nature of this friendly 

 tie is still unknown. Perhaps in this case the 

 material which is set free by one species as it 

 feeds and grows is the only thing which its 

 associate can live and thrive upon. 



