6 PREFACE 



which is defending itself or from the bacterium which is causing 

 the infection, both of which sources have been suggested by dif- 

 ferent authors. 



It may be that the reader will find sometimes in the course of 

 this discussion that I have multiplied evidence by repetition, 

 or that it is necessary to make an effort to follow certain of the 

 experiments, so that he is lost in a maze, not only of new phenom- 

 mena for which he is as yet unprepared, but also in phenomena 

 of extreme complexity. The difficulties of exposition of the sub- 

 ject will readily be comprehended if we realize that up to the 

 present time Bacteriology has been considered as a " problem 

 of two bodies," bacterium and medium, whether the medium be 

 the organism parasitized or a culture fluid. And this problem 

 of the two bodies has been indeed complex. But it is of necessity 

 much less complicated than the "problem of three bodies" with 

 which we must now be concerned, where we must recognize the 

 interactions between the medium culture medium or organism 

 parasitized, the bacterium parasitizing this medium, and the 

 ultramicrobial bacteriophage parasitizing the bacterium. 



Pan's, July 1, 1921. 



