164 THE BACTERIOPHAGE AND ITS BEHAVIOR 



virulence for one strain is obtained (when working with organisms be- 

 longing to homogeneous races — we will see later the significance of 

 this) virulence is likewise enhanced for strains which were unsuited to 

 the development of this virulence. The reason for this fact is that when 

 the virulence is once increased, the resistance of the bacterium, which 

 previously would have been able to withstand a low virulence, is over- 

 come by a higher virulence. Upon this point again the story of patho- 

 genic bacteria provides us with ample examples of facts of the same 

 nature. 



Before ending this section I would like to call attention to an error 

 of fact which has been made by Seiffert."^ This author has assumed 

 that the increase in the virulence of the bacteriophage effected by suc- 

 cessive passages at the expense of bacteria is due to an "adaptation" 

 on the part of the bacteria and not to an adaptation of the bacterio- 

 phage. With regard to "theories" of this type Pasteur stated, "they 

 are under no particular obligations as regards the facts." 



Just how valid such a theory is may be shown by the following, which 

 indicates exactly the sequence of events leading to the adaptation. In- 

 oculate a drop of filtrate containing bacteriophage corpuscles of low 

 virulence into a suspension of bacteria. After an appropriate time 

 filter through a candle. This effectively discards all of the bacteria 

 which have been subjected to contact with the bacteriophage cor- 

 puscles. Introduce this filtrate, containing no bacteria whatever, into 

 a second suspension of young bacteria — bacteria which have never yet 

 been in contact with the bacteriophage. When bacteriophagy has 

 occurred filter again and discard once more all of the bacteria which 

 have been in contact with the bacteriophage corpuscles. Carry out 

 in this manner a series of passages, introducing a filtration to ehminate 

 the bacteria between each passage. None of the bacteria of the sus- 

 pension in which one passage is made can be carried over into the 

 suspension of the following passage. Under such conditions, since the 

 bacteria are not involved in the passages, how can they undergo adapta- 

 tion? Such an adaptation as a being may acquire can be transmitted 

 only when there are descendents. 



It is easy to understand that the fact that the virulence of the bacterio- 

 phage is subject to increase is disturbing to the partisans of certain 

 theories which we must discuss later. Nevertheless, if objections are 

 raised to the concept which I have evolved, the opposing arguments 

 should, at least, take the facts into consideration. 



