354 THE BACTERIOPHAGE AND ITS BEHAVIOR 



non. Indeed, there is much reason to believe that the bacteriophage is 

 the being which possesses this character to the highest degree. Further- 

 more, this character of variability is always proportionate to the more 

 or less outspoken faculty of adaptation. 



8. THE BACTERIOPHAGE CORPUSCLE: A LIVING ULTRAVIRUS 



The first of all the scientific classifications is that which separates the 

 beings known to man into two great categories; inert beings on the one 

 hand, living beings on the other. This classification is based upon the 

 fact that the living beings present certain properties which the others 

 lack. Just as soon as it is determined that a being possesses, or does 

 not possess, these properties, it is irrevocably placed in one or the other 

 of these two categories. 



If, perchance, it is found that a being, possessing the characters which 

 classify it as "living," presents certain peculiarities that have up to 

 that time not been detected in another being so classed, this in no way 

 throws its nature into question. For just as soon as it possesses the 

 "criteria" of fife it is necessarily living, and the only choice is to recog- 

 nize that the peculiarity which it presents is compatible with life. This 

 sort of thing has occurred more than once before now in the history of 

 science. 



Experiment shows that the bacteriophage corpuscle, an autonomous 

 being,* possesses the powers of assimilation in a heterologous medium 

 and of adaptation, and thus, necessarily, the consequences of these 

 two powers — the faculties of multiplication and of variability of 

 characters. 



This combination of characters constitutes the complete criterion of 

 life. The bacteriophage corpuscle, conforming to this criterion in every 

 respect, is a living being. This is no longer an hypothesis; it is a certainty. 



It is this living being, this infravisible microbe, to which I have given 



* The demonstration of the living nature of the bacteriophage under the form 

 in which it is given in this chapter was published for the first time in 1923.^*^ It 

 has been repeated, notably in the text published in the United States (Immunity 

 in Natural Infectious Disease). Following its publication some of the discussion 

 stopped. Nevertheless, there are still some writers who attempt to show that 

 certain facts "fit in better" if one believes that the "lytic principle" is derived 

 from the bacterium. None of them have discussed the proofs that I have 

 adduced; none have even alluded to them. 



Their attitude forces me to suggest that logically they might consider point 

 by point the experimental facts which serve as the basis of my demonstration be- 

 fore they continue their study and their discussion of the nature of the bacterio- 

 phage. 



