170 INTRODUCTION 



Finally, in dissolving the bacteria, can it not be an indirect 

 factor in naturally acquired antitoxic immunity? 



These are the points which we will consider in the second part 

 of this monograph. 



It would seem that the only method which ought to be followed 

 in investigating the relation between immunity and a principle 

 to which one may attribute a protective power ought to be founded 

 on the observation of natural disease, and that the parallelism 

 between the state of the patient and the presence, and the potency, 

 of the supposed protective principle, ought to serve as the cri- 

 terion for determining its true role. If a parallelism exists, it 

 may be regarded in the possible relation of cause and effect, and 

 one can then turn to the counter-test for confirmation. If a 

 parallelism does not exist, the relation of cause and effect cannot 

 be invoked, and the principle under consideration cannot play 

 an active role in the processes of recovery. This is the method 

 of investigation, the only logical one it seems to me, that I have 

 applied in investigating the relationship between the bacterio- 

 phage and immunity. I consider, in fact, that a theory of im- 

 munity based only on simple observation or on comparison, always 

 remains subject to discussion. For simple observation readily 

 leads to error, especially when the observations are made on 

 refractory animals and are not found to be confirmed when applied 

 to a susceptible animal. It is certainly much easier to experi- 

 ment in the laboratory with caged animals; to study the immunity 

 against the cholera vibrio, for example, on a guinea pig which is 

 resistant to the disease naturally, than to run everywhere in 

 search of epizootics in order to study the disease in its normal 

 environment. But common sense alone is adequate to make it 

 apparent that the first method can prove nothing, and that only 

 observation of the natural disease, complemented by experimen- 

 tation on an animal susceptible to it, can give results that have 

 an absolute value. It may indeed seem strange that we use the 

 word "immunization" in speaking of a refractory animal, since 

 the refractory state already represents immunity carried to its 

 highest degree. 



I wish to be free of such criticism and will thus follow an order 

 which seems to me the most logical. We will observe first, 



