172 INTRODUCTION 



evolution. A being which evolves is necessarily a being which 

 lives, which adapts itself, and which conquers. From the instant 

 that it ceases to adapt itself to evolve it dies. Evolution is 

 always conducted according to the law of least effort. The multi- 

 cellular organisms have profited by securing for their defense 

 the parasitism of the bacteriophage for the bacteria; which is 

 only a chapter in the universal struggle. 



If, among all living beings, the bacteria alone escaped para- 

 sitism, where would we arrive? It is very simple. One of two 

 things would take place. Either evolution would not extend 

 beyond the stage of the unicellular being, or evolution would be 

 accomplished in another manner and immunity would be as- 

 sured by other means; a simple matter of adaptation. The bac- 

 teriophage does not exist for the defense of the superior organ- 

 ism against the bacteria, it exists simply because in the course of 

 evolution certain germs have parasitized others. 



Nothing in nature exists simply for an end, for nature is not 

 an end. That there exist on the earth thinking beings, or that 

 they might not have been, is a perfectly negligible incident. Is 

 this point of view "finalistic"? But what does it matter; a scien- 

 tific theory is true or false according to the proofs upon which it 

 is founded. 



Each time that we will speak in the course of this discussion 

 of "antibacterial immunity" it is essential to understand "anti- 

 bacterial immunity in a susceptible individual." These observa- 

 tions and experiments, as I have already remarked, are concerned 

 with this and this alone. 



Up to the present I have paid little attention to the phenomena 

 of immunity in the refractory animal. It indeed seems, in 

 general, in the special type of immunity which characterizes the 

 refractory state, that the elimination of bacteria which may gain 

 access to the body, and which because of the refractory state are 

 not pathogenic, is effected by phagocytosis. In this special 

 case, defense by the bacteriophage could not possess, the greater 

 part of the time, the opportunity to act. Phagocytosis is ac- 

 complished too rapidly to allow the bacteriophage time to in- 

 crease its virulence toward the bacterium which is introduced 

 into the organism. 



