316 PASTEUR: THE HISTORY OF A MIND 



do not understand why its absence no longer interferes 

 with growth when the number of microbes increase 

 which have need of it for their development. On the 

 contrary, it is much more comprehensible why the 

 presence of an injurious substance can stop a small 

 detachment of the enemy, and not a large troop. 



It is useless to dwell upon the discussion of these ex- 

 planations of immunity, both of which may indeed have 

 their part in the phenomenon, but cannot play a stellar 

 role. Strictly speaking, they are sufficient to explain 

 the immunity produced by vaccination, but they weaken 

 when it is a question of explaining the duration of immu- 

 nity. How can we admit the persistence for years of 

 this injurious element, or the absence of the necessary 

 element, when nutrition and destructive metabo- 

 lism bring and remove such varied elements. The 

 element duration is represented in the tissues, not by 

 the chemical substances which compose them, but by 

 their permanent form by the cell. 



The tw^o explanations which we have just considered 

 are not the only ones which have been proposed. There 

 have been successively attributed to the humors, and 

 to the liquids of the animal economy, a destructive 

 power for microbes, an attenuating power, an anti- 

 toxic power, all these powers depending solely on con- 

 ditions of the physico-chemical order. Without entering 

 into a detail which however important it is, would be 

 out of place here, it can be said that all these theories 

 have shown themselves to be powerless to explain the 

 great fact of the creation and the persistence of inmiunity. 

 As to the creation of this property in the individual, 

 either it has been found that the liquids in circulation 

 or the humors that occur in the interior of the body, 

 did not have the destructive, attenuating, or antitoxic 

 power which we find in them outside of the organism, 



