312 PASTEUR: THE HISTORY OF A MIND 



beginning we ascribed variations in virulence to the 

 microbe itself and there was there a vast field for evolu- 

 tions, but it did not embrace all the possible ones. We 

 have been obliged to add to it those which come from 

 the variation of the living organisms in which the mi- 

 crobes establish themselves, and the virulence which we 

 see results from an infinite number of combinations of 

 these two causes of variation. 



IX 



CHEMICAL AND HUMORAL THEORIES OF 

 IMMUNITY 



From what we have just said it follows that the word 

 virulence has no meaning either in relation to the microbe 

 or to the host. It signifies little more than the relation 

 between strength and resistance, without telling us 

 anything about the absolute value of these two forces. 

 A microbe which does not kill a given animal or does not 

 make it ill is devoid of virulence with respect to that 

 animal, and one might believe from this statement that 

 all the truths which we have discovered are naivetes, or 

 mere definitions of words; that would be making a great 

 mistake. What we have discovered, in reality, is a new 

 field of study. With respect to this or that animal, such 

 or such a microbe may remain harmless for many 

 reasons. It cannot develop in its tissues, or, if it does 

 develop, deposits there no injurious substances, or even, 

 perhaps, produces beneficial effects, leading to increased 

 resistance. The field of hypotheses is unlimited. Let 

 us see what experience offers, and let us observe how 

 much the field of experiment has been extended, thanks 

 to Pasteur. 



Here are normal sheep inoculated, some with the 



