FOWL CHOLEEA. 219 



in the least resemble each other. But, glancing back 

 over Pasteur's work, are not the diseases of silkworms, 

 pebrine and flacherie, also virulent diseases ? Thus, 

 in so many things, through so many studies, the same 

 connection holds good. Each discovery of Pasteur's is 

 linked to those which precede it, and is the rigorous 

 verification by experimental method of a preconceived 

 idea. 



' Nothing can be done,' said he one day, ' without 

 preconceived ideas; only there must be the wisdom 

 not to accept their deductions beyond what experi- 

 ments confirm. Preconceived ideas, subjected to the 

 severe control of experimentation, are the vivifying 

 flame of scientific observation, whilst fixed ideas are 

 its danger. Do you remember the fine saying of 

 Bossuet ? " The greatest sign of an ill-regulated 

 mind is to believe things because you wish them to be 

 so." To choose a road, to stop habitually and to ask 

 whether you have not gone astray, that is the true 

 method.' 



It is this method which conducted him in 1880 

 to that wonderful discovery, the attenuation of COIL- 

 tagia. What certain of these contagia are, we have 

 already seen. We shall now learn what they become 

 in the hands of Pasteur. 



