156 PASTEUR: THE HISTORY OF A MIND 



saw that the number of those containing corpuscles 

 increased more and more. Among the worms, the cor- 

 puscles were rare. In the chrysalids, especially in the 

 older ones, the corpuscles were frequent. Finally, not a 

 single one of the moths was free from them, and they 

 were there in profusion. 



The question seemed, therefore, to be cleared up, for 

 how could one interpret this double observation other- 

 wise than by saying : there is a disease which can weaken 

 the worm in the absence of the corpuscle, but of which 

 the corpuscle is the tardy evidence. The two broods 

 have suffered from this disease but the first has been 

 attacked only when the worms were near the cocoon 

 stage, and this brood has succeeded well although it 

 has been a little diseased. In the second, the disease 

 has attacked the worms more severely, and it is for this 

 reason that this brood has been languishing and has 

 almost miscarried. 



This interpretation, we know to-day, is inexact, and, 

 consequently, it was perilous. Its danger was that it 

 led to a practical conclusion which Pasteur did not hesi- 

 tate to draw. From the moment that the corpuscle 

 appeared thus as the evidence of an advanced disease, 

 it is clear that it would be more advantageous to obtain 

 eggs from the non-corpuscular moths rather than from 

 the corpuscular moths. The first might be diseased, but 

 they would have been so for a shorter period and prob- 

 ably less seriously. "To say that the disease should 

 be regarded as affecting by preference the chrysalid and 

 the moth is only to say that at this age it manifests itself 

 more apparently and also without doubt more danger- 

 ously for its posterity." It is thus that Pasteur, starting 

 from a false idea, immediately put the capstone upon a 

 method of egg-selection which became theoretically and 

 practically, still better when the false idea which had 





