176 PASTEUR: THE HISTORY OF A MIND 



festly hereditary character. As we have said, it ravaged 

 certain lots of eggs, which were sometimes distributed 

 among a number of silk-growers, and bred, owing to 

 this fact, under the most varied conditions, and which, 

 nevertheless, were attacked at the same period, in the 

 same age of the worm, as if they had all brought with 

 them a germ of destruction. Most frequently after the 

 fourth molt, during the period of voracity called the "big 

 gorge," when the healthy worms greedily devour the 

 foliage given them, the diseased worms were seen to be 

 indifferent to the provender, crawling over the leaf 

 without attacking it, even avoiding it, and having the 

 appearance of seeking a tranquil corner in which to die. 

 When dead, the worm sometimes softened and rotted, 

 but sometimes remained firm and hard, so that one must 

 touch it to be certain it was dead. At other times, when 

 the disease attacked the worm more slowly, it climbed 

 the heather, but with difficulty, slowly spun its cocoon, 

 sometimes did not finish it but left it in the condition of 

 skin, and died without changing into a chrysalis or a 

 moth. 



In recalling the conditions under which the production 

 of the eggs showing this hereditary predisposition to the 

 disease of the morts-flats had taken place, Pasteur 

 remembered suddenly that one of the cultures had not 

 been entirely satisfactory at the time the worms climbed 

 up to undergo their transformation. The worms had 

 climbed up soft, had dragged themselves at this time. 

 Here Pasteur reaped the advantage of that constant and 

 penetrating supervision which he exercised over every- 

 thing. In a year, he had become an excellent breeder 

 of silkworms. If he observed well, he knew also how to 

 draw conclusions, and immediately he reflected that the 

 eggs subject to this hereditary sensitiveness to the morts- 

 flats must have come from those apparently successful 



