140 LOUIS PASTEUR 



couraged by a priori opinions, and by assertions which 

 were more or less guesswork. He was resolved not to 

 abandon his preconceived idea until experiment had 

 pronounced upon it with precision. All scientific re- 

 search, in order to be undertaken and followed up 

 with success, should have, as point of departure, a 

 preconceived idea, an hypothesis which we must seek 

 to verify by experiment. To judge of the value of 

 the facts which Pasteur had just announced, it was 

 necessary to know if there existed the relation of cause 

 and effect between the corpuscles and the disease. 

 This was the great point to be elucidated. 



But if, without preliminary groping, he had dis- 

 covered the way to be pursued, Pasteur subsequently 

 brought to bear his rare prudence as an experimen- 

 talist. For five years he returned annually for some 

 months to Alais. The little house nestling among 

 the trees called Pont-Guisquet became at the same 

 time his habitation and his silkworm nursery. It is 

 hemmed in by mountains, up the sides of which ter- 

 races rise, one above the other, planted with mulberry 

 trees. The solitude was profound. Madame Pasteur 

 and her daughter constituted themselves silkworm- 

 rearers performing their part in earnest, not only 

 gathering the leaves of the mulberry trees, but also 

 taking part in all the experiments. The assistants 

 of the Ecole Normale, Duclaux, Maillot, Gernez, 



