184 PASTEUR : THE HISTORY OF A MIND 



the morts-flats, and we have united by a common bond 

 all the modes of appearance of the disease, both that 

 which is encountered in the sporadic state on sound eggs 

 exempt from all hereditary predisposition, and that 

 which rages among the descendants of a brood where 

 the morts-flats has been. 



The disease of the morts-flats is then, like that of the 

 corpuscles, a contagious disease, but sometimes be- 

 cause its germ is so common, it may become a disease 

 which appears to be spontaneous, sporadic or epidemic, 

 benign or disastrous, and the origin of which a super- 

 ficial observer might, with reason, attribute to the com- 

 mon conditions of cold, heat, humidity, electricity, so 

 often invoked by ancient medicine. More enlightened 

 now, we can say: No, these common influences neither 

 are the disease nor make the disease; they open the door 

 for it and give it scope. In -this case, and in all the cases 

 where one is led to lay on them the blame, we find on 

 close investigation a germ, more or less widespread, 

 ordinarily kept within bounds by natural laws, but able 

 when conditions change, when its virulence is exalted, 

 when its host is enfeebled, to invade the territory which 

 was barred to it up to that time. The bacillus of flach- 

 erie is always present, but the sound worm is able to 

 defend itself on the side where it is threatened. It will 

 not, perhaps, resist a wound an unusual way for con- 

 tagion; it will better resist an introduction into the di- 

 gestive tract, but still there must not be too many ba- 

 cilli, nor too virulent ones. 



For the same reasons the hereditary predisposition 

 no longer makes for the same certainty of action as it 

 does in pebrine. There it resulted from the deposition 

 in the egg of a germ the development of which was as- 

 sured; here there is no transmission of the germ. We 

 find, it is true, in the stomach of the chrysalis (a stomach 



