314 PASTEUR: THE HISTORY OF A MIND 



is man alone able to contract syphilis? 1 These are 

 questions which one did not even dream of putting to 

 himself. But after living viruses were discovered and 

 their conditions of growth were known, man could ask 

 himself why they develop here and not there, on the 

 French sheep and not on the Algerian sheep, both of 

 which are, however, authentic sheep. 



For an answer to this embarrassing question, Pasteur 

 sought quite naturally, as any man of science would do, 

 in his experience and memory. They were, it is true, the 

 experience and the memory of a chemist, and the question 

 did not remain long in the field where he first placed it. 

 But all theory is good which foresees new facts, and 

 however inexact it appears to-day, the explanation of 

 Pasteur has had that merit. 



He knew, through his long experience with fermenta- 

 tions, that even when one works in vitro the smallest cir- 

 cumstances suffice to permit or to hinder the development 

 of a microbe. When he saw certain of them demand veal- 

 bouillon and certain others fowl-bouillon, it did not sur- 

 prise him that a particular disease was peculiar to a 

 particular species, and another disease to another species. 

 Neither was it astonishing, knowing how hard to please 

 the microbes are on questions of temperature, that the 

 chilled fowl should contract anthrax, while at its ordinary 

 temperature it remained unaffected. Finally, knowing, 

 as we have said, that the chicken cholera microbe refuses 

 to develop again in a medium in which it has already 

 lived, why be astonished that it should refuse to live 

 again in an organism which it has already invaded? 

 There was, in these exclusively physical or chemical 



1 We now know from the studies of Metchnikoff that syphilis is in- 

 oculable into apes, and from those of Noguchi into rabbits the living 

 testicles of which are the best culture medium for the propagation of the 

 Treponema pallidum the protozoan cause of syphilis. Trs. 



