XXXvi INTRODUCTION. 



in the body, but essential to the growth of the parasite, 

 as to render the body unfit for the production of a 

 second crop. The soil is exhausted, and, until the 

 lost constituent is restored, the body is protected 

 from any further attack of the same disorder. Such 

 an explanation of non-recurrent diseases naturally 

 presents itself to a thorough believer in the germ 

 theory, and such was the solution which, in reply to 

 a question, I ventured to offer nearly fifteen years 

 ago to an eminent London physician. To exhaust a 

 soil, however, a parasite less vigorous and destructive 

 than the really virulent one may suffice ; and if, after 

 having by means of a feebler organism exhausted the 

 soil, without fatal result, the most highly virulent 

 parasite be introduced into the system, it will prove 

 powerless. This, in the language of the germ theory, 

 is the whole secret of vaccination. 



The general problem, of which Jenner's discovery 

 was a particular case, has been grasped by Pasteur, in 

 a manner, and with results, which five short years ago 

 were simply unimaginable. How much ' accident ' 

 had to do with shaping the course of his enquiries 

 I know not. A mind like his resembles a photo- 

 graphic plate, which is ready to accept and develop 

 luminous impressions, sought and unsought. In the 

 chapter on fowl cholera is described how Pasteur first 

 obtained his attenuated virus. By successive cultiva- 

 tions of the parasite he showed, that after it had been 



