18 BOARD OF AGRICULTURE. 



nature of the modifications thus produced, and the exact 

 conditions which are essential to these modifications, are 

 even yet problems which confront the scientist and puzzle the 

 philosopher; but we doubt not they are vital changes, — a 

 sort of adaptation to the environment ; the germ remains the 

 same in form, but its requirements are diflerent. 



It was a long time before it occurred to the investisrator 

 that perhaps the conditions of life which were necessary to 

 transform a very virulent virus into a comparatively harmless 

 one, might be supplied in the laboratory of the biologist as 

 well as in the laboratory of nature — that in fact this empir- 

 ical practice might be made the foundation of a truly scien- 

 tific one. The conception was most important ; it is one of 

 those great advances of science which prove the stepping- 

 stone to achievements innumerable ; it marks an epoch in 

 medicine, and so long as men and animals are the prey of 

 the microscopic parasites which constitute the virus of con- 

 tagious diseases, the name of Pasteur will be remembered as 

 one of the greatest public benefactors which the world has 

 so far produced. 



This discovery, however, was not reached at a single 

 bound ; it was the outcome of methods of research per- 

 fected by years of application in allied branches of science. As 

 we look back to-day we can see that it depends upon many dis- 

 coveries which at first sight appear to have but little connec- 

 tion with it. It was necessary to cultivate the disease germs 

 free from contamination with atmospheric germs before they 

 could be studied ; but this could never have been accom- 

 plished until the theory of spontaneous generation had been 

 exploded, since no pure species could ever be cultivated free 

 from contamination if other similar organisms were contin- 

 ually arising. And so we learned that putrefaction was 

 caused by microscopic organisms ; that these could only 

 arise from other similar things which had previously existed ; 

 that a broth in which such organisms had been destroyed by 

 heat, and from which others were excluded, would remain 

 sweet indefinitely, and might be used for cultivating and 

 studying germs of various kinds. Apparatus for such work 

 was perfected ; and, finally, when science had advanced to 

 the proper point, the man who, of all others, had been con- 



