45 



in virulence of the disease-producing organism 

 may be brought about — by conducting their 

 artificial cultivation in a particular way, and by 

 transmission through the system of an animal 

 differing in nature from that in which the disease 

 naturally occurs. 



When the chain of discoveries reached the point 

 of showing that bacilli could be reared outside the 

 body in an artificial soil or cultivating medium, a 

 great advance was made towards obtaining a full 

 knowledge about them, as it placed the observer 

 in a more favourable position for the successful 

 prosecution of research by enabling him to vary 

 and control his conditions in a manner that could 

 not otherwise have been effected. Although much 

 has been accomplished, it must be said much still 

 remains to be done. In the case of a few bacilli 

 the life history has been pretty clearly made out. 

 Cultivated in a certain way they retain their 

 virulence, no matter through how many succes- 

 sions they pass. The last product in a series of 

 successive cultivations is as virulent as the parent 

 stock. By modifying the conditions under which 



