ESSAYS ON BACTERIOLOGY. 83 



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chemical products of germs acting upon the living or- 

 ganism. In the endeavor to imitate or improve upon 

 nature in this direction the tendency has been unmis- 

 takably to make use of what may be called the natural 

 laboratory, that is, the animal body. 



Several important discoveries have combined to 

 give us the new serum therapy. 



1. It was found that the blood and other body 

 fluids had a certain amount of bactericidal power. 

 This may be said to have been the starting point of the 

 long Hue of investigations which have centered in the 

 blood. 



2. It was found that, imitating nature, animals 

 could be rendered immune to certain diseases by in- 

 troducing into their bodies the germs or the toxic 

 products of the specific germs which caused the dis- 

 eases. 



3. Carrying these experiments further, it was 

 found that by a process of progressive inoculations 

 from weaker to more virulent germs, or of injections 

 of smaller to larger quantities of toxins, a progressive 

 degree of resisting power to those germs or toxins 

 could be developed. The degree to which this rise of 

 resisting power can be carried is indeed astonishing. 



4. When the agency of this artificially induced 

 resisting power came to be investigated, it was found 

 to apparently have its seat in the blood. 



5. And now the surprising and epoch-making dis- 

 covery was added that this resisting power, thus arti- 



