DUST AXD DISEASE. 287 



that the true ferments are organized beings, which find in 

 the reputed ferments their necessary food. 



Side by side with these researches and discoveries, and 

 fortified by them and others, has run the germ-theory of 

 epidemic disease. The notion was expressed by Kirchcr 

 and favored by Linngeus, that epidemic diseases are due 

 to germs which float in the atmosphere, enter the body, and 

 produce disturbance by the development within the body 

 of parasitic life. While it was still struggling against 

 great odds, this theory found an expounder and a defender 

 in the President of this institution. At a time when most 

 of his medical brethren considered it a wild dream, Sir 

 Henry Holland contended that some form of the germ- 

 theory was probably true. The strength of this theory 

 consists in the perfect parallelism of the phenomena of con 

 tagious disease with those of life. As a planted acorn 

 gives birth to an oak competent to produce a whole crop 

 of acorns, each gifted with the power of reproducing its 

 parent-tree ; and as thus from a single seedling a whole 

 forest may spring ; so, it is contended, these epidemic dis 

 eases literally plant their seeds, grow, and shake abroad 

 new germs, which, meeting in the human body their proper 

 food and temperature, finally take possession of whole 

 populations. There is nothing to my knowledge in pure 

 chemistry which resembles the power of self-multiplication 

 possessed by the matter which produces epidemic disease. 

 If you sow wheat you do not get barley ; if you sow small 

 pox you do not get scarlet fever, but small-pox indefinitely 

 multiplied, and nothing else. The matter of each con 

 tagious disease reproduces itself as rigidly as if it were (as 

 Miss Nightingale puts it) dog or cat. 



*arasitic Diseases of Silk-worms. Pasteups Researches. 



It is admitted on all hands that some diseases are the 

 product of parasitic growth. Both in man and lower crea- 



