THE CAUSE OF INFECTIOUS DISEASE. 265 



can distinguish, species and genera among bac- 

 teria and other minute organisms. Such con- 

 stancy as we observe, however, is not the mysti- 

 cal constancy of " specific " essences, but a con- 

 stancy made possible by the permanence of the 

 environment. Micro-organisms change with 

 the changes in their surroundings, and the 

 placing of this fact on a sure footing consti- 

 tutes the great advance that modern bacteri- 

 ology has made beyond the standpoint reached 

 by Koch. 



Just as the human being possessed of a def- 

 inite organization is compelled continually to 

 adapt himself to changing conditions of life, so 

 is the microbe also constrained to the same task. 

 In the majority of men the bodily constitution 

 is always oscillating, manifesting now increase, 

 now decrease of a definite disposition toward 

 disease. The microbes also vary according to 

 the conditions imposed upon them and display 

 increased or decreased capacity to grow or form 

 poisons in the human body, capacity in other 

 words to remove with greater or less difficulty 

 the resistance inherent in the human organi- 

 zation. Accordingly we observe the occurrence 

 sometimes of mild, sometimes of severe epi- 

 demics, and in every epidemic, along with the 

 " typical " cases, we find especially grave or 



