9? 



fever to the hypothetical tonic influence of sewer-gas 

 upon the bacteria inhabiting the alimentary canal, etc. 



The evidence in support of this hypothesis consists of 

 deductions by analogy and of experimental observations. 

 Since the life of an organism is the resultant of many 

 forces, it is a priori evident that a modification of one or 

 more of these forces may be followed by a change of the 

 resultant life. In the higher plants and animals we have 

 abundant evidence to this effect ; the domestic pigeon 

 and the dahlia are examples rendered familiar to us by 

 Darwin. Such modifications, it is true, require time ; 

 but in biology time is measured by generations, not by 

 years ; and since from one bacterium a second may be 

 produced in thirty to sixty minutes, it is evident that a 

 day may induce in these organisms the effects of a thou- 

 sand years in man. The evidence by analogy with higher 

 organisms supports then the theory in question. 



This same principle modification of function by 

 changes of environment which Grawitz and Buchner 

 had vainly attempted to demonstrate, seems to have 

 been demonstrated by Pasteur in his studies upon pro- 

 tective vaccination. He asserts that the microbes which 

 he regards as the morbid agents in chicken-cholera can 

 be deprived of their virulence by successive cultures in 

 contact with air ; so that a given quantity of such culture 

 fluid causes effects far less severe than the same quantity 

 before such modification. Pasteur subsequently applied 

 the same principle to the mitigation of anthrax virus ; 

 indeed the list has been still further extended by himself 

 and others. Since in all these cases the same general 

 principle is illustrated, it will suffice for our present pur- 

 pose to consider the mitigation of anthrax virus for the 

 preventive vaccination of sheep. 



Pasteur's theory is this : the anthrax rods, as found in 

 the blood of an animal dead of the disease, when placed 

 in a suitable liquid maintained at a temperature of 42-43 

 C. grow as usual into threads, but do not produce spores. 

 After a certain time their vitality is lost ; when transferred 

 to another flask, kept tinder the same conditions, they do 

 5 



