THE CAUSE OF INFECTIOUS DISEASE. 269 



tions create a larger number of the same or 

 similar dispositions toward disease. 



That the " specificity " of the disease germs 

 is a phenomenon of adaptation, and hence not 

 an essence is manifest also from the fact that 

 the parasites adapt themselves to given condi- 

 tions of life not only in their mode of action 

 but in their form. Koch has shown that the 

 anthrax bacteria develop their characteristic 

 form of rods only in their parasitic phase. The 

 tubercle bacilli have such a strongly marked 

 capacity of adaptation that Maffucci and Koch 

 even distinguished as separate species or vari- 

 eties the germs of mammalian and avian tu- 

 berculosis. Fischel and Hueppe, however, by 

 the choice of suitable parasitic and saprophy tic 

 conditions of life succeeded in converting each 

 kind, one into the other, and thus in proving 

 that it is the similarity or difference in condi- 

 tions which ultimately brings about the great 

 divergence. 



Finally,, in other cases of which accurate 

 studies were made long since, especially among 

 the higher animal and plant parasites, a close 

 adaptation to the conditions of life is manifested 

 in the fact that a parasite, in order to complete 

 its development, needs not only an interchange 

 of parasitic and saprophytic modes of life, like 

 the facultative parasites among bacteria, but 



