252 BACTERIA IN INFECTIOUS DISEASES. 



must produce similar results; and there can be 

 little doubt that physiological varieties, or breeds, 

 of the different species of bacteria are constantly 

 being produced and destroyed by the operation of 

 natural causes. Under the influence of a favorable 

 temperature and of abundant pabulum, these mi- 

 nute plants multiply abundantly ; and, in accord- 

 ance with the laws of natural selection, there must 

 be a constant tendency among them to develop 

 those characters which are most favorable to their 

 preservation, e. g., a capacity for rapid multi- 

 plication, and to adapt themselves to their envi- 

 ronment. 



If we suppose that under certain circumstances 

 the conditions relating to environment approach 

 those which would be found within the body of a 

 living animal, we can easily understand how a 

 micro-organism, which has adapted itself to these 

 conditions, may become a pathogenic organism, 

 when by any chance it is introduced into the cir- 

 culation of such an animal. The culture-fluid 

 blood and temperature being favorable, it is only 

 a question of superiority by vital resistance on the 

 one hand, or by reproductive activity on the 

 other. 



That harmless species of bacteria may develop 

 pathogenic properties in the manner indicated, 

 seems extremely probable ; and we should a priori 

 expect that such a result would occur more 

 frequently in the tropics, where the elevated 

 temperature and abundance of organic pabulum 



