CA USES OF DISEASE. 2 2 3 



days, as is seen in the various forms of malarial 

 fevers. 



The variations in the periods of incubation, eruption, 

 and crisis, correspond with the life-history of the various 

 micro-organisms as ascertained by means of experiments. 

 When bacteria are introduced into cultivation-media, 

 such as solutions of gelatine, agar-agar, chicken-broth, 

 and the like, there is always a latent period which varies 

 with different bacteria. When the micro-organisms com- 

 mence to grow it is often possible to identify its nature 

 from the mode of growth and behaviour towards the 

 medium, independently of its microscopical characters. 



Inasmuch as the varieties of bacteria present intrinsic 

 differences in this respect, it is not unreasonable to sup- 

 pose that the variations in the period of incubation, 

 character of the eruption, and effects upon an organism 

 should vary according to the nature of the parasite intro- 

 duced. 



Bacteria, like other forms of life, present extreme 

 variety, and the differences between innocent and malig- 

 nant forms of bacteria, in so far as their effects upon an 

 organism is concerned, are very great. Nevertheless, we 

 can pass by insensible gradations from one form to the 

 other : indeed, the history of micro-organisms shows 

 clearly enough, as in the case of animal parasites, that 

 pathogenic bacteria have been slowly evolved from non- 

 pathogenic forms, and have slowly acquired the power of 

 flourishing upon living bodies when the environment is 

 suitable. This, of course, raises the all-important ques- 

 tion, What is suitable environment? Micro-organisms 

 exhibit wonderful vitality and seem to be omnipresent. 



