CAUSES OF DISEASE. 223 
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. 
