136 THE AMERICAN MONTHLY [May 
reason why they can not exist in nature elsewhere than 
upon or within the animal body. Hence, we find animal 
species living as parasites upon other animals, and hav- 
ing no individuals of their species living a non-parasitic 
existence. They have developed and have been modifi- 
ed since they began their existence as parasites, just as 
the species of animals living free in nature have been 
modified. Consequently, if an animal becomes infected 
with lice or mites at the present day it must get them 
from some other animal which bears them. 
The adaptation and modification of the bacteria and 
protozoa which cause the contagious diseases has proba- 
bly occurred in much the same manner as that of the lar- 
ger animal parasites which we have been considering. 
The glanders bacillus has lived a parasitic existence in 
the bodies of animals of the horse kind for many thousand 
years. It is no longer able to multiply or live for any 
considerable time in nature outside of the animal body. 
It is therefore a strictly parasitic organism. The bacilus 
of tuberculosis is even further developed as a parasite 
than the bacillus of glanders, as it is much more difficult 
to cultivate in the laboratory even under the most care- 
fully adjusted conditions. There is no reason to suppose 
that any bacilli exist in nature having the same biological 
characteristics as have the glanders and tuberculosis ba- 
- eilli. 
The exact form of the rabies virus has never been sat- 
isfactorily determined, but what we know of it leads to 
the conclusion that it is a parasitic organism of some kind, 
which has been modified by thousands of years of exist- 
ence within the animal body, and which has no counter- 
part elsewhere in nature. Inoculation with it is easy ; it 
has specialized as to the conditions of life to such an ex- 
tent that it multiplies only in the brain, spinal cord, nerve 
trunks, and a few glands; it can not be made to grow out- 
side of the body by any methods now known. All these 
