214 EVOLUTION AND DISEASE. 
Bacteria—using this term in a general sense—require 
high powers of the microscope and suitable methods of 
preparation for their satisfactory detection and identifica- 
tion: they appear as minute rods, or small rounded 
bodies ; the latter are usually termed micrococci. In 
order to prove that any given bacterium is the cause of 
a particular disease, it is necessary to ascertain its 
constant presence in the blood or tissues of animals 
suffering from the disease, and then to cultivate it in 
media apart from the living body through several 
generations ; on introducing the product of such cul- 
tivations into an animal, the disease should make its 
appearance and the micro-organisms occur in the 
lesions. 
Pathogenic bacteria, when introduced into an organism 
either accidentally or designedly, give rise to trouble 
which may be either local or general. The local signs 
are peculiar, for the disease may in some cases be 
restricted to the part at which the morbid agents were 
introduced, or the tissues at the seat of inoculation may 
serve as a focus in which the bacteria germinate and are 
subsequently disseminated through the system generally, 
producing profound disturbances and not infrequently 
death. 
Before studying in detail the effects of the introduction 
of bacteria into an organism, it will be necessary to 
review the leading facts connected with the evolution of 
the inflammatory process as manifested by a complex 
organism. 
The simplest animal known—the amceba—is a 
microscopic mass of nucleated protoplasm capable of 
