THE CLASSIFICATION OF BACTERIA. 261 
We can hardly imagine that the multitude of bacte- 
rial varieties which now exist have always existed. 
The probability is very strong, that with succeeding 
generations and changing conditions new bacterial 
varieties have developed with new characteristics. 
From time to time the changing conditions under 
which life progressed probably exposed certain animals 
to the invasion of varieties which never before had 
gained access to them. If the bacteria found the soil 
suitable, and also some means of transmission to other 
animals equally susceptible, a pathogenic species became 
established which at first, perhaps, found conditions 
only occasionally favorable to it, but later became more 
parasitic in its characteristics. Thus in some such way 
a multitude of bacterial groups arose, some of which 
accustomed themselves to the conditions present in the 
soil, others to those in fishes, others to those in birds, 
and others still to those in man. 
These are, however, theories—what has been actually 
observed in the few years during which bacteria have 
been studied? In this short time the pathogenic spe- 
cies as observed in disease have kept practically unal- 
tered. The diphtheria bacilli are the same to-day as 
when Léffler discovered them in 1884, and the dis- 
ease itself is evidently the same as history shows it to 
have been before the time of Christ. The same is true 
for tuberculosis, smallpox, hydrophobia, leprosy, ete. 
Under practically unchanged conditions, therefore, as 
exist in the bodies of men, bacteria which have once 
become established as parasites continue so long as they 
remain to retain their peculiar (specific) characteristics. 
Whether new disease varieties, such as the influenza 
bacillus, are coming into existence from time to time, is, 
