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 Loffler 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, etc. 

 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, 



