38 PRINCIPLES OF BACTERIOLOGY 



is at times arbitrary in placing some varieties under one generic division 

 and others closely allied in another. It has also the objection, already 

 noted, that it is only one of several classifications already in use, and 

 until an authoritative body agrees on some one it seems unwise in 

 such a volume as this to change the usually employed names for 

 others which are, perhaps, intrinsically better. Another important 

 reason for waiting is that with the increase of our knowledge we are 

 constantly changing the position of different bacteria. Thus, such a 

 well-known germ as the tubercle bacillus is now found to produce, 

 under certain conditions, long, thread-like branching forms ; so that it 

 ceases to be under the classification of Migula, either a bacillus or bac- 

 terium. We will, therefore, simply use in this book the older, less scien- 

 tific nomenclature, of classing all rod forms as bacilli and all spiral 

 forms as spirilla, and consider together, in so far as is practicable, cer- 

 tain groups of bacteria whose members are closely allied to each other 

 in some one or more important directions. 



Permanence of Bacterial Species. When we come to study special 

 varieties or groups of bacteria, such as the bacilli which produce typhoid 

 fever, diphtheria, and tuberculosis, it is of great importance for us to 

 determine, if possible, to what extent the peculiar characteristics which 

 each of these groups of bacteria possess are permanent in the generations 

 which develop from them. 



We cannot believe that the multitude of bacterial 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 some 

 means of transmission to other animals equally susceptible, a parasitic 

 species became established which at first, perhaps, found conditions 

 only occasionally favorable to it. Thus in some such way a multitude 

 of bacterial groups arose, some of which accustomed themselves to 

 the conditions present in living plants, others to those in fishes, others 

 to those in birds, and others still to those in man and the higher 

 animals. 



These are, however, theories. W 7 hat has been actually observed in the 

 few years during which bacteria have been studied? In this short time 

 the pathogenic species as observed in disease have remained practically 

 unaltered. The diphtheria bacilli are the same to-day as when Loeffler 

 discovered them in 1884, and the disease itself is evidently the same as 

 history shows it to have been before the time of Christ. The same 

 permanence of disease type is true for tuberculosis, smallpox, hydro- 

 phobia, leprosy, etc. Under practically unchanged conditions, there- 

 fore, 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, 



