VARIETIES OF ORGANISMS. 17 



made to resemble very closely in its cultural as- 

 pects another species of this organism. On the 

 whole, these are not large variations, and identifi- 

 cation can be accomplished by one or more of the 

 biologic reactions, such as the agglutination, bac- 

 teriolytic or opsonic tests. 



In some instances the conditions are such that varieties. 

 different species appear to represent only differ- 

 ent grades or types of virulence of the same or- 

 ganism. Those acid-fast bacilli which resemble 

 the bacillus of tuberculosis form the most striking 

 example of this. Bacilli of this character are com- 

 monly found in various grasses or clover, which 

 are used as food by cattle. Such organisms have 

 a low grade of virulence, and when injected into 

 animals cause the formation of only a nodule of 

 granulation tissue, sometimes with the presence 

 of giant cells, and the process heals readily. It is 

 possible that certain of these organisms of more 

 than usual virulence, or which may have acquired 

 such virulence by residence in the alimentary tract 

 of the ox, have retained their new pathogenic 

 power as a permanent character, and that bovine 

 tuberculosis originated in this way. It would be 

 none the less logical to assume that the bacillus 

 of human tuberculosis was derived from the bo- 

 vine type in a similar manner. This can only be 

 a subject for speculation, however. 



Bacteriologists frequently are able to place a 

 number of species in a "group," the members of 

 which resemble each other more or less closely, as 

 the colon group, or the dysentery group. The 

 members of a group may vary widely in their ^ 



pathogenic power, whereas in other instances they 

 produce similar diseases. Thus, Novy has shown 



