VARIATIONS AND VARIETIES 201 



one of the commonest phenomena noticed among disease- 

 producing bacteria. And we also know that by artificial 

 procedures many of the highly; pathogenic organisms may 

 be in part or wholly deprived of their powers to cause the 

 lesions peculiar to the activities of the normal organisms, 

 retaining at the same time all other peculiarities common to 

 the species. 



The manifold studies upon infection and immunity have 

 placed at our disposal methods by which it is possible to 

 detect differences among closely related types of the same 

 species that cannot be revealed in any other way. 



Such differences appear to be idioplasmic, if the word is 

 appropriate to bacteria, and though slight quantitative 

 fluctuations may be noted, the strains characterized by them 

 have them as fixed, inherent peculiarities. 



We may regard them therefore not as indicating modi- 

 fications of a component common to the species, but rather 

 as specific components possessed by some varieties of a 

 species and not by others; not necessarily as new characters 

 evolving from environmental influences, insofar as can be 

 determined, but as natural, fundamental components revealed 

 only by newer adequate methods of investigation. 



Through the use of certain immunologic methods or tests 

 it is now possible to subdivide most of the pathogenic species 

 into distinct sub-groups the one group differing from the 

 other only in the presence or absence of those components 

 necessary to complete the differential reactions. 



To make this clear: If one immunize an animal from 

 Bacillus typhosus the blood serum of such animal if brought 

 together with Bacillus typhosus causes the bacilli to gather 

 in distinct clumps, whereas, if such serum be brought in 

 contact with Bacillus coli communis, or with Bacillus dysen- 



