CHAPTER X 



SUMMAKT 



It will be evident from the foregoing pages that practically 

 every character of bacteria is liable to vary at different times 

 and under different conditions. These variations are of two 

 kinds, spontaneous or "intrinsic" — that is to say due to 

 tendencies inherent in the organism itself — and impressed 

 as a result of external influences. These modifying influences 

 have been enumerated (Chapter II) and examples given of 

 the variations they produce. 



In many cases an organism may appear to vary although 

 no variation actually takes place, and in other cases what 

 appears to be a "spontaneous " variation is actually an " im- 

 pressed" variation due to external influences which have not 

 been recognised by the observer. These various sources of 

 error have been enumerated and discussed in Chapter III. 



A tendency to vary in a particular way — either spontane- 

 ously or in response to external stimuli — may be so charac- 

 teristic of a certain organism as to be in itself almost specific 

 in character, and so far from confusing its identity may 

 actually make this more apparent. The pleomorphism of 

 B. diphtheriae, the tendency of 8. scarlatinae to assume 

 a bacillary shape, the tendency of B. paratyphoid B to form 

 papillae on raffinose agar, will serve as examples. 



No single property of bacteria can be regarded as specific 

 nor does the occurrence of variation in respect to any one 

 quality or function, or to several of them simultaneously, 

 necessarily imply a loss of specific character on the part of the 

 organism concerned. This is well illustrated by the mor- 

 phology of bacteria. A certain appearance may be spoken 

 of as, "characteristic." This does not mean that it is in- 

 variable but merely that the organism shows a tendency to 



