30 A CONSIDERATION OF THE EVIDENCE [CH. in 



alone such as its morphology where naked eye and micro- 

 scopic appearances are relied upon, or its virulence in the 

 case of animal inoculation, or its fermenting power when the 

 culture is tested by being "put through the sugars." The 

 organism may be atypical in respect to the particular character 

 the observer depends upon for its detection, and its subsequent 

 discovery will then lead to erroneous conclusions. A knowledge 

 of the extent to which organisms may be atypical in one or 

 other character is the best safeguard against such an over- 

 sight. 



The most thorough identification is demanded at the con- 

 clusion of an experiment no less than at its commencement, and 

 the strictest rules must be observed before the continuity of 

 two forms differing from each other is regarded as established. 

 Such continuity may be impossible to prove even when we 

 are dealing with a " pure culture." A certain number of 

 organisms in a pure culture may undergo variation while the 

 rest of the strain remain true to type. From time to time, 

 as the conditions of growth change, now the variants may 

 predominate almost to the exclusion of the original stock, 

 and now the original stock may predominate almost to the 

 exclusion of the variants, so that, following the variation, 

 reversion may appear to take place, and yet there may 

 actually be no continuity in the latter case between the 

 variants which are dying out and the original stock which is 

 again asserting itself. 



3. In the living tissues the possibility of secondary in- 

 vasion must be borne in mind. For example, the leptothrix 

 forms which McDonald (1908) describes in the spinal fluid 

 in cerebrospinal fever, as this writer himself recognises, 

 cannot be regarded as morphological variants of the meningo- 

 coccus without definite proof of identity. 



Again, the pathogenic effects in a given case must not be 

 attributed to an organism isolated from the tissues unless 

 adequate proof is forthcoming of its being in fact the cause 

 and not a secondary invader. 



Forbes in 1903 drew attention to the frequency with which 

 diphtheria bacilli were to be found in the ear discharges of 



