i - 



Dr. E. W. A. Walker. 



[May 16, 



development of particular series of reactions. When a given streptococcus 

 has reached the particular (or one of the particular) series of reactions 

 favoured by particular surroundings, it is quite possible that under continued 

 cultivation in the same surroundings it may exhibit no changes of reaction 

 during long periods of time. And it might even be somewhat slow to change 

 when introduced into a new environment. 



It appears to me that this hypothesis of adaptation to the particular 

 environment affords a more probable explanation of the differences observed 

 among streptococci in the test reactions than the theory of fixed types. 

 For example, the differences observed between the common streptococci 

 of the saliva and those of the faeces may find their explanation along 

 these lines, since it must be presumed that the faecal streptococci originally 

 reach the intestine by descending from the mouth. The change apparently 

 is a very gradual one, for Andrewes and Horder state (1906, 7), with regard to 

 the reactions of these streptococcal types, that " S. salivarius passes by 

 insensible gradations into S. fcecalis." 



Further, it may be regarded as at least possible that the streptococci 

 present in saliva actually acquire the S. std i carina type (or types) during and 

 owing to their residence in the mouth. If this is not the case, how does so 

 comparatively great a degree of uniformity of reactions as is described come 

 to occur among the streptococci found in an open cavity like the mouth ? Or 

 again, how comes it that the common streptococcus of the air does not appear 

 among the types stated to be most common in the saliva ? 



The fact that, although the total number of types described in the saliva 

 and in faeces is very considerable (in saliva 48 and in faeces 40), yet the great 

 majority of all the streptococci found have been shown to fall into a few main 

 groups, itself suggests the possibility that a persistent adaptation goes on in 

 these situations, as a result of which the streptococci entering the mouth or 

 reaching the intestine constantly tend to assume particular series of 

 reactions. 



The findings in a number of my own earlier experiments led me to suggest 

 the possibility that in particular surroundings streptococci tend to assume 

 particular types in relation to Gordon's tests — in fact, that the reactions 

 differentiate particular environments rather than particular varieties of 

 streptococci. The evidence, which is somewhat striking and undoubtedly 

 suggestive, is shown in Tables V, VI, VII, and VIII. 



