36 ADAPTATION AND DISEASE 



we can by careful passage through selected animals so augment 

 the virulence that eventually the hundredth or the thousandth 

 of a drop of a twelve-hour culture, or even much less than this, 

 may cause the death of strong adults in six hours or less. 



It is quite possible that these streptococci may eventually 

 afford us the best material for demonstrating the development 

 of pathogenic from harmless bacteria. They are singularly 

 widespread in nature : they are common on the skin, where 

 the temperature is lower than it is in the passages of the body ; 

 they are constantly. present in the mouth, and, as pointed out 

 by Andrewes, the majority of cultures obtained from the mouths 

 of healthy individuals are harmless when inoculated into lower 

 animals. Certain it is, also, that after long years of work and 

 the investigations of numerous trained observers, we seem to 

 be almost as far off as ever from establishing the dividing lines 

 between different species — if species proper they are — of strepto- 

 cocci. Of late, it is true, we are accustoming ourselves to recog- 

 nize and attribute particular properties to Streptococcus faecalis, 

 Streptococcus equinus, haemolytic streptococci, etc. ; we have 

 no doubt regarding the existence, and, what is more, the utility 

 of recognizing these " strains." But if we depend on fermenta- 

 tion tests (and it is only by biochemical and not by morpho- 

 logical properties that we can distinguish between the forms), 

 then, as shown by Ainley Walker, continued growth in the 

 laboratory results in changes of the fermentation tests, so that 

 there is as much evidence that we deal with the different strains 

 of one species which exhibit differences in consequence of their 

 previous habitat, environment, and history, as there is that 

 they constitute distinct species. For a time it looked as though 

 Rosenow, of Chicago, had definitely settled this point, and by 

 passage through animals had converted non-haemolytic into 

 haemolytic strains, and even streptococci into pneumococci 

 (which have many properties in common). But Holman, of 

 Pittsburgh, has brought forward such forcible criticism of Rose- 

 now's results, calling attention to the frequency of combined 

 growths of more than one strain or species of streptococci, and 

 the need for pure cultures gained from isolated cocci, that before 

 Rosenow's work can be accepted in its entirety, this laborious 

 confirmatory work must be undertaken. Saying this, I firmly 

 believe that Rosenow is along the right fines — and pragmatically 



