144 BACTERIOLOGY. 



tion that pleomorphic species may be rela- 

 tively constant in form, and, conversely, that 

 the forms of simple species may be very vari- 

 able. Buchner, Gruber and MetschnikofT 

 have discovered important examples of this 

 rule. 



If the activities of bacteria be artificially 

 modified, if for instance, pathogenic bacteria 

 be so attenuated that they have little . or no 

 effect upon animals, in such a case morpho- 

 logical changes in the germ may be regarded 

 as degenerative ; the species may not only be 

 less virulent, but also grow more feebly. It 

 is possible, however, to destroy the power of 

 the " specific " action upon animal organisms 

 while the bacteria themselves remain vigorous 

 or indeed grow even more vigorously. In 

 some cases of this sort, disease germs adapt 

 themselves exclusively to decomposition and 

 become mere saprophytes. The ability of 

 bacteria to form pigments, fermentation- 

 products and poisons and to provoke disease 

 is proved by accurate investigation to be sim- 

 ply a quality of adaptation, and a knowledge 

 of this fact makes it possible for us to discuss 

 scientifically the question, involved as it is at 

 every point, of the causes of disease (Ch. IV.). 

 Koch's belief that the " cause " of disease 

 is the constancy of the " specific " disease-pro- 



