126 BACTERIOLOGY. 



down to Ferdinand Cohn, Josef Schroter and 

 Robert Koch there have always been investiga- 

 tors who supposed a rigid constancy of form 

 and species. These observers, moreover, ' 

 established genera and species as had been 

 done in the first instance, directly according 

 to form, and thus came about the conception 

 of form-genera and form-species, the individual 

 forms of which were supposed to be always the 

 same. So long as this idea prevailed, when- 

 ever novel forms were observed after an alter- 

 ation of the conditions of nutrition, they were 

 referred to the intrusion of germs of other 

 species ; the occurrence of unusual fermenta- 

 tive effects was also taken for a proof that for- 

 eign germs had stolen in. Definite forms and 

 uniform effects were supposed always to coin- 

 cide. Under the influence of this conception 

 F. Cohn divided bacteria, according to the 

 effects they produced, into pigment-forming or 

 chromogenic, fermentation-provoking or zymo- 

 genic and disease-producing or pathogenic. 

 Thus by the aid of physiological characteris- 

 tics, he obtained a more extended application 

 for his classification into form-species, espe- 

 cially as regards those forms that, on account 

 of their minuteness, did not admit of sharp 

 distinction. Many of the species thus obtained 

 are, it is true, collective, and with more 



