ROBERT KOCH 377 



be expected between particles which border on the invisible. With re- 

 gard to these differences, I refer not only to the size and form of the 

 bacteria, but also to the conditions of their growth, which can be 

 best recognized by observing their situation and grouping. I there- 

 fore study not only the individual alone, but the whole group of 

 bacteria, and would, for example, consider a micrococcus which in 

 one species of animal occurred only in masses (i. e., in a zooglsea 

 form), as different from another which in the same variety of animal, 

 under the same conditions of life, was only met with as isolated in- 

 dividuals. Attention must also be paid to the physiological effect, of 

 which I scarcely know a more striking example than the case of 

 the bacillus and the chain-like micrococcus growing together in the 

 cellular tissue of the ear ; the one passing into the blood and penetrat- 

 ing into the white blood corpuscles, the other spreading out slowly 

 into the tissues in its vicinity and destroying everything around about ; 

 or again, the case of the septicaemic and pysemic micrococci of the 

 rabbit in their different relations to the blood; or lastly, the bacilli 

 only extending over the surface of the aural cartilage in the erysipe- 

 talous disease, as contrasted with the bacillus anthracis, likewise in- 

 oculated on the rabbit's ear, but quickly passing into the blood. 



As, however, there corresponds to each of the diseases investigated 

 a form of bacterium distinctly characterized by its physiological ac- 

 tion, by its conditions of growth, size, and form, which, however often 

 the disease be transmitted from one animal to another, always re- 

 mains the same and never passes over into another form, e. g., from 

 the spherical to the rod shaped, we must in the meantime regard these 

 different forms of pathogenic bacteria as distinct and constant species. 



This is, however, an assertion that will be much disputed by bot- 

 anists, to whose special province this subject really belongs. 



Amongst those botanists who have written against the subdivision 

 of bacteria into species, is Nageli, who says, *T have for ten years ex- 

 amined thousands of different forms of bacteria, and I have not yet 

 seen any absolute necessity for dividing them even into two distinct 

 species." 



Brefeld also states that he can only admit the existence of specific 

 forms justifying the formation of distinct species when the whole 

 history of development has been traced by cultivation from spore to 

 spore in the most nutritive fluids. 



