$ iv.] The question of species. 33 



hour. Those to which the conditions are unfavourable may be 

 seen, if a single specimen is watched continuously, to be dis- 

 solved and to disappear entirely in a few hours. By the com- 

 bination of phenomena of this kind the character of any given 

 mixture may be totally changed in a short space of time. 



It is obvious that difficulties such as these do not invalidate 

 our postulate, but on the contrary bring it out into sharper 

 relief. Those who altogether deny the existence of species in 

 Bacteria, with Billroth and Nageli at their head, have in fact 

 never undertaken a direct observation of continuity of develop- 

 ment, and they are therefore not justified in denying their exist- 

 ence. Billroth has accurately examined and compared the 

 forms, but has never continuously followed and checked the 

 changes in a preparation or culture ; the observation has been 

 interrupted by intervals of sufficient length to allow of various 

 things happening unobserved. Nageli, as far as can be gathered 

 from his publications, has not closely examined the forms at all, 

 but grounds his conclusions, even when they are morphological, 

 on non-morphological observations with respect to phenomena 

 of decomposition on the great scale. One instance of this mode 

 of dealing with the subject may be mentioned. Nageli remarks 

 that milk which has not been boiled turns sour when left standing 

 for a time, but that boiled milk becomes bitter (9). He admits 

 that the sourness is due to the presence of a Bacterium. He 

 considers the bitterness to be the result of a change in the 

 action of the same Bacterium caused by the boiling a ' trans- 

 formation of the definite ferment-nature of a single Fungus into 

 a ferment of another kind.' Here it is assumed that a single 

 Bacterium-species is present in the unboiled milk ; the question 

 is not asked whether there may not perhaps be several species in 

 it, some of which predominate before, the others after the boil- 

 ing, and whether the different changes in the milk may not be 

 thus explained. But Hueppe's more recent researches have 

 shown that such is really the true state of the case (10). Of the 

 various Bacteria-forms present in the unboiled milk, Micrococcus 



