CONSTANCY AND MUTABILITY OF FUNGI. 681 



remain unnoticed in a living condition in milk which had 

 been insufficiently sterilised ; and thus he showed that it was 

 probable that in these experiments we did not have to do with 

 variation in the characters of the same species, but with the 

 effect of different species of bacteria. 



The more the methods of pure cultivation of bacteria have 

 been improved, and the more complete our knowledge as to 

 the biology of the bacteria has become, so much the more 

 certainly do all other experiments' lead us to the conviction 

 that the specific vital characters of the lower fungi can be 

 retained in a similar manner as in the higher organisms, and 

 are not subject to any extensive variation. Even Buchner 

 in his more recent investigations recognises the physiological 

 attributes of the bacteria as sufficiently constant to enable 

 him to base on them a distinction between the individual 

 species. 



It is true that during a long period of time, just Formation of 

 as in the case of the higher plants, a real formation of van ' 

 varieties may occur in the lower fungi, the physiological 

 characters being chiefly affected. This variation does 

 not, however, arise as the direct result of definite ex- 

 ternal conditions, but is due originally to a certain 

 tendency to variation which often only leads to an 

 ephemeral existence of the somewhat abnormal ex- 

 amples, and thus to no important consequences ; at 

 times, however, when by chance the external conditions 

 are such tbat these abnormal examples are, as the result 

 of the abnormality in their physiological characters, 

 especially able to grow concurrently with other organ- 

 isms, and are not weaker than individuals which have 

 not these properties, we may have a continuation of 

 these abnormalities through a series of generations, and 

 thus the formation of a new variety. In many of the 

 lower fungi, however, the tendency to this form of 

 variation is extremely slight ; more especially we must 

 accept it as undoubted with regard to most of the ex- 

 citing agents of fermentation that they have retained 

 the same physiological characters and the same mode 

 of breaking up the fermenting materials for thousands 

 of years. 



In one respect, it is true, a character of the bacteria 

 has been observed which does not apparently follow the 



