CONSTANCY AND MUTABILITY OF FUNGI. 677 



differentiate the bacteria on the basis of their morpho- 

 logical characters, more especially as in the case of the 

 bacteria we have to deal with extremely slight and deli" 

 cate differences in form ; and hence we must give up the 

 former classifications based essentially on such differences. 

 But as a matter of fact these various alterations in form 

 do not lead to the disappearance of the species, but only 

 serve, by the use of more complete means of observation, 

 for a more accurate differentiation of the individual 

 species. 



After the lapse of long periods of time it is probable True varieties 

 that a third kind of morphological alteration may occur 

 in the lower fungi, and lead to the formation of varieties 

 and new species ; these alterations, however, appear as 

 a rule to occur in a similarly slow and inappreciable 

 manner as in the higher plants. Very distinct evidence 

 has recently been produced which shows that the form 

 of certain bacteria has undergone extraordinarily little 

 variation even in the course of hundreds and thousands 

 of years. In thin sections of the petrified roots of coni- 

 fers from the coal period, van Tieghem* was able to 

 demonstrate the characteristic butyric acid bacillus; 

 and Zopf and Miller t found in the tartar on the teeth 

 of Egyptian mummies the same forms of bacteria which 

 can at the present time be demonstrated as the ordinary 

 inhabitants of the mouth. These examples, however, 

 are only of value for the species in question, and we 

 cannot without further information draw the conclusion 

 that all fungi have a similarly slight tendency to undergo 

 variation. 



Nageli, Buchner, Wernich, and others have formerly Differences in 



asserted that under the influence of external conditions a * he views as 

 , , . ,. . p , ,, . . to the van- 



very marked variation in torm occurs, and that one species ability of 



can be converted by variations in the conditions of its form, 

 existence into another species, characterised by other mor- 

 phological characters and other modes of growth (thus the 

 anthrax bacilli were converted into a so-called transition 

 form, and then into true hay bacilli, see p. 241). The 



* Compt. rend., 1879. f Arch.f. exp. Pathol. u. Pharm., 1882. 



