A.\D HKKEDS OF BA( Tl'.RIA 



1 1 1 



revolutionary investigations on yeasts that many different species of SaccJiaro- 

 niyccs were recogni/cd. The SaccJiaromyccs ccrci'isiac of beer- wort, and 

 the vS". cllipsoiiicns of vinous fermentation, and a few others, were the only 

 species known. To-day the species S. ccrci'isinc is divided into hundreds 

 of ' races ' or varieties, and the same has happened to the wine ferments. In 

 these technical processes, some of which arc as old as human culture itself, 

 changes have been going on for thousands of years similar to those changes 

 we have caused artificially and intentionally in our domesticated plants 

 and animals. But while we have no difficulty now in distinguishing from 

 one another the innumerable kinds of cultivated plants, the determination 

 of racial characters among micro-organisms is a task of great difficulty. 

 Morphological data are almost useless, and we have to fall back upon physio- 

 logical peculiarities such as the degree of warmth required (different optima), 

 the nature and amount of by-products, or the specific fermentative power. 



FlG. 24. Fermentation bacteria, a-c, acetic bacteria trom E. Chr. Hansen. a, Bacillus ace/i ; &, B. Pas- 

 tc nrian its , c, B. KutsinfianUS \ d, B. acidi lactici\ the commonest cause of lactic fermentation ; e, Closiridium 

 biityricum, an anaerobic butyric ferment, ' granulose ' bearing ; to the right spore in spindle-shaped rod. _/| PlfC- 

 tridhun pahidosuni, anaerobic ferment from marsh water, corresponds in form to the methane bacteria, and to 

 some of the butyric bacteria. Magn. a-f 1000. 



Moreover, it must not be forgotten that new varieties are continually 

 arising, old ones dying out and being replaced by others with recently 

 acquired powers. The shortness of the time necessary for the production 

 of a new race of organisms is shown by such plants as the chrysanthemum 

 and the potato. Although the culture of the potato is only two or three 

 hundred years old, there are no less than five hundred varieties in exis- 

 tence, distinguished by shape and colour, by their flavour, by the relative 

 amount of albumen and starch, and other points. Among bacteria, too, 

 there are forms that, like the yeasts, have been cultivated from time 

 immemorial in cheese-making, for instance, and in vinegar fermentation. 

 These are now cultivated in pure form in our laboratories. 



The difficulty of classification among micro-organisms is further 

 increased by the fact that many morphologically different species possess 



