THE WORLD OF MICROSCOPIC LIFE 49 



mals down to and including the Protozoa them- 

 selves. 



It is with the minute forms of plant life that the 

 researches of Pasteur were mainly concerned, and 

 the importance of these greatly exceeds that of the 

 Protozoa, great as this may be. There can be no 

 doubt that in the absence of these primitive plants, 

 all higher life on the globe would be impossible. 

 Like the Protozoa, the Protophyta or one-celled 

 plants, belong to many groups. There are the 

 minute green algae, which are common inhabitants 

 of both fresh and salt water, the more primitive 

 blue-green algae, the diatoms with their beautifully 

 sculptured silicious shells, the desmids, and the 

 many other forms which we cannot even mention 

 in this cursory sketch. 



One group of primitive fungi, the yeast plants, 

 are, however, of more than usual interest to us in 

 this history on account of their relation to the 

 problem of fermentation studied by Pasteur. Man- 

 kind has made use of yeasts from the earliest times 

 without suspecting that their activity is due to the 

 life of minute plants. Examination of actively fer- 

 menting beer or wine, and especially the growth 

 that often appears floating upon the surface, re- 

 veals multitudes of spherical or oval bodies which 



