188 VARIATION OF SPECIES i 



they were mixed types which only tended very slightly to induce 

 bottom fermentation. 



These experiments show that the distinction set up between top and 

 bottom yeasts does not exist; in reality, one may find both cells of 

 top and bottom yeasts in the same culture. It may be that the 

 conditions in the environment favor the development of one type, as 

 with Saccharomyces turbidans incubated at 5 C. The bottom yeast 

 may thus change into a top yeast, and perhaps top yeasts into bottom 

 yeasts. 



In summarizing, it is apparent that the same species of yeast may 

 undergo important variations in morphology and function. Thus, a 

 new series of varieties and types may be created in which particular 

 characteristics are maintained for a certain time, be it indefinitely. 

 Thus also, with regard to physiological functions, the fermenting func- 

 tion is susceptible in a certain measure; of being enfeebled or increased, 

 or a top yeast may change into a bottom yeast. Is one justified, in 

 light of these data, in refusing to differentiate between species? Cer- 

 tainly not. We may use the temperature limits for the formation of 

 ascospores, scum and for budding, and the action toward different 

 sugars. The characteristics of the ascospores and germination main- 

 tain themselves without undergoing modification. We may conclude 

 that if the species is not definitely constant in yeast, it is as constant 

 as with the other plants. More difficulties are encountered with the 

 yeasts on account of the variability of a great number of character- 

 istics. 



"All of the variations of species seem to be more doubted than 

 with the higher plants; variations are more rapid with the yeasts. 

 Further, the specific characters are less definite than in the higher 

 plants and one may distinguish less easily those which are constant 

 and specific from those which are not. One may encounter yeasts 

 scarcely more variable than the higher plants, but their generation 

 time is much shorter and, consequently, the phenomena of variation 

 appear more quickly. Here the investigator may be witness to re- 

 markable transformations in a short time." Such are the words which 

 Hansen has used with regard to this question. 



