VARIATION OF SPECIES 187 



cells of the bottom yeast continued to give a bottom fermentation 

 and those of top yeasts a top fermentation. This seemed to show 

 that there was no transformation from a bottom yeast into a top 

 yeast but probably a selection; at a temperature of 5 C., the cells 

 from the top yeast developed alone while the cells of the bottom 

 yeast remained stationary. Their properties however were not 

 modified. Saccharomyces lurbidans offers, then, as Hansen de- 

 scribed in 1883, characteristics of a bottom yeast. Such changes 

 have been induced since that time that part of the cells in a culture 

 cause a bottom fermentation and part a top fermentation. This 

 change had been induced spontaneously while the yeast was in Han- 

 sen's laboratory and without apparent cause. Furthermore this 

 characteristic seems to have been retained, since Hansen, on later 

 observations, found the same proportion of cells of both types of 

 yeast. This phenomenon is related to the mutations of de Vries. 



Studies on other bottom yeasts by Hansen have also given some 

 evidence of a change from bottom to top yeast. He isolated 1000 

 cells from the yeast Johannisberg II ', which, through a number of 

 generations, had given a bottom fermentation. He cultivated these 

 separately; 984 gave a true bottom fermentation while 16 produced 

 an intermediary fermentation. The cells inducing bottom fermenta- 

 tion tended to remain constant while those in the top fermentations 

 tended to be changed. 



Two of the 16 cultures which induced an intermediary fermenta- 

 tion, were studied further. Here are the results for one of them: of 

 100 cells, 5 gave a top fermentation, 55 an intermediary fermentation 

 and 40 a bottom fermentation. Other observations were made on the 

 5 cells which produced the top fermentation. Of 100 cells, 78 gave a 

 top fermentation, 9 an intermediary fermentation and 13 a bottom 

 fermentation. In this particular case there was a more or less definite 

 movement toward the top fermentation. Similar results have been 

 obtained by using ascospores in place of the vegetative cells. 



Saccharomyces carlbergensis and monascencis gave similar results. 



On the contrary, Hansen did not observe this tendency on the part 

 of the top yeasts to transform into bottom yeasts. They seemed to be 

 much more stable. He was not able to transform Saccharomyces vali- 

 dus, a typical top yeast, into one which would produce a bottom fer- 

 mentation. In one analysis, he could find only 3 " bottom cells " in 

 100 and he- could not increase this number. In another analysis, out 

 of 1529 cells only one was separated which induced bottom fermenta- 

 tion. With Saccharomyces cerevisiae, another top yeast, out of 2423 

 cells, Hansen found only 7 which gave a bottom fermentation and a 

 more careful study of the vegetation formed by these indicated that 



