RESEARCHES ON YEASTS. 101 



better, but was always less stable (in the sense used above) 

 than that obtained by means of the original yeast. 



It is now several years since I succeeded in producing the 

 first varieties or possibly, even new species of Saccharo- 

 myces. Although they have since been cultivated under very 

 different conditions, they have remained constant ; the newly- 

 acquired properties have been perpetuated through numerous 

 generations.* 1 



On glancing at the results described in this chapter, it is 

 seen that the following conditions determined the direction in 

 which my studies had to be continued : In order to obtain a 

 definite starting point, it was, in the first place, necessary to 

 ensure the absolute purity of the cultures experimented with ; 

 I therefore elaborated the methods described on p. 5, and by 

 means of which I was enabled to start always from the single 

 cell. The next point was to investigate more closely whether 

 there were differences in the cultures so obtained, and, if so, 

 to determine further what the differences were. Since the 

 characters given by previous workers for the recognition of 

 species are false, it became necessary to discover new ones, 

 and for this purpose I treated the question from a botanical 

 standpoint. It was necessary to complete these studies 

 before an experimental investigation of the complicated ques- 

 tions of variation could be successfully undertaken. Since 

 1882 I have from time to time made experiments in this 

 direction, but it was only in 1888 that my investigations were 

 sufficiently advanced to enable me to take up this new problem 

 as a main point in my plan of work. This does not, however, 

 in the least imply that I gave up my old line of study. 



The above investigations on the question of variation 

 again opened up new paths for the study of the Saccharo- 

 mycetes. Though the results hitherto obtained are at present 

 essentially of theoretical interest, yet some of them have 



* In a special treatise on which I am at the present time engaged, I will give 

 a detailed account of the variation phenomena and of the factors and laws which 

 influence them. 



