92 RESEARCHES ON YEASTS. 



beers less stable than the others at this stage, although at 

 the end of the storage period they are far more stable. 



4. ON VARIATION. 



The characters which we make use of for distinguishing 

 different species of animals and plants have no absolute 

 validity, but are constant only under certain conditions. It 

 is especially to the epoch-making works of Darwin, that we 

 are indebted for the doctrine of variation. The greatest and 

 most difficult work, however, still remains to be accomplished, 

 namely, the determination of the active factors with a view 

 to the final elucidation of the laws regulating variation. 



As long ago as 1883, I pointed out in some of my first 

 studies on the Saccharomycetes how varieties could be pro- 

 duced under different conditions, and by degrees I published 

 a series of communications on the subject, more especially 

 interwoven in my researches in the physiology and mor- 

 phology of alcoholic ferments quoted above. 



The changes may be partly of a more or less temporary 

 nature, and partly permanent ; in the latter case, the pro- 

 perties being reproduced through endless generations and 

 under various conditions of culture. In the following, I give 

 examples of both kinds taken from my investigations, and 

 I have selected such as may prove of interest not only to the 

 theorist, but also to the practical zymotechnologist. Those 

 readers who wish for further information, are referred to the 

 original treatises. 



If we wish to prepare a pitching yeast for industrial 

 application, it is not sufficient that it is an absolutely pure 

 culture of the desired race of yeast, but it must also be in 

 such a condition that it will behave in a normal manner in 

 practice, either at once, or at any rate after a very short time. 

 In breweries, the question of attenuation and brightening 

 plays an important part. Imperfect brightening and strong 



