Induced Mutational Changes in Yeast 133 



nutrition and to changed conditions of existence that lead to 

 alterations in the chain of biochemical reactions and in the 

 corresponding structural elements of the cell. This conclusion 

 is very simple and at the same time natural, since the con- 

 stantly observed close interrelationship of the organism with 

 the conditions of its existence testifies to the fact that these 

 conditions are not only the most essential source of develop- 

 ment but are also the cause of changes in the organism. The 

 experimental data on the increase of the fermentative 

 properties of yeast, the increase being connected with the 

 acquisition of new hereditary stable functions (under the 

 influence of the specific substrate), confirms this assumption. 

 These are, undoubtedly, positive hereditary changes. The evolu- 

 tional significance of this type of change is unquestionable. 



Directed changes in the ability to ferment galactose under 

 the influence of the substrate (galactose) were observed by 

 Lindegren and Pittman (1953). Lindegren (1956) believes that 

 what happened, in this case, was the restoration of the 

 recessive locus which evidently had formerly been active but 

 had been damaged and inactivated ; so that the non -functioning 

 recessive gene again became functional and active (dominant). 

 Although in this case we speak about restoration of the lost 

 function and, according to Lindegren, restoration takes place 

 due to mutations under the influence of galactose but not of 

 lactose, we failed to detect this type of mutation occurring 

 spontaneously. These data, if considered along the lines 

 already mentioned, can be considered as confirmation of the 

 directed hereditary changes of the fermentative properties of 

 yeast, under the influence of the specific substrate, which 

 were observed in our experiments (Kossikov, 1948, 1951, 

 1952). 



Conclusion 



(1) Our experiments carried out in the course of the last 

 eight years, on directed changes of fermentative properties of 

 S. globosus, S. paradoxus and other yeasts under the influence 

 of a specific substrate, have shown that in these cases single 



