Variations in Paramecium Aurelia. 



65 



The production of new lines is caused by endomixis, the reorgani- 

 zation process in Paramecium, so closely related to the sexual 

 phenomenon of conjugation and still so essentially different from 

 this process by being effected in one cell and not affording the 

 introduction of foreign chromatin into the cell. We have to 

 conclude that even in so called "asexually" propagated lines 

 heritable variations can be produced by directed selection. But 

 why do these new lines not oftener appear in nature or in the 

 laboratory? First different observers have given different meas- 

 urements for strains of Paramecium, isolated from populations; 

 Jollos reports the breaking up of a line, and Jennings the appear- 

 ance of lines with new heritable characters after conjugation. 

 Owing to the difficult culture conditions he could, at that time, 

 not prove definitely that the breaking up in different new lines 

 is effected by conjugation. Now it is possible to do so. But 

 the role of the organization process in question is twofold: En- 

 domixis gives rise to new combinations that can be selected, but 

 also stabilizes the line at each occurrence. If variations in mean 

 length appear between 37 and 45 units, those lines will thrive best 

 either in mass cultures or isolated lines, which can adapt them- 

 selves the quickest to the given environment. I have observed 

 that all descendants of the O lines with 37, 43 and 45 units die in 

 the constant environment I choose for all these experiments. 

 The 41 mean length and 11 mean breadth had the best chances 

 to survive in the same environment. The uniformity, a word 

 that should be better used than constancy in the appearances of 

 the genotype, is a product of the influences of the chosen environ- 

 ment and of the adaptability of the different recombinations 

 appearing after endomixis. These recombinations will become 

 "heritable," if the environment remains nearly constant, else 

 other recombinations appear after endomixis that are better 

 adapted to the new conditions. 



What specific effects conjugation, the other reorganization 

 process, has in the life of Paramecium is still unknown. 



8 Jollos, V., Biol. Cenlralbl., 33, 222-236. 



