294 HEREDITY IN THE PROTOZOA 



certain families whose members possessed a larger 

 number of nuclei but were smaller in diameter. 

 It is evident, therefore, that other factors than 

 nuclei number are also present. Possibly the amount 

 of chromatin in the nuclei of these families may be 

 different. 



Since selection was also effective in Arcella 

 dentata, which has consistently only two nuclei, the 

 results here cannot be ascribed to an increase in 

 the number of nuclei, but Jennings has suggested 

 for Difflugia that "the substances determining the 

 hereditary characters may be distributed with less 

 accuracy than in the higher organisms, so that the 

 two products of fission may often receive parts 

 that are not equivalent." Hegner thinks that this 

 may be true for Arcella and adds "The sudden 

 large heritable change (mutations) w^ould, according 

 to this suggestion, be due to large qualitative in- 

 equalities, and the smaller heritable variations to 

 smaller qualitative inequalities during nuclear di- 

 vision." Root (1918) has likewise carried out se- 

 lection experiments with Centropyxis and has re- 

 corded changes following selection. 



How these changes are brought about in the 

 Protozoa is by no means evident. Jennings has not 

 committed himself to any one interpretation. If 

 we were to accept an old and now discredited view 

 as to the influence of selection on any "fluctuating" 

 character, it might appear that that view was con- 

 firmed, namely, that any change that appears in 

 an individual will serve as a starting point for 



