HEREDITY IN THE PROTOZOA 293 



ful, therefore, whether one can safely appeal to 

 unequal division of the chromidial mass as the 

 means by which the selective process becomes 

 effective. On the other hand, there is evidence 

 from other observations that Hegner has made 

 that may seem to furnish at least a clue to the 

 agent through which the selective process takes 

 place. Let us examine this evidence. 



In one species, Arcella j^olypora, there is a variable 

 number of nuclei ranging from 3 to 10. The di- 

 ameter of the shells correspondingly ranges from 25 

 to 33 units. The number of these nuclei may 

 change, and if large or small individuals are selected 

 it is found that the larger individuals have more 

 nuclei than the smaller ones. The change takes 

 place before or during the selection period, but is 

 not supposed to be directly affected by the selection 

 itself. The larger individuals that are picked are 

 larger because they have more nuclei, and the 

 small ones are smaller because they have lost one 

 or more nuclei. The change occurs at irregular 

 intervals, and consists generally in an increase or 

 decrease of one nucleus. "In several cases the 

 number of nuclei doubled from 3 to 6." It was 

 found also that the diameter of the shell and the 

 number of spines are closely correlated, so that 

 when selection was made for more spines, the size 

 also increased, and vice versa. It is possible, there- 

 fore, that when selection of specimens took place, 

 what was really selected was variations in the 

 total nuclear mass. It is true that tliere were 



