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 thjs evidence. 
In one species, Arcella polypora, 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 there were 
