90 ROBERT W. HEGNER 



the characteristics of the shell. No stages appeared in my cul- 

 tures resembling those described by the investigators mentioned 

 above, so it was impossible to determine whether the removal of 

 chromidia has any effect upon the formation of secondary nuclei 

 and upon the characteristics of the generations that appear after 

 conjugation. 



8. The relations between internal changes and external characteristics 



One of the most striking results of the cutting experiments 

 performed on Arcella dentata is the discovery that a large inter- 

 nal change is only slowly expressed by the external characters. 

 Thus, when a uninucleate specimen becomes binucleate, its sub- 

 sequent offspring, which are also binucleate, do not at once be- 

 come as large nor possess as many spines as normal binucleates, 

 but show only a small increase in these respects over their par- 

 ent. Additional increases in the second, third, and fourth gen- 

 erations, however, finally lead to the attainment of the normal 

 binucleate condition, and when this state is reached no further 

 increase takes place. 



This gradual change following nuclear doubling looks very 

 much like variation in a definite direction (orthogenesis) , and, if 

 the nuclei could not be seen, as is the case in most of the other 

 shelled rhizopods, one might interpret the results as due to suc- 

 cessive internal changes rather than to a single modification. 

 The writer believes that heritable variations are all discontinuous, 

 but that they differ from one another in degree. The reactions 

 of Arcella to internal changes indicate, however, that unless nu- 

 clear conditions are closely followed, it is impossible to deter- 

 mine in shelled rhizopods and other similar organisms whether 

 gradual variations in a definite direction are due to small or to 

 large internal differences. Such variations, therefore, as those 

 observed by Jennings in Difflugia and by Root in Centropyxis 

 may have been due to abrupt changes in the nuclear condition 

 that were gradually being expressed in successive generations by 

 the external characters. That is, they were really what are gen- 

 erally known as mutations and not variations of the Darwinian 

 type. 



