No. 606] MUTATION IN DIDINIUM NASUTUM 359 



By referring to the table it will be seen that the mu- 

 tation investigated originated, as previously stated, at 

 the close of a period of extraordinarily high rate of fission 

 and immediately after a short period of very high death- 

 rate in which all but one of the lines died out. At the 

 beginning of this period and at the close of the preceding 

 period the individuals were very small and showed all 

 the characteristics in behavior common to individuals 

 about to conjugate. Whether or not anything in the 

 nature of a nuclear reorganization in preparation for con- 

 jugation occurred in these didinia is not known, but or- 

 dinarily such phenomena do not begin until some time 

 after union takes place in conjugating specimens. More- 

 over, the period between fissions was not long enough to 

 admit of much in the way of reorganization aside from 

 what ordinarily occurs during the process of fission. 

 Whether or not the ancestors of the mutants were actually 

 homozygous is not known. If they were not the mutation 

 may possibly have been due to a rearrangement of unit 

 characters represented in the chromosomes during fission 

 resulting in a change in dominance. However, if this did 

 actually take place it is not in accord with the results ob- 

 tained in very extensive investigations, all of which seem 

 to show that changes in dominance do not occur in asexual 

 reproduction. It is probable, therefore, that the mutation 

 was due to a direct effect of the environment on the physi- 

 ological processes in the organism and not to inherited 

 nuclear phenomena largely independent of the immediate 

 environment. 



The mutation theory so ably champioiiod by DoVries 

 has of late lost greatly in prestige, owing largely to the 

 contention that the plants {(Enothera) in which DeVries 

 discovered mutations were hybrids. If the conclusion 

 reached in this work proves to be correct it will strongly 

 support the theory in question. It will demonstrate that 

 marked variations may appear suddenly in organisms re- 

 producing asexually, that such variations may be heritable 

 and that they may have a decided evolutionary value. 



