VARIATION AND HEREDITY 181 



which was formed by a complete fusion of two conjugants. This ab- 

 normal animal underwent fission 367 times for 405 days, but finally 

 reverted back to normal forms, without reversion to double form. 



Jennings (1941) outlined five types of long-lasting inherited 

 changes during vegetative reproduction, as follows: (1) changes that 

 occur in the course of normal life history, immaturity to sexual ma- 

 turity which involves many generations; (2) degenerative changes 

 resulting from existence under unfavorable conditions; (3) adaptive 

 changes or inherited acclimitization or immunity ; (4) changes which 

 are neither adaptive nor degenerative, occurring under specific en- 

 vironmental conditions; and (5) changes in form, size, and other 

 characters, which are apparently not due to environment. 



Whatever exact mechanism by which the long-lasting modifica- 

 tions are brought about may be, they are difficult to distinguish 

 from permanent modification or mutation, since they persist for 

 hundreds of generations, and cases of mutation have in most instan- 

 ces not been followed by sufficiently long enough pure-line cultures 

 to definitely establish them as such. 



Jollos observed that if Paramecium were subjected to environ- 

 mental change during late stages of conjugation, certain individuals, 

 if not all, become permanently changed. Possibly the recombining and 

 reorganizing nuclear materials are affected in such a way that the 

 hereditary constitution or genotype becomes altered. MacDougall 

 subjected Chilodonella uncinata to ultraviolet rays and produced 

 many changes which were placed in three groups: (1) abnormalities 

 which caused the death of the organism; (2) temporary variations 

 which disappeared by the third generation ; and (3) variations which 

 were inherited through successive generations and hence considered 

 as mutations. The mutants were triploid, tetraploid, and tailed 

 diploid forms (Fig. 86), which bred true for a variable length of time 

 in pure-line cultures, either being lost or dying off finally. The tailed 

 form differed from the normal form in the body shape, in the number 

 of ciliary rows and contractile vacuoles, and in the mode of move- 

 ment, but during conjugation it showed the diploid number of chro- 

 mosomes as in the typical form. The tailed mutant remained true 

 and underwent 20 conjugations during ten months. 



In biparental inheritance, the nuclei of two individuals partici- 

 pate in producing new combinations which would naturally bring 

 about diverse genetic constitutions. The new combination is ac- 

 complished either by sexual fusion in Sarcodina, Mastigophora, and 

 Sporozoa, or by conjugation in Euciliata and Suctoria. 



The genetics of sexual fusion is only known in a few forms. Perhaps 



