526 



THE AMERICAN NATURALIST [Vol. XLII 



comprising individuals left over after the daily isolations, 

 there has been every opportunity for its occurrence. Of 

 course there has been no possibility of conjugation in the 

 four direct lines of the culture on account of the daily 

 isolations. This result agrees with that which I obtained 

 with my cultures of various hypotrichous forms— and I 

 believe it suggests strongly that conjugation must be re- 

 garded as a more or less variable phenomenon which 

 occurs in the life history when conditions are adverse for 

 the normal life of the organism, and which is not neces- 

 sary under the conditions of a varied environment. I 

 believe it is customary to regard conjugation as of far 

 more frequent occurrence than it actually is in the life 

 history of "wild" individuals, because it is brought to the 

 attention in laboratory cultures and "hay infusions" 

 which pass through a series of changes— changes which 

 inevitably bring about conditions unfavorable to the con- 

 tinued reproduction of the organisms, and which are com- 

 pensated for by conjugation. 



No period of marked physiological depression is indi- 

 cated by the division-rate of this culture during the first 

 year of its life; but well-defined morphological changes 

 have occurred. These cytological variations, chiefly 

 nuclear, demand further study. It is evident, however, 

 that the relation of the rate of division to the so-called 

 "normal" condition of the nuclear apparatus of Para- 

 mecium is not substantiated by this culture, as profound 

 nuclear changes apparently do not affect the rate of divi- 

 sion. I believe from a study of this culture and "wild" 

 cultures in large laboratory aquaria, that various nuclear 

 changes which are not at present recognized occur 

 normally in the life history of Paramecium, and that 

 possibly when conjugation between two individuals is 

 prevented, either under the conditions of culture or in the 

 "wild" state, a rearrangement of the nuclear apparatus 

 is resorted to which may be analogous to endogamy, or 

 conjugation of nuclei within the original cell. 



