368 HOYT S. HOPKINS 



On the other hand, there is reason for believing that the 

 internal factors may be slowly modified with time, perhaps by 

 the prolonged adjustment of the organisms to a new environ- 

 ment. Strains which early in their history showed a marked 

 tendency to conjugate periodically lose this power as time pro- 

 gresses. Evidence bearing on this point has been presented: 

 for Par. caudatum, race 17a on page 349; race 38, page 352, 

 and for Par. aurelia, race 3, page 360; race 11, page 362, races 

 23 and 25, page 365. Those races which, early in their history, 

 showed a tendency to conjugate at frequent intervals, and in 

 large numbers at each epidemic, retain their susceptibility longer 

 . than those races which at first conjugated only at longer intervals 

 and in smaller numbers. The susceptibility of such cultures 

 in which conjugation is in abeyance can often be partially re- 

 stored by subjecting them to a long period of dormancy. Thus, 

 a culture of Par. aurelia, 23i, renewed on January 10, 1920, after 

 a five-month period of dormancy, gave conjugants on January 

 16th. From an exconjugant a new culture, 23k, was started, 

 and in this conjugation was repeated under essentially uniform 

 conditions. (In several new exconjugant lines, 231, (1) to (6), 

 however, conjugation did not occur.) This shows that the very 

 long dormant period to which the organisms of this race were 

 exposed had restored their power to conjugate periodically, as 

 in the original culture, 23a, of this race (p. 364). 



2. GROWTH FACTORS INFLUENCING CONJUGATION 



On the basis of the above observations, and by the aid of 

 suppplementary experiments, I shall attempt to analyze the 

 conditions which are known to influence the occurrence of con- 

 jugation in Paramecium. Three outstanding factors or relation- 

 ships call for explanation. These are, 1) the relation of fission- 

 rate to the onset of conjugation; 2) the state of dormancy, and, 

 3) the periodicity which characterizes freely conjugating strains. 



A comparative study of division-rate as such, in the various 

 races of Paramecium here treated, shows that there is no simple 

 correlation between high or low division-rate and a strong ten- 

 dency to conjugate. Among those races having a high rate of 



