514 G. N. CALKINS AND L. H. GREGORY 



The conclusion is evident from these data that variations in 

 size of Paramecium caudatum do not furnish sufficient evidence 

 to warrant the conclusion that variations are the result of am- 

 phimixis or, a fortiori, that variability is the primary purpose of 

 conjugation. In order to obtain a basis for such an argument, 

 many series similar to our series J, should be studied in the same 

 way, and if then the modes should fall in different regions of the 

 curve (as they do when aurelia and caudatum are compared), 

 then there might be justifiable grounds for conclusions as to 

 heredity of size. 



E. Rejuvenation or variability f 



The fundamental purpose of conjugation offers an interesting 

 problem for speculation almost as endless apparently, as the vital- 

 ity of Paramecium. The matter has been so frequently discussed 

 in papers by Jennings, Woodruff, Calkins and a host of others 

 that we shall limit the present discussion to the consideration of 

 points raised by the observations made in recent experiments. 



Jennings, whose careful and thorough methods of experimen- 

 tation cannot be criticized, has recently come to the conclusion 

 that ''conjugation does not rejuvenate in any simple, direct way. 

 What it does is to produce variation; to produce a greater num- 

 ber of different combinations, having different properties" ('12, 

 p. 573). This conclusion was based largely on the experiment 

 of dividing a conjugating race into two portions. In one of these 

 portions the individuals were allowed to conjugate, and did con- 

 jugate four times in the course of a number of months. The in- 

 dividuals of the other portion were cultivated during the same 

 period in slides and were not permitted to conjugate. The latter 

 series weakened; the individuals "multiplied slowly and irregu- 

 larly and many died." Some of them were then allowed to 

 conjugate with the result that some strains appeared that were 

 more vigorous than the parents; others were weak and died out 

 but even the stronger ones ''were notably less vigorous than the 

 strains which had been kept throughout under more natural con- 

 ditions and had conjugated frequently." We believe that these 



