206 



HEREDITY AND SEX 



SHOWING DECREASE OF VIGOR, AS MEASURED BY VARIOUS CHAR- 

 ACTERS, IN Six SUCCESSIVELY INBRED PARTHENOGENETIC LINES 

 OF Hydatina senta 



number of times the first daughter was too weak to 

 become the mother of a new line. It is clear that 

 inbreeding did not lead to an increase in vigor. 



In paramcecium there is also some new evidence. 

 Calkins in 1904 brought about the conjugation of two in- 

 dividuals of a weak race in the 354th generation. From 

 one of the conjugants a new line was obtained that 

 went through another cycle of at least 376 generations 

 in culture, while during the same time and under sim- 

 ilar conditions the weakened race from which the con- 

 jugants were derived underwent only 277 generations. 



Jennings has recently reported an experiment in 

 which some paramcecia, intentionally weakened by 

 breeding in a small amount of culture fluid, were 



