FERTILITY 207 



allowed to conjugate. Most of the lines that descended 

 from several pairs showed no improvement but soon 

 died out. In only one case was an individual produced 

 that was benefited by the process. 



Jennings' results are, however, peculiar in one very 

 important respect. He did not use a race that had run 

 down as a result of a long succession of generations, but 

 a race that he had weakened by keeping under poor 

 conditions. We do not know that the result in this 

 case is the same as that in senile races or inbred races 

 of other workers. It is not certain that the hereditary 

 complex was affected in the way in which that complex 

 is changed by inbreeding. He may have injured some 

 other part of the mechanism. 



Jennings interprets conjugation in paramcecium to 

 mean that a recombination of the hereditary factors 

 takes place. Some of these combinations may be more 

 favorable for a given environment than are others. 

 Since these will produce more offspring, they will soon 

 become the predominant race. 



The next diagram (Fig. 100) will serve to recall the 

 principal facts in regard to conjugation in paramce- 

 cium. Two individuals are represented by black and 

 white circles. At the time of conjugation the small 

 or micronucleus in each divides (B), each then divides 

 again (C). Four nuclei are produced. One of these 

 micronuclei, the one that lies nearest the fusion point, 

 divides once more, and one of the halves passes into the 

 other individual and fuses there with another nucleus. 

 The process is mutual. Separation of the two indi- 

 viduals then takes place and two ex-con jugants are 

 formed. Each has a new double nucleus. This nu- 



