Diversity Produced by Conjugation 163 



The descendants of the ex-con jugants do not by any 

 means always show some families that exceed the fission 

 rate in the descendants of the non-conjugants. In nine ex- 

 tensive cultures (described in my paper of 1913, page 364) 

 in which this matter was studied, in but three did the 

 maximum for the con jugants exceed that for the non-con- 

 jugants, while in all cases the minimum and the average for 

 the conjugants were below those for the non-conjugants. 



We have set forth the relations found in Paramecium, the 

 organism most thoroughly studied from this point of view. 

 In Uroleptus, according to Calkins, those that have conju- 

 gated usually show an increased rate of reproduction. The 

 production among the combinations resulting from mating, 

 of families with an increased vigor of reproduction (usual 

 in some infusoria, occasional or rare in others) of course 

 justifies the statement that in those cases conjugation has 

 caused rejuvenescence, as maintained by Calkins. Consis- 

 tency with the facts compels one, however, to say that while 

 mating sometimes produces rejuvenescence, in most of the 

 individuals that mate, it produces, in the majority of the 

 known species, not rejuvenescence, but its opposite, the ani- 

 mals being less vigorous and multiplying less rapidly than 

 would have been the case if they had not mated. 



But why should mating produce rejuvenescence in some 

 cases and not in others? In other words, why has mating 

 such diverse hereditary results in different cases? This ques- 

 tion we shall take up in our next lecture ; here we may indi- 

 cate merely the general nature of the answer which we shall 

 reach. We should expect conjugation to give rejuvenescence 

 just as endomixis does, since in it too the old macronucleus 

 is replaced by the unused micronucleus. But in conjugation 

 there are likewise produced new combinations of the parental 

 characters. Some of these new combinations are vigorous ; 



