Distinctive Results of Mating 151 



culties of the complex process ; and as we shall try to show, 

 by frequent incompatibilities between the parts united. 



In rare lines in Paramecium, and other infusoria, and 

 commonly in Uroleptus, according to Calkins, all these diffi- 

 culties are overcome, so that the replacement of the worn 

 nucleus by the reserve has the same effect in renewing vitality 

 that it has when its occurs without mating. There is thus 

 far no evidence either from Calkins' experiment with Urolep- 

 tus, or from any others, that the mating, as distinguished 

 from the replacement of tLc macronucleus by the micronuc- 

 leus, produces rejuvenescence. Calkins finds that in Urolep- 

 tus this replacement without conjugation restores vitality 

 as it does with conjugation. 



What then are the distinctive results of mating, as com- 

 pared with that replacement which occurs without mating? 

 On this the theory of rejuvenescence has not thus far cast 

 light. The thing to do under such circumstances is to drop 

 for the time any definite preconceived theory, and examine 

 with care the facts ; sometimes these carry a theory of their 

 own! Just what differences can we find between individuals 

 that have mated and those that have not? 



In our examination of the question of diversity of sex in 

 Paramecium (Lecture 5), we mentioned a difference that was 

 found in that organism. After mating, the two individuals 

 and their offspring have become more alike than the two 

 stocks were before mating. This, as before remarked, is not 

 unnatural, for each individual after mating is, if it may 

 be so expressed, partly made up from the other individual. 

 In all of the offspring of each of the mates, half the nucleus 

 has come from the other mate. It is therefore not surprising 

 that the two sets of offspring are much alike; they are all 

 children of the same family ; they are as closely related as 

 are the brothers and sisters of a human family. 



Here then we have in these lower organisms a distinctive 



