442 Evolution and Adaptation 



two organisms, decrepid with old age, could renew their 

 youth by uniting. Two spent rockets, he says, cannot be 

 imagined to form a new one by combining. There is ap- 

 parent soundness in this argument, if the implication is taken 

 in a narrow physical sense. If, on the other hand, the egg is 

 so constituted that at a certain stage in its development an 

 outside change is required to introduce a new phase, then 

 the conception of rejuvenescence does not appear in quite 

 so absurd a light. 



This hypothesis of rejuvenescence is based mainly on cer- 

 tain processes that take place in the life history of some of 

 the unicellular animals. Let us now see what this evidence is. 

 The results of certain experiments carried out by Maupas on 

 some of the ciliate protozoans have been fruitful in arousing 

 discussion as to the ultimate meaning of the sexual process. 

 Maupas' experiments consisted in isolating single individuals, 

 and in following the history of the descendants that were 

 produced non-sexually by division. He found that the de- 

 scendants of an individual kept on dividing, but showed no 

 tendency to unite with each other. After a large number of 

 generations had been passed through (in Stylonychia pustu- 

 lata, between 128 and 175 ; in Leucophys patida, 300 to 450; 

 and in Onychodromus grandis, 140 to 230 generations), the 

 division began to slow down, and finally came to a stand- 

 still. Maupas found that if he took one of these run-down 

 individuals, and placed it with another in the same condition 

 from another culture, that had had a different parentage, the 

 two would unite and the so-called process of conjugation take 

 place. This process consists for the species used, in the tem- 

 porary union and partial fusion of the protoplasm of the two 

 individuals, of an interchange of micronuclei, and of a fusion, 

 in each individual, of the micronucleus received from the 

 other individual with one of its own. The individuals then 

 separate, and a new nucleus (or nuclei) is formed out of the 

 fused pair. 



