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- 
fata, between 128 and 175; in Leucophys patila, 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. 
