THE EVOLUTION OF SEX 9 



mysterious; unless it can be made more explicit, it 

 does not seem in accord with our physico-chemical 

 conceptions. Jennings, who has more recently studied 

 in greater detail the process of division and conjugation 

 in paramcecium, has found evidence on which to base 

 a more explicit statement as to the meaning of rejuve- 

 nescence through conjugation. 



Jennings' work is safeguarded at every turn by care- 

 ful controls, and owing in large part to these controls 

 his results make the interpretations more certain. He 

 found in a vigorous race, that conjugated at rather 

 definite intervals, that after conjugation the division 

 rate was not greater than it had been before, but on 

 the contrary was slower — a fact known, as he points 

 out, to Maupas and to Hertwig. Conjugation does 

 not rejuvenate in this sense. 



Jennings states that, since his race was at the be- 

 ginning vigorous, the objection might be raised that 

 the conditions were not entirely fulfilled, for his pred- 

 ecessors had concluded that it is a weakened race that 

 was saved from annihilation by the process. In order 

 to meet this objection he took some individuals from 

 his stock and reared them in a small amount of culture 

 fluid on a slide. After a time they became weakened 

 and their rate of division was retarded. He then al- 

 lowed them to conjugate, and reared the conjugants. 

 Most of these were not benefited in the least by the 

 process, and soon died. A few improved and began 

 to multiply, but even then not so fast as paramoecia in 

 the control cultures that had been prevented from con- 

 jugating. Still others gave intermediate rates of 

 division. 



