374 H. S. JENNINGS 



hvo such stocks of diverse origin. There is thus not even any indi- 

 rect evidence that conjugation rejuvenates, since the stocks tliat 

 conjugated underwent the same fate as those that did not. 



So far as I have been able to discover, there is no experimental 

 evidence from any other source that conjugation rejuvenates. 

 In Miss Cull's paper entitled ''Rejuvenescence as the result of 

 conjugation" ('07), the evidence consists merely in showing that 

 a considerable fraction of those that had conjugated continued 

 thereafter to multiply. But control experiments show, as set 

 forth in the body of the present paper, J^hat they would have con- 

 tinued equally if they had not conjugated; in fact a larger propor- 

 tion would have continued to multiply if they had not been 

 allowed to conjugate. There is thus in these results no evidence 

 of rejuvenescence through conjugation; and this must be said of 

 all observations which merely show that some of the ex-conju- 

 gants continue to multiply. Control experiments with animals 

 prevented from conjugating are necessary for a correct under- 

 standing of the results. 



In the long series of studies set forth in the present paper as 

 Experiments 5 to 14, the effects of conjugation were studied when 

 one division of a race is allowed to conjugate frequently, while 

 another is kept from conjugating; also the effects of conjugation 

 in a race that is actually depressed. As to the first point, the 

 animals that did not conjugate were found throughout to be more 

 vigorous than those that conjugated frequently. 



With regard to the effects of conjugation in a depressed race, 

 it is to be recalled that Maupas had repeatedly tried this experi- 

 ment, finding always that conjugation has no beneficial effect 

 under such conditions. The question might then be regarded as 

 settled, since there is no expectation of beneficial effect even 

 accepting the views of the great upholder of the theory of reju- 

 venescence; positive results would be directly opposed to the 

 experimental results of Maupas. 



Yet it was in one of these experiments alone that any result was 

 reached that could possibly lend themselves to an attempt to 

 maintain that conjugation has a beneficial effect on vigor and 

 vitality. In Experiment 12 the stock was so depressed that it 



