570 THE POPULAR SCIENCE MONTHLY 



The way to test this question is to have a set of the animals of the 

 same parentage and history; to divide these into two groups, and to 

 allow one group to conjugate, the other not. Then keeping the two 

 groups under the same conditions, what difference is found to be caused 

 by the conjugation? 



In carrying out such experiments, the control set, those that have 

 not conjugated, are fully as necessary as the other; otherwise we can 

 not tell whether the phenomena shown by those that have conjugated 

 are really due to the conjugation or not. Neglect to have this control 

 set has led to erroneous conclusions in some of the work previously done. 



Comparative experiments of this character I have tried many times 

 with large numbers of individuals. As the animals begin to conjugate, 

 they first come in contact and stick together at the anterior end, though 

 the process can not be consummated till the more posterior regions 

 become united. At this point then I intervened, separated the two be- 

 fore union was complete, and removed each to a drop of water by itself. 

 Other pairs were allowed to complete conjugation, then the members 

 were isolated in the same way. The two sets were then kept under the 

 same conditions and their propagation was followed exactly. The two 

 differ in no other respect save that one set has conjugated, while the 

 other has not. What difference is caused by conjugation? 



1. We find that the animals which were ready to conjugate, which 

 were actually attempting to do so, are by no means in a depressed, 

 degenerated condition, unable to multiply farther. On the contrary, if 

 they are not allowed to conjugate, each continues to multiply with 

 undiminished vigor. Conjugation is then not necessary for further 

 multiplication. And we can by no means assume that because indi- 

 viduals are ready to conjugate, they are therefore in a degenerate or 

 senile condition. Nor can we assume, as has been done by some authors, 

 that if the animals continue to multiply after conjugation, this shows 

 that conjugation has had a rejuvenating effect, for the same specimens 

 continue equally without conjugation. 



This fact, taken in connection with the results of Woodruff, explains 

 Maupas's supposed positive evidence that conjugation produces reju- 

 venescence, as also the more recent results of Miss Cull.^ In Maupas's 

 case, which is the one that has been mainly relied upon as demon- 

 strating rejuvenescence, after the animals had become sickly (this being 

 due, as Woodruff's work shows, to the fact that they had lived long under 

 conditions not fully adapted to them), he tried mating one of them 

 with a wild specimen. He then took one from this pair, and found that 

 it was strong and well, so that it multiplied for 316 generations. 

 Maupas supposed that this was due to the fact that conjugation had 

 occurred. I believe it is fairly clear that the result was not due to the 



^ Cull, Sara White, "Rejuvenescence as a Eesult of Conjugation," Joiirn. 

 of Exper. Zool, 1907, 4, 85-89. 



