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 before 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 individuals 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 rejuvenescence, as also the more recent results of Miss Cull. In Maupas's case, which is the one that has been mainly relied upon as demonstrating 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 Result of Conjugation," Journ. of Exper. Zool, 1907, 4, 85-89.