360 H. S. JENNINGS 



factors, gives a truer insight than many imperfect experiments. 

 Such a model experiment is, for present purposes, Experiment 1. 

 In this experiment none of the Unes descended from non-con ju- 

 gants (split pairs) died out during the five weeks of the experiment. 

 Of the lines descended from conjugants, though kept under exactly 

 the same conditions, 38 per cent died out during the same period. 

 In the other experiments some of both sets died, though as a rule 

 with more deaths among the lines derived from conjugants. 



In two out of the ten or twelve experiments in which this 

 matter was tested the usual relation was reversed; in both these 

 experiments we are dealing with exceptional conditions. In 

 Experiment 3 the temperature was abnormally high, standing 

 much of the time above 32°C. I have found by long experience 

 in Baltimore that it is not possible to carry on slide cultures of 

 Paramecium at such a temperature; from whatever source, the 

 animals rapidly die out. Thus, in this experiment the conditions 

 were so bad that a large proportion of both conjugants and non- 

 conjugants died within the four days of the experiment. But 

 under these conditions the lines descended from non-conjugants 

 died out still more rapidly than those descended from conjugants. 

 Of the former 68.6 per cent died in the four days; of the latter, 

 but 23.4 per cent. The difference seemed clearly due to the 

 furious rapidity at which the non-conjugants multiplied, while 

 the conjugants (as is the rule after conjugation) divided but 

 slowly. There is little doubt but that under usual temperature 

 conditions the advantage would have been, in this case also, with 

 the non-conjugants. The fact, however, that conjugation may 

 be physiologically advantageous under very exceptional condi- 

 tions is an important one. 



The other case in which the advantage was with the conjugants 

 is Experiment 12. Here we are dealing with a much depressed 

 stock, in which reproduction is slow and mortality high before 

 conjugation. Such a stock can hardly be induced to conjugate; 

 so that but three pairs could be obtained from it. With the six 

 lines derived from these were compared ten lines derived from 

 non-conjugants of the same culture. It is important to note that 

 the latter were not split pairs; in other words, they were not 



