350 S. O. MAST 



sequently evident that here greater variation was found in four 

 non-con jugant lines than was found in the ex-con jugant and non- 

 conjugant hues mentioned above. Obviously then, the fact 

 that the rate of fission differed in these lines adds extremely 

 little support to the conclusion that conjugation causes an in- 

 crease in vitality in protozoa. 



Concerning the significance of the evidence which Calkins pre- 

 sents in support of the contention that encystment at times causes 

 an increase in the rate of fission, the following may be said: 

 Calkins kept didinia continuously in spring water which was 

 frequently changed. To this he added from day to day a few 

 paramecia. He maintains that, under such conditions, the 

 rate of fission decreased while the rate of encystment increased 

 until. after the production of 84 to 148 generations all were en- 

 cysted. He then poured off the old water and added ''fresh 

 water and Paramecium." This caused the didinia to come out 

 of the cysts after which, he maintains, the rate of fission was 

 again practically the same as it had been in the beginning. 

 And he holds that this shows that encystment results in 

 rejuvenescence. 



The result of Calkins' experiments seem to show fairly clearly 

 that there is a decrease in the rate of fission under the conditions 

 stated and that there is an increase after encystment, but they do 

 not seem to me to show that this increase is necessarily due to 

 encystment. To recover the race from encystment Calkins, as 

 stated above, replaced the solution by fresh water (he does not 

 say what kind) and paramecia (presumably many). Thus he 

 subjected the organisms to a marked change of environment. 

 Now, Calkins and others have shown that just such changes 

 cause definite increase in the rate of fission in Paramecium under 

 conditions similar to those in which the didinia were before 

 encystment. Is it not therefore reasonable to assume that the 

 increase in the rate of fission in Didinium noted by Calkins was 

 due to the change in the environment rather than to encyst- 

 ment? At any rate, we have demonstrated that Didinium can 

 produce over 1000 generations without any apparent periods of 

 depression except at the close and that after having produced 



