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THE AMERICAN NATURALIST [Vol. LI 



and encystment. During the coldest weather it was suffi- 

 cient to transfer every other day, but during the warmest 

 weather it was found advantageous to transfer twice a 

 day. Nothing was sterilized in these experiments, but the 

 same pipet was used in all transfers and the watch-glasses 

 not in use were exposed to the air and allowed to dry. 

 Moreover, from time to time the didinia in each line in 

 either group were transferred directly to the watch- 

 glasses from which the didinia of the other group had 

 just been taken. In these dishes there always remained 

 considerable solution, in some instances a drop or more. 

 Furthermore, in a few cases didinia from the more rapidly 

 dividing lines were transferred directly without the addi- 

 tion of fresh food to dishes in which more slowly dividing 

 lines had died or from which all of the didinia had been 

 removed. 



Such treatment had no appreciable effect 6n the relative 

 rate of fission in the two races. It is obviously evident, 

 therefore, that the difference in the rate observed was not 

 due to difference in the bacterial contents of the solution 

 if there really was any such difference, nor was it de- 

 pendent upon selection, natural or otherwise, for mem- 

 bers of the more rapidly dividing pairs were always trans- 

 ferred in all lines. And the number of lines lost by death 

 and encystment was essentially the same in both. In the 

 one 30 were lost by death and 8 by encystment, in the 

 other 33 by death and 3 by encystment. Assuming that 

 the weaker lines died out in every case, it is exident that 

 in this respect both races were subjected to practically the 

 same sort of selection. And since all of the cultures were 

 subjected to the same conditions otherwise, it is clear that 

 the difference in the rate of fission in the two races must 

 have been due to the constitution of the organisms. 



We have consequently demonstrated that marked vari- 

 ations in the rate of fission may appear quite suddenly 

 in the progeny of a single individual without conjugation 

 or encystpient, that some of these variations are heritable, 

 and that they can probably be produced by subjecting the 

 individuals to abnormally high temperature. 



