ENDOMIXIS IN PARAMECIUM AURELIA. 445 



This experiment, in which different races were bred under 

 the same varied culture conditions, shows an almost perfect 

 synchronism of endomixis in all the races; that is, in A bred 

 throughout without interruption on the varied medium; in 

 bred until its death near the end of the experiment on the same 

 medium and again in the new isolated from the main culture; 

 in B, which was isolated from 'wild' material during the progress 

 of the work; and finally in AE, which was restarted twice from 

 a sister culture bred under "constant" environmental conditions. 



The most reasonable conclusion, on the basis of this experi- 

 ment, to account for the fact that the different races immediately 

 showed endomixis synchronously, is that the general culture 

 conditions initially influenced the appearance of the process; 

 that is, brought about its consummation a few days earlier or 

 later than it would have appeared under the former environment 

 of the races, and that, once established, the rhythmic period 

 characteristic of the species persisted and maintained the syn- 

 chronism of endomixis. If this conclusion is justified then it 

 must be assumed that the synchronism of B is due to the chance 

 isolation of this race at just the same period in the rhythm which 

 A, AE and were at at the time, or so near this period that the 

 environmental change immediately made it coincide with that 

 of the other races. The alternate hypothesis would be that 

 there is in all races of this species a definitely established syn- 

 chronism which holds under all normal environmental conditions. 

 But such a theory would require more than one series of experi- 

 ments to render its discussion profitable! 



B. 



In this set of experiments a study was made of the periodicity 

 of rhythms and endomixis in different races of Paramecium 

 aurelia when bred under the same practically constant culture 

 conditions, as already described. The results are shown graph- 

 ically in Fig. 2. 



Subcultures At and AEt (both derived from the same race, I, 

 about 1,000 generations previously) showed endomixis in pre- 

 served specimens at periods 4 and 9 and AEt also at period 

 15. Neither culture again underwent the process during a 



