T. M. SONNEBORN 275 



more) in E. harpa. The existence of a considerable immature pe- 

 riod is hardly to be considered fortuitous. As set forth above, it 

 exists also in T. pyriformis, in P. bursaria, and in outbreeding 

 varieties of P. aurelia. Its meaning appears to be to discourage 

 inbreeding by making mating impossible when the organism is 

 most likely to be near its relatives and by giving time to spread 

 and meet a stranger that would make a suitable mate. Of other 

 stages in the life cycle, nothing is known except that there is a 

 long mature period and a long life cycle, if there is a terminating 

 cycle at all, as I believe there is. All that is known is characteristic 

 of outbreeders. 



Third, Kimball mentioned that there was high mortality in his 

 crosses, all of which involved inbreeding. As we have seen in 

 the accounts of Paramecium and Tetrahymena, inbreeding mor- 

 tality is a sign of an outbreeder. 



All these strong indications that E. patella is an outbreeder 

 make me conclude that the superficially fumbling, inefficient, 

 and contradictory features may be misleading us. The major 

 difficulty seems to be the fact that mating types which can cross 

 also induce each other to self. This is no way for an outbreeder 

 to behave. Perhaps it is largely a laboratory phenomenon and 

 rare or nonexistent in nature. In the laboratory these observa- 

 tions were made in depression slides, i.e., in about 1 ml. of fluid 

 containing large numbers of euplotes. Contrast this with a pond 

 or river. The contrast is important, for we deal with extracellular 

 hormones acting through the surrounding fluid medium. Surely, 

 in a pond the dilution of the hormones would be enormous and 

 rapid. There would be a sharp concentration gradient away from 

 the position of the secreting animal. Further, the concentration of 

 animals producing hormones would be expected to be very much 

 less than in the laboratory. Consequently, the hormones could 

 probably work only for short distances, possibly only on two 

 euplotes that were close together, if not in actual contact. I can 

 see little likelihood of the hormonal mechanism leading in nature 

 to anything like the amount of selfing observed in the laboratory. 



However, there may well be some selfing in nature as well. 

 This is indicated by two facts. First, selfing was observed re- 



