250 PROTOZOA 



masked to a considerable degree by outbreeding. Jennings' efforts 

 to carry on series of successive inbred generations by sib matings 

 were frustrated by rapidly increasing death rates which ap- 

 proached 100%. 



The exception referred to above concerns the closest possible 

 inbreeding, matings between the two types that self-differentiate 

 within a clone. This will be referred to as selfing. No greater 

 mortality appeared after such selfing than when one or the other 

 type was outbred. However, there is some indication that the 

 clones produced by selfing age more rapidly than clones produced 

 by outbreeding: the former may give at ages of 12 to 18 months 

 as much death after conjugation as the latter do at ages of 3 to 4 

 years. This, like the earlier occurrence of self-differentiation, in- 

 dicates shorter stages in the life cycle after selfing than after out- 

 crossing. The higher death rate from sib matings than from 

 selfing or outcrosses led Jennings to remark, "There is in clonal 

 self-fertilization some biological relation that prevents it from 

 resulting in high mortality" (Jennings, 1944c, p. 195). 



What this biological relation may be is of great interest, and 

 this will be considered later. Here it suffices to note that it oper- 

 ates adaptively. The organism, as an outbreeder, survives in- 

 breeding but poorly; yet, to survive at all, it requires some form 

 of inbreeding as a last resort to forestall death of those members 

 of a clone that have failed to find a suitable mate. It has managed 

 to accomplish this by facilitating selfing through self-differentia- 

 tion and it has somehow endowed this saving sort of mating witli 

 protection against the usually disastrous effects of other sorts of 

 inbreeding. Whether something like this also operates in the 

 sellings that occur during senility in variety 15 of /\ aurelia needs 

 to be investigated. It is even possible that a similar basis under- 

 lies the perplexing observations of Pringle (1955) and Johnson 

 (1955) on inbreeding and outbreeding deaths in P. caudatum. 



The Mechanism of Mating Type Determination and Its Bear- 

 ing on the Breeding System. Jennings (1942) maintained that 

 the mating types are determined by the chromosomal constitu- 

 tion. He based this conclusion on two facts: (1) conjugation 

 brings about identical genotypes in the two clones of a synclone; 



