T. M. SONNEBORN 247 



over, caryonides that mature as selfers are totally unknown in 

 P. bursaria. Both the absence of selfing caryonides and the usual 

 synclonal uniformity in mating type serve to reduce the probabil- 

 ity of mating between close relatives. 



3. Senility, autogamy, and selfing. Senility is marked by de- 

 creasing fertility, as in other species. However, unlike most vari- 

 eties of P. aurelia, variety 1 of P. bursaria is able to mate readily 

 throughout senility, almost until the clone dies. The probability 

 of death after conjugation is low, usually less than 10%, during 

 maturity; but with the onset of senility, as a rule at an age of 30 

 to 40 months, the probability of death after conjugation begins 

 to rise and continues to rise, often eventually approaching 100%. 

 There is much variation in the details from clone to clone, but 

 representative results are about 80% mortality after conjugation 

 at 4 years of age and 100%; or nearly so at 5 to 5/2 years of age 

 and thereafter. In the laboratory the period of senility lasts 4 to 5 

 years, making the total life cycle 8 to 9 years. (Of course many 

 clones are weak and die immediately or soon after conjugation; 

 the present account refers only to clones of full vigor. ) 



Comparable decreasing fertility with age is found in other Cili- 

 ates. Most varieties of P. aurelia, those that undergo autogamy, 

 exhibit an age-correlated mounting death rate after autogamy; 

 and varieties 15 and 16 show the same after selfing during senil- 

 ity. The varieties that undergo autogamy during senility can also 

 conjugate during that stage, but conjugation is difficult to obtain. 

 In this case, the parallel to the situation in P. bursaria is exact, 

 for the probability of death after these conjugations also increases 

 with the advance of senility. 



The long period during which conjugation, but only cross 

 conjugation, can occur in the life cycle of P. bursaria, 7 to 8 years, 

 suggests how rare opportunities for crossing may be in nature. 

 Long ago Weismann suggested that the length of the reproduc- 

 tive period in organisms is adjusted by natural selection to permit 

 the production of just about a sufficient number of offspring to 

 perpetuate the species. Even allowing for sexual progeny that fail 

 to survive either because of inherent defects or because of extinc- 

 tion due to external causes, Weismanns suggestion, if valid, 



