T. M. SONNEBORN 215 



spect as varieties 10 and 14, which were used at the start as the 

 examples of the most extreme inbreeders. They both belong to 

 group B and lack a period of immaturity. Hence they are capable 

 of mating when they are most likely to be near close relatives of 

 complementary mating type. Their period of maturity, however, 

 is short, about 6 days or 25 fissions, but this is twice as long as in 

 varieties 10 and 14. Such a short mature period might be consid- 

 ered to provide relatively little opportunity for conjugation to 

 occur. Yet, if all the individuals lived and food remained abun- 

 dant, the two clones of complementary mating type from a pair 

 of conjugants would yield many millions, possibly even billions, of 

 progeny before famine would induce 100% autogamy rather than 

 ripeness for mating. Applying the maximum density of population 

 calculated from Pringle's data on variety 9, it would take a lake 

 with a surface area of a square mile and an average depth of 25 

 feet to house so large a population. It would seem, therefore, that 

 even with so short a period of maturity, the progeny of a single 

 pair of conjugants would be likely to deplete the food and be ripe 

 to conjugate before maturity had passed. This would certainly 

 happen in the second generation for it would start, not with a 

 single pair of conjugants, but with a large population. This and 

 all such calculations fail to take into account predation and other 

 factors that eliminate part of the population and permit the resi- 

 due to multiply further before exhausting the food supply. On 

 the other hand, it also ignores competitors for food. Such errors 

 will have to be corrected before conclusions as to the relative 

 frequency of conjugation and autogamy in nature can be placed 

 on a firm basis. Meanwhile, considerations based on the limited 

 available knowledge have value in exposing needed types of fur- 

 ther investigation. For example, it is known that although mating 

 type changes relatively rarely at autogamy and conjugation, 

 changes are more frequent in one direction at autogamy and in 

 the opposite direction at conjugation in variety 4 at 27°. There 

 is need to try to discover the various factors involved in determin- 

 ing the relative frequencies of the two mating types in a popula- 

 tion, their quantitative effects, and the equilibria attained under 



