PROTOZOA 



The period of maturity is probably even longer than the im- 

 mature period in these varieties. An outbreeder, especially one 

 with no known self-controlled method of getting from one body 

 of water to another, doubtless needs much time to find a suitable 

 mate. The chance of finding a suitable mate is increased in va- 

 riety 16 because there are three other mating types. In all other 

 varieties, there would be only one. Obviously if the chance of 

 finding any stranger belonging to the same variety is low, there 

 is a selective advantage in a system of multiple mating types 

 which greatly increases the probability that the first stranger en- 

 countered will, if mature, be a suitable mate. No such advantage 

 attaches to multiple mating types in inbreeders. Lacking imma- 

 turity and having either the group A or B method of mating type 

 inheritance, they would still have prospective mates at hand 

 even with a two-type system. 



Two considerations apply to both the periods of immaturity 

 and maturity. First, varieties 15 and 16 have lower fission rates 

 than varieties 10 and 14. This makes both immaturity and 

 maturity last longer in time than is indicated by merely com- 

 paring the numbers of fissions in these periods with those in the 

 inbreeders, for at low fission rates it obviously takes longer to 

 use up the number of fissions allotted to each stage of life. The 

 times available for migration and for finding a suitable mate are 

 thus to this extent increased. Second, various collectors (e.g., 

 Pringle, 1956) have reported that some varieties of P. aurelia 

 can be found in nature only during summer and fall, in certain 

 regions, and that the numbers found in samples of equal volume 

 have an annual rise and decline. In varieties 15 and 16, with im- 

 maturity and maturity lasting through one, two, or more such 

 cycles, this might drastically reduce the number of caryonides 

 inhabiting a particular body of water and so would perhaps re- 

 duce the probability that caryonides of complementary mating 

 type and common descent would persist in the same body of 

 water until they reached maturity and attained a sufficiently high 

 population density to be likely to encounter one another. 



Finally, varieties 15 and 16 arc the only ones in which autog- 

 ,ini\ appears to be replaced by old-age selfing. Thus, even this 



