304 PROTOZOA 



fertilization during senility (setting in outbreeders, autogamy in 

 inbreeders), and the method of mating type inheritance (syn- 

 clonal uniformity in outbreeders, its absence in inbreeders ) . The 

 breeding system actually determined depends on the particular 

 combination of these features; when an inbreeding and an out- 

 breeding feature occur together, one may negate the other. Thus 

 the combination of a period of immaturity with a very brief ma- 

 ture period followed by autogamy leads to inbreeding in spite 

 of the usual association of immaturity with outbreeding. Other 

 examples are also cited. 



Each local population examined in one inbreeding variety has 

 a uniquely different chromosome set. This seems to be the basis 

 of F2 mortality following crosses between populations. Strongly 

 inbreeding varieties are thus a loose assemblage of highly isolated 

 local populations, each of which has a status that just falls short 

 of being comparable to that of a whole outbreeding variety. An 

 outbreeding variety represents a clear-cut level of biological or- 

 ganization; inbreeding varieties do not. In the latter, the local 

 population, not the variety, is the significant unit of biological 

 organization. 



Data are given to indicate the low population densities even 

 at the season of maximal populations; and to show that, within 

 a population of certain varieties, mating is not random; inbreed- 

 ing prevails. Other observations indicate that migration into and 

 out of a single body of water is a very rare event: the composi- 

 tion of a population remains constant over the years. Specula- 

 tions on means of migration and its consequences under various 

 conditions are presented. 



The varieties are divisible into two groups, A and B, on the 

 basis of differences in mating type inheritance 1 . This is paralleled 

 by differences in the serotype system and the presence or absence 

 of killer strains. 1 It seems to represent an early evolutionary cleav- 

 age in P. aurelia. Similarities in serotypic and mating type speci- 

 ficities are greater within group A and within group B than 

 between groups. Geographical distribution of the varieties in- 

 dicates that within each group a widely distributed variety has 

 given rise near the temperature limits of its range 1 to narrowly 



