T. M. SONNEBORN 303 



nor can any combination of the others serve this purpose. Most 

 of the traits just distinguish a few "roups of varieties. 



Only mating type specificity is sufficient to identify all varie- 

 ties. Serologic specificity may, with fuller knowledge, eventually 

 serve the same purpose. Great labor, technical knowledge, skill, 

 and elaborate materials are required to use either mating types 

 or serotypes for identification. Either living standard cultures of 

 all mating types or hundreds of specific antisera are required. The 

 difficulties and hazards involved in identification clearly preclude 

 assigning species names to the varieties. 



Objections are also raised against considering the varieties to 

 be species. The term "syngen" is proposed to replace "biological 

 species." 



Breeding Systems and Evolution in P. aurelia. Fertilization 

 serves two functions: genetic recombination and initiating new 

 life cycles. Death (genetic, somatic, or both) eventually occurs 

 in the absence of fertilization. Fertilization occurs either as con- 

 jugation or as autogamy. Varieties that undergo autogamy do 

 so at a later stage of life (senility) than the one (maturity) in 

 which they are ripe for conjugation. Varieties that lack autogamy 

 are adapted only to cross conjugation during maturity, but to 

 cross conjugation and self-conjugation during senility. Natural 

 selection has thus given prior choice to conjugation and cross 

 conjugation. The relative adaptation to cross conjugation and 

 to selfing or autogamy differs greatly among the varieties. Few 

 are strong outbreeders, many are intermediate, and many are 

 strong inbreeders. The different breeding systems of different 

 varieties are supported by differences in their life features. Out- 

 breeders have long periods of immaturity, maturity, senility, and 

 total life. This gives opportunity to migrate away from relatives 

 before mating and to have much time to find a suitable mate. In- 

 breeders have short lives, with no immature period and a very 

 brief mature period. They must mate quickly with the close rela- 

 tives near at hand or undergo autogamy. Other adaptations to the 

 breeding system, fully explained in the text, concern the number 

 of mating types (two in inbreeders, more in outbreeders), the 

 fission rate (low in outbreeders, high in inbreeders), the form of 



