T. M. SONNEBORN 245 



tion called variety 5 never yielded more than one mating type. 



4. Other differences. At 26.5°, conjugation is completed in 

 20 to 22 hours in variety 1, but it lasts for 32 hours or more in 

 varieties 2, 4, and 6. Variety 3 shows no daily periodicity in sexual 

 reactivity; but varieties 1 and 2 are most reactive around noon 

 and are not reactive at night (Jennings, 1939; Wichterman, 1948; 

 Ehret, 1953). Reliable varietal differences in size, shape, or fine 

 anatomy have not been reported. Strains of the same variety ap- 

 pear to differ more commonly in size, shape, and certain physio- 

 logical traits than do strains of the same variety in either P. aure- 

 lia or P. caudatum, 



5. Status of the problem of varietal identification. All the 

 differences among varieties occurring in the same region involve 

 mating types and conjugation. This imposes great labor and long- 

 continued laboratory investigation on the part of anyone who 

 sets out to identify a strain and therefore is, in my opinion, not 

 suitable as a basis for species naming. However, it is true that 

 with labor and time the varieties could be identified without 

 recourse to living standard cultures. The requirements are per- 

 haps not so great as they would be for a comparable accomplish- 

 ment in P. aurelia or P. caudatum, but they are still much too 

 great for routine taxonomic purposes. 



Life Features and the Breeding System. The life features of 

 P. bursar ia form a striking contrast to those which characterize 

 most varieties of P. aurelia and P. caudatum. The differences are 

 by no means fortuitous. They are all clearly understandable as 

 adaptations to or consequences of a different breeding system. In 

 brief, P. bursaria is an outbreeder, while most varieties of the 

 other two species are inbreeders to a greater or lesser degree. In 

 this frame of reference, the life features of P. bursaria and their 

 contrast with those of most varieties of the other two species 

 make excellent sense. 



The life history of P. bursaria is documented in great detail in 

 a series of papers by Jennings ( 1944a,b,c,d; 1945). Most of this 

 work was done on variety 1. My comments are therefore to be 

 understood as applying chiefly to that variety. There is no indica- 

 tion in these papers of any differences in the major life features 



