T. M. SONNEBORN 239 



in which there is free flow of genes among populations of a va- 

 riety, but none between varieties. The extent to which the same 

 difficulties exist in the American "varieties" of group 2 is un- 

 known. 



The preceding relations constitute the feature of special inter- 

 est in P. caudatum to which reference was made at the start of 

 this section. It is the possibility to which Pringle referred, namely, 

 that group 2 or a part of it may be a group of differentiated local 

 populations more or less isolated by Fl mortality. This, as he 

 emphasizes, is different from the F2 mortality which serves to aid 

 in isolating local populations of some varieties of P. aurelia. It is 

 obviously most important for our understanding of evolution and 

 speciation in P. caudatum to have this situation clarified. 



Breeding Systems. The observations on the European popula- 

 tions belonging to group 2, as already indicated, point to inbreed- 

 ing as the prevailing system in that material. There are other 

 indications that inbreeding is widespread among the varieties of 

 P. caudatum. With few possible exceptions, no immature periods 

 have been found thus far, though the methods Gilman has em- 

 ployed would not detect periods of immaturity of much less than 

 one month. Selfing is common in some varieties ( Gilman, 1941 ) 

 and it may occur more often in cultures of one prevailing mating 

 type than in cultures of the other. The method of mating type 

 inheritance is apparently the kind characteristic of the group B 

 varieties of P. aurelia, at least in six of the seven varieties exam- 

 ined (Gilman, personal communication). This would assure the 

 presence of animals of both mating types in a synclone. All these 

 features speak for inbreeding. Of the varieties thus far examined, 

 all fall into the same pattern with the possible exception of vari- 

 ety 3. The latter may show a number of outbreeding features, 

 but the picture is not yet clear. 



With such widespread occurrence of inbreeding features, au- 

 togamy might be expected. It, or what can now be interpreted as 

 autogamy, was reported by Erdmann and Woodruff (1916), by 

 Jollos (1916), and by Giese and Arkoosh (1939). On the other 

 hand, Gilman (1941 and unpublished), Pringle ( 1955), and John- 

 son ( 1955) have searched for autogamy and have been unable to 



