266 PROTOZOA 



during immaturity is highly effective in stabilizing potential sett- 

 ers as a pure type. They pointed out that early selfing is a devel- 

 opmental stage characterized by progressive limitation of 

 expressible mating potentialities. Early selfing has also been 

 reported in variety 6 by Gruchy ( 1955 ) and may well occur in 

 other varieties. 



This early selfing is to be distinguished from age-induced 

 selfing, just as it was in P. aurelia. The selfers collected from 

 nature would probably in most cases be past the incompletely 

 determined early stage of development. Moreover, they would 

 probably have been much exposed to the stabilizing action of 

 starvation. In fact, since conjugation is induced by starvation, 

 the exconjugants would at once be exposed to this stabilizing 

 influence and early selfing would be much rarer in nature than 

 in the laboratory where exconjugants are routinely provided 

 with excess food. Finally, the available reports indicate that 

 selfers collected from nature are not stabilized as one type by 

 starvation in the laboratory. A sharp distinction between the two 

 kinds of selfers should therefore be made. 



To sum up the situation on varietal differences in periods of 

 the life cycle, very little is yet known about this, except for the 

 existence of differences in length of the immature period. How- 

 ever, from what is known about the marked difference between 

 varieties 1 and 2 in the method of mating type inheritance and 

 in geographical distribution, one may expect them to differ in 

 breeding system and in many features of their lives adapted to 

 this difference. Since the first two varieties to be studied show 

 such marked differences, it is to be expected that others also will. 

 This is already suggested by the observations on selfers and 

 amicronucleates found in nature. 



4. Are the varieties of T. pyriformis readily identifiable? Pres- 

 ent knowledge provides only one way to identify the varieties 

 and that is by their mating reactions. Other differences are scarce 

 and of little or no use in identification. There is, if anything, 

 even less ground for assigning them specific names than there 

 was in P. bursaria or P. aurelia. The question of whether they 

 should be considered biological species is in principle again the 



