T. M. SONNEBORN 271 



ture period may be a few days shorter is of less significance. Even 

 without an immature period, the method of mating type inherit- 

 ance inhibits the closest form of conjugation inbreeding. Variety 

 2 is also distinguished by having more known mating types than 

 any other variety and, as we have seen, multiplicity of mating 

 types is of adaptive value to an outbreeder. Finally, variety 2 has 

 the widest known latitudinal distribution in nature, and we have 

 previously seen how this too is correlated with outbreeding. The 

 fact that the first two varieties to be studied in detail show such 

 marked differences in features associated with different breeding 

 systems leads to the supposition that the species T. pyriformis, 

 like P. aurelia, consists of varieties with a large array of breeding 

 patterns, perhaps from rather extreme outbreeders to extreme in- 

 breeders. 



Euplotes 



Species, Varieties and Mating Types. Until Kimball ( 1939, 

 1942) discovered mating types in Euplotes patella and worked 

 out the breeding relations among many collections of organisms 

 that closely resembled or were identical with E. patella, there 

 was great confusion in the literature concerning those species of 

 Euplotes that look much like E. patella. Sorting his collections 

 into those that would or would not interbreed, Kimball came 

 out with at least five groups. One group did not conjugate. Each 

 of the other four included a number of collections that could 

 conjugate in mixtures with each other, but no strain of one 

 group would conjugate with any strain of another group. Pierson 

 (1943) made a careful morphological study of Kimball's five 

 groups of strains and of some additional strains belonging to these 

 groups. On the whole (exception noted below), the groups were 

 found to be morphologically distinct. The one which was not 

 observed to conjugate was identified as E. woodruffi. One proved 

 to be a new species, E. aedicukitus. One was E. eurystomus. Two, 

 this is the exception, could not be distinguished morphologically; 

 both were E. patella. There may have been more than one group 

 in some of the other species already mentioned, but this is not 

 clear from the published account. There is also some vagueness 



