T. M. SONNEBORN 163 



whether each variety may be considered to have a potentially 

 common gene pool. 



The next section enumerates the differences that distinguish 

 the varieties and inquires whether these differences suffice, for 

 practical purposes, in the recognition and identification of the 

 varieties. At this point, the question of whether the varieties 

 should be given specific names is considered. Then follows a 

 section that deals with evolution in P. aurelia at various levels 

 and attempts to relate the biological differences among groups 

 of varieties, among varieties of a group, and among strains of a 

 variety to the systems of breeding, the various degrees of in- 

 breeding and outbreeding. This key section ends with a con- 

 sideration of the relation of systems of breeding to the species 

 problem. 



Mating Types and Varieties. The primary basis for recog- 

 nizing the existence of distinct varieties in Ciliates is the spec- 

 ificity of the mating types, i.e., the specific selectivity of the 

 breeding relations. On this basis, 16 varieties of P. aurelia can 

 now be recognized. Varieties 1 to 7 were discovered by Sonne- 

 born (1938, 1942b), variety 8 by Sonneborn and Dippell (1946), 

 variety 9 by Beale and Schneller (1954), varieties 10 to 15 by 

 Sonneborn (1956a; unpubl.). What is here referred to as variety 

 16 was described under the name of P. midtimicronucleatum by 

 Giese (1941). 3 



The number of mating types per variety is remarkably uniform. 

 As a rule, there are two and only two interbreeding mating 

 types in a variety. Until recently, only one type was known in 

 variety 7; but only one strain of that variety had been found. 

 Sonneborn (1956a) found the missing mating type, along with 

 the one previously known, in each of several new strains of 

 variety 7. Only variety 16 remains as the exception to the rule. 

 According to Giese (1941), it has four mating types, each of 

 which can mate with the other three. Were it not for this unique 

 feature, there would be reason to suppose that Giese's variety is 

 identical with Sonneborn's variety 15. 3 



Mating relations between varieties follow a usual rule, to which 



