236 PROTOZOA 



passes from one breeding system to the next, in this sequence: 

 obligatory outbreeder, facultative out- and inbreeder, obligatory 

 inbreeder, exclusively asexual reproduction. Nearly the whole 

 sequence is found among the varieties of P. aurelia; they embrace 

 the range from nearly obligatory outbreeder to nearly obligatory 

 inbreeder. Exclusively asexual reproduction is not found, for sur- 

 vival depends upon fertilization or something associated with it. 

 The near-obligatory outbreeders have a wide geographic range 

 both in longitude and latitude and relatively few closely related 

 varieties of this sort are found in the same territory. The more 

 one approaches obligatory inbreeding, the larger the number of 

 varieties and the more restricted their range, especially in lati- 

 tude. Toward the end of the series, each local population becomes 

 almost as distinctive, differentiated, and isolated as a variety of 

 outbreeders. Correspondingly, the further one proceeds down the 

 sequence of breeding systems, the more difficult the species prob- 

 lem becomes until, in terms of the biological species concept, it 

 bursts into nothingness when the closest inbreeding becomes 

 obligatory or when sexuality is altogether abandoned. 



Toward the end of this paper, we shall take up these extreme 

 situations. But first we turn to the other species of Ciliates in 

 which mating types are known. The points that have been made 

 in the detailed analysis of P. aurelia will serve as a touchstone for 

 orientation and interpretation in reviewing the status of knowl- 

 edge in the other Ciliates. 



Paramecium caudatum 



The Varieties of P. caudal urn and Their Distribution. In gen- 

 eral, the situation in P. caudatum bears many resemblances to 

 the one just set forth in detail for P. aurelia; but there is, as will 

 appear, at least one feature of special interest. Like P. aurelia, 

 P. caudatum consists of a number of varieties or syngens. Each of 

 these varieties has two mating types. At least 16 varieties have 

 been found: varieties 1 and 3, independently by Oilman (1939) 

 and by lliwatashi (1949); varieties 12 and 13 by Iliwatashi 

 ( L949); and at least twelve more by Gihnan (1939,' 1941, 1949, 

 L950 3 L954, L956a,b). Gihnan (1950) reported the relation of the 



