T. M. SONNEBORN 297 



reproduction and justifies the attempt to generalize these con- 

 cepts. 



The progessive series of changes accompanying the sequence 

 of breeding systems in the Ciliates includes chiefly the following: 

 the number of closely related syngens per species increases; the 

 range of distribution of a syngen, especially in latitude, becomes 

 more and more restricted; the number of local populations per 

 syngen becomes smaller and smaller; the genetic divergence of 

 each local population becomes greater and greater. Thus, the 

 outbreeding species Paramecium bursaria includes only three 

 syngens in North America, while the inbreeding species P. cauda- 

 tum includes 12, and others include more, perhaps many more. 

 Outbreeding syngens, like variety 1 of P. bursaria, variety 15 of 

 P. aurelia, and variety 2 of Tetrahymena pyriformis, cover a 

 range from cold to very warm latitudes; while inbreeding syn- 

 gens, such as variety 10 or 14 of P. aurelia, may be restricted to 

 a small number of populations near the Gulf of Mexico. An out- 

 breeding syngen, such as variety 1 of P. bursaria, is genetically 

 so much alike throughout its range that crosses between widely 

 separated populations yield far more viable progenies than in- 

 breeding, and no differences in chromosome morphology or size 

 are found throughout its range. Different populations of an in- 

 breeding syngen, such as variety 4 of P. aurelia, are able to inter- 

 breed in the laboratory, but the F2 shows high nonviability, 

 sometimes approaching 100%, in contrast to the virtual absence 

 of nonviability after inbreeding within a population. Further, 

 each of its local populations that has been examined has a unique 

 chromosome number in an aneuploid series and shows unique 

 chromosome morphology and size. Syngens with intermediate 

 breeding systems, such as many varieties of P. aurelia, are inter- 

 mediate in range and in the genetic divergence of their popula- 

 tions. The extreme narrowness of range and divergence of 

 populations may be shown by the P. caudatum of Europe; each 

 local population gives more or less Fl nonviability when crossed 

 with other local populations. But that situation is not yet entirely 

 clear. 



Actual, as contrasted with potential, gene flow thus becomes 



