T. M. SONNEBORN 173 



4, which yield 85% mortality in the F2, and also between strains 

 172 and 51, which yield almost the highest F2 mortality observed, 

 about 96%. 



Thus, with respect to F2 mortality alone, there appears to be 

 a virtually uninterrupted series of gradations from crosses be- 

 tween varieties down to crosses between different clones of the 

 same strain. The former approach 100% F2 mortality; the latter 

 usually show 0% F2 mortality, at least in some varieties. But the 

 series is not continuous when other features are taken into 

 account. The survivors in the F2 or backcrosses show no im- 

 provement in viability at subsequent autogamies or backcrosses 

 following intervarietal crosses; but the mortality decreases to 

 zero at the second autogamy or after several backcrosses follow- 

 ing interstrain crosses within a variety. The survivors in the 

 later generations are not by any means of normal vigor after 

 intervarietal crosses; they are quite normal in vigor after inter- 

 strain crosses within a variety. All intervarietal crosses that can 

 be made give essentially the same low survival in the F2 and 

 backcrosses; it is impossible for the genes of two varieties to be 

 recombined in normal vigorous combinations. Although different 

 strains of the same variety sometimes yield as low F2 survival 

 as do diverse varieties, their genes can always be recombined in 

 such combinations. The sharp break between varieties is thus 

 never matched by the break between strains of the same variety. 

 Among the latter, gene flow is readily possible; among the former, 

 it is nonexistent or almost so. 



Paramecium aurelia: Differences among Varieties 

 and Problem of Identification 



The preceding section showed that each of the 16 varieties of 

 P. aurelia has a potentially common gene pool which is effec- 

 tively cut off from the gene pool of every other variety. Each 

 variety therefore qualifies as a species according to the modern 

 biological species concept. Two questions now arise. ( 1 ) Should 

 the term "variety" be replaced by "species" in referring to them 

 henceforth? (2) Should each former variety be baptized with a 

 different specific name, while the old "species" P. aurelia becomes 



