T. M. SONNEBORN 217 



varieties of karyotypic diversities among local populations. Clearly 

 this situation could develop only if crosses between local popula- 

 tions were rare events. Otherwise, interbreeding would swamp out 

 existing karyotypic diversities and prevent new ones from arising, 

 as we shall later find to be true in certain outbreeding Ciliates. 

 Thus Dippell's studies lead to the view that F2 mortality follow- 

 ing strain crosses may be one of the most potent evidences of the 

 existence of regular close inbreeding in a variety. 



The general picture of variety 4 that now emerges is of a vari- 

 ety which is a loose assemblage of innumerable highly isolated 

 and highly inbred local populations. Grouping these together as 

 a variety is to some degree artificial. Although it is true that they 

 are potentially able to interbreed and do so in the laboratory, 

 everything we know about them points strongly to the conclusion 

 that such interbreeding occurs very rarely in nature, almost negli- 

 gibly except perhaps on a very long-range view. In effect, each 

 local population of variety 4 just falls short of having the same 

 status as a variety among outbreeders. The same conclusions ap- 

 ply even more extremely to varieties 8, 10, and 14, perhaps less 

 so to variety 1 and some others. 



The importance of these conclusions in relation to problems of 

 species, breeding systems, and evolution in P. aurelia is self- 

 evident. Some varieties are in effect virtually obligatory inbreed- 

 ers, and each isolate in nature is an entity of some biological 

 stature. For such varieties, the local population is the natural 

 biological unit of organization, although still a part of a larger 

 unit (the variety) the members of which are in actuality all but 

 completely cut off from one another. 



This situation raises many problems about which we can only 

 speculate at present; most of the information needed for more 

 solid analysis is lacking, though at least much of it is obtainable. 

 Soma speculation may stimulate efforts to supply the needed in- 

 formation. The problem of how paramecia get from one body of 

 water to another, as their distribution proves they must, is an old 

 but still unanswered one. Cysts capable of withstanding desicca- 

 tion have never been satisfactorily proved to occur in Parame- 

 cium. Human transportation of water must be a minor and recent 



