T. M. SONNEBORN 301 



constant and restricted conditions of life, as is done by sexual 

 inbreeders and asexual diploids. With the choice of one or the 

 other fundamental alternative go a host of further adaptations 

 to the system of breeding or reproduction, as set forth above in 

 detail for the Ciliates. The superficially chaotic differences in the 

 features of life of different closely related syngens acquire deep 

 significance as systematically selected means to the great end of 

 successful survival. The correlated problems of speciation that 

 tax the taxonomist and generate concepts from the geneticist are 

 trivial man-made difficulties that divert attention from the funda- 

 mental beauty and simplicity of nature's ways of meeting nature's 

 problems. 



Summary 



Statement of Problems and General Background. (1) Prob- 

 lems. Can biological species, i.e., the common gene pools, of 

 Ciliates be readily identified? Should they be assigned species 

 names? Should they be considered as species? Is it possible to 

 arrive at concepts of species and asexual equivalents of "biolog- 

 ical species" which would be generally applicable to inbreeders 

 and asexual organisms as well as to outbreeders? What is the 

 bearing of breeding systems and methods of reproduction on 

 species and evolutionary problems? What is the relation of the 

 varied life features of Ciliates to their breeding systems? 



2. Background. Inbreeders and outbreeders among the Cili- 

 ates were early recognized. Since the discovery of mating types 

 (1937), each taxonomic species has been found to include a 

 number of "biological species" or varieties. Each variety includes 

 a number of populations which are potentially able to interbreed; 

 but the varieties are sexually or genetically isolated. Information 

 is fullest on several taxonomic species of Paramecium. The chief 

 differential characters of these species are given. 



The Specificity of Mating Types and Isolation of Varieties in 

 P. aurelia. Of the 16 known varieties, all have two mating types 

 except variety 16 which has four. 3 Although flow of genes can 

 occur in the laboratory between different populations of the 

 same variety, the F2 generation shows in some varieties con- 



