T. M. SONNEBORN 289 



most of the last hundred years, it has been tacitly assumed that 

 these two aspects of species coincide and that, since the evolu- 

 tionary unit often represents a real level of biological organiza- 

 tion, species exist as real entities, not as human constructs made 

 for the convenience of biologists. Modern researches, on the con- 

 trary, have shown that the evolutionary unit is not always readily 

 recognizable. In such cases, the proponents of the modern biolog- 

 ical species concept admit the dual implications of the term 

 species by distinguishing named species, those which are readily 

 recognizable, from sibling species (Mayr, 1948), those which are 

 often not named because they are not readily recognizable. This 

 leads to the totally illogical procedure of referring to a group of 

 species as a species, i.e., a group of unnamed sibling species has 

 a single species name. Moreover, by reserving species names for 

 readily recognized groups, this procedure tacitly admits that the 

 convenience of biologists must first of all be served by the term 

 species. Agreeing with this, I propose, in the interest of logic and 

 clarity, to use a different term, syngen, for the evolutionary unit. 

 The same group of organisms would be both a species and a syn- 

 gen if both the criteria of ready recognition and minimal irre- 

 versible evolutionary divergence were met. Otherwise, not. The 

 reluctance of proponents of the modern biological species concept 

 to take this logical step seems to be based on their love of the 

 word species. Its early connection with evolution theory, its wide 

 usage, and general familiarity with it make it a great prize to cap- 

 ture and invest with absolute existence as a definite level of bio- 

 logical organization of the greatest evolutionary significance. This 

 cannot be done without sacrifice of logic and introduction of con- 

 fusion. 



Once the brave but hopeless effort of the proponents of the 

 modern biological species concept is abandoned, it will again be 

 generally recognized that species in sexual and asexual organisms 

 have been and can be defined on essentially the same principle. 

 The principle is simply minimal irreversible evolutionary di- 

 vergence that yields readily recognizable difference. With this 

 principle guiding him, no greater task is placed upon the skill of 

 the taxonomist in classifying asexual organisms into species than 



