T. M. SONNEBOHN 295 



a species and a syngen. The reverence now accorded, often un- 

 justly, to the term species would in time be transferred to the 

 term syngen, or whatever term may be adopted, because of its 

 greater evolutionary significance. The taxonomist and the ge- 

 neticist would both be free to perform their different tasks with 

 a logical and consistent terminology, neither being frustrated by 

 conflict with the other. The major remaining problem is whether 

 the two terms can be generalized. 



Generalization of the term species for the unit of identification 

 offers no great difficulty in principle. It simply requires a verbal 

 statement which embodies current sound taxonomic practice. 

 The first requirement is that the species be readily and visibly 

 identifiable as a group of organisms manifesting discontinuity in 

 morphology with all other groups. Secondly, the level of visible 

 discontinuity ideally is the simplest one judged to be untrans- 

 gressible by genetic means, either recombination or mutation. 

 Groups of organisms that are judged to show minimal, irrever- 

 sible, visible divergence thus are assigned to different species. 



The generalization of syngen is more difficult. Failure to recog- 

 nize the equivalent of the syngen in organisms that cannot out- 

 breed is due to the identification of the method of ascertainment 

 with the thing ascertained. The discreteness and reality of the 

 latter is based upon the genetic complexity of the features that 

 mark off a syngen from closely related syngens. This is what pre- 

 vents gene flow between them, and it is the reason that the test 

 of gene flow serves for ascertainment. Conceptually therefore a 

 syngen can be defined by the complexity of its genetic distinct- 

 ness from other syngens. A syngen, like a species, has thus passed 

 the threshold of irreversible evolutionary divergence; but, unlike 

 a species, it need not show readily recognized visible differentia- 

 tion. This level of biological organization is difficult to discover 

 in any kind of organism, but it can be done with any kind. With 

 outbreeders, the method is to test for gene flow. With asexual 

 organisms, the method is to compare different strains of a species 

 from every possible point of view and, with the fullest possible 

 array of facts, to arrive at judgments on discontinuities and the 

 probable complexity of their genetic basis. In spite of the differ- 



