T. M. SONNEBORN 293 



and the modern biological species concept; (2) the relation of 

 breeding systems and methods of reproduction to the species 

 problem; and (3) the significance of differences in the major fea- 

 tures of life. These topics have been touched upon repeatedly 

 throughout this paper in connection with the treatment of the 

 various organisms discussed. The following resume brings to- 

 gether the relevant points on each topic. 



The Problem of Taxonomic Species and the Modern Biological 

 Species Concept. The modern biological species concept of a 

 potentially common gene pool is in principle limited to a fraction 

 of existing organisms and can in practice be applied to but a 

 small fraction of these. It excludes all purely asexual organisms 

 and all obligatory self-fertilizers. Into this category fall perhaps 

 half or more of the Protozoa and a large number of plants in all 

 the major groups, as well as many invertebrates. Among the or- 

 ganisms to which the biological species concept applies in prin- 

 ciple, relatively few can ever be sorted with assurance into such 

 species because the task of accomplishing this in any one organ- 

 ism is so great. 



The fact that closely related biological species sometimes can- 

 not be readily identified leads to the logical inconsistency of 

 grouping a number of species into a species. Thus the species 

 Paramecium aiirelia consists of at least 16 biological species (re- 

 ferred to as varieties in this paper). These can be identified only 

 under special conditions which are not generally available or only 

 with great labor and prolonged research. Biological species of 

 this sort are referred to as sibling species (Mayr, 1948) and may 

 remain unnamed. A single species name may be used for the 

 whole group of them. Regardless of the mental reservations about 

 what the "real" species are in such cases, this procedure is tanta- 

 mount to admitting that identifiability cannot be ignored in a 

 workable species concept. Beginning with logical inconsistency 

 we end in terminological inconsistency, using the word species to 

 denote three different things: taxonomic species not analyzed as 

 to gene flow, biological species, and groups of closely related and 

 practically unidentifiable sibling species. 



The enthusiasm for the biological species concept in spite of 



