A TAXONOMIC STUDY ON PYEOSOMA — METCALF AND HOPKINS. 197 



If some of these forms were to be exterminated, leaving the gaps 

 between species more evident, the specific value of the remaining 

 complexes of qualities would be more clearly seen, but no more real. 



But the treatment of each of these complexes of characters as a 

 true species to be described and illustrated compels us to almost 

 endless labor and produces results intelligible only to special students 

 who have the minutest details of structure clearly in mind. It is 

 not a practical solution of the taxonomic problem. One must 

 adopt a more conventional conception of species than this. 



In the genus Pyrosoma, so far as now known, there is one major line 

 of cleavage, apparently not obscured by intercrossing. This may be 

 indicated by recognizing two subgenera, Pyrosomata jixata and 

 Pyrosomata ambulata, and we find that these prove to be sharply 

 distinct. Within each subgenus so recognized, there are divergent 

 groups of forms, and between some of these groups intergradation is 

 not observed or is far from complete. These groups may conven- 

 iently be classed as species. Minor divergencies within these groups, 

 where intergradation is imperfect, may be given subspecific value. 

 There remain still, especially in the group we name atlanticum, a 

 number of forms which diverge considerably at the extremes, but 

 completely intergrade through intermediate forms. What shall we 

 do with these? We cannot describe each condition observed. 

 This would mean a separate description for almost each colony. 

 We compromise by describing as a "forma " dipleurosoma, one of the 

 most interesting conditions, and grouping the rest under the con- 

 venient but not scientifically classificatory term "intermedium.'" 



What is the meaning of the taxonomic conditions described for 

 Pyrosoma ? What has been the history of the genus ? Were the 

 lines of cleavage into "subgenera," "species" and "subspecies," 

 which we find to-day, established long ago, to become partially 

 obscured by a more recent period of mutation accompanied by inter- 

 breeding, or are the subdivisions of the genus becoming established 

 in the midst of present conditions ? I do not see that the data we 

 possess can be so analyzed as to answer these questions for us. We 

 do not even know whether mutation is continuing at present. Until 

 we can answer this most fundamental question there seems little 

 chance of even valuable conjecture as to the further questions that 

 suggest themselves. 



There are, of course, many parallels to the condition of flux seen 

 in Pyrosoma, especially in the atlanticum group. Perhaps as strik- 

 ing an example as any is seen in Neretina virginea (Metcalf, 1904), 

 whose color and color pattern show the greatest divergence in dif- 

 ferent individuals and the most complete intergradation between any 

 two forms one may select. Similarly the silver spot butterflies 

 (Argynnis), which have been analyzed into many species, show 

 complete intergradation between the divergent forms. The con- 



