158 



The Journal ok Hkrhdity 



processes for maintaining the networks 

 of descent of the species. 



Why the interweaving of lines of 

 descent in specific groups is necessary to 

 maintain the jjower of celkilar organiza- 

 tion we do not know, but neither do we 

 know why the overla])ping of metal 

 plates generates the electric current in a 

 voltaic pile. The facts are established 

 by repeated observation, and should 

 not be disregarded in our attempts to 

 understand related phenomena. Of 

 course, these complexities of specific 

 organization and sexuality are very 

 unwelcome ideas to those who are about 

 to solve the problems of evolution and 

 heredity by simple experimental and 

 statistical methods, but no truly bio- 

 logical investigation can disregard the 

 fundamental fact that organisms exist 

 in s])ecies. 



That categories of classification of 

 species arc artificial is not an indication 

 that the groups themselves have no real 

 existence. As well might we say that 

 continents and islands have no real 

 existence because their shore-lines are 

 not definitely fixed and do not coincide 

 with parallels of latitude and longitude 

 which geograi^hers use to determine loca- 

 tions. The object of biological classifi- 

 cation is to find our w^ay about in the 

 endless diversities of organic nature, 

 and for this purpose many conventional 

 devices are employed, but these should 

 not be allowed to conceal the more essen- 

 tial facts. 



FINDING THE LIMITS OF SPECIES. 



Students of classification are always 

 seeking diagnostic differences between 

 the various species and higher grovips, 

 for the task of discovering such charac- 

 ters and framing them into words is very 

 difficult. Hence taxonomy makes only 

 gradual progress, like other dej^artments 

 of science. Yet the formal nature of the 

 concepts and categories of classifica- 

 tion does not affect the concrete nature 

 of the groui)s that are being classified. 

 The categories of the biologist are arti- 

 ficial like those of the geograi)her, but 

 not more so. Geograi:)hers do not agree 

 in the groujnng of the arehii)elagoes of 

 the Pacific Ocean, but this is not con- 



sidered a reason for denying that islands 

 exist or that some islands are close 

 together and others far apart. Species 

 are biological islands, in a sea of non- 

 existence. The higher categories of 

 classification, such as genera, families 

 and orders, are more conceptual than 

 species, in that they do not form coher- 

 ent networks of lines of descent, but even 

 this deficiency does not render them pure 

 abstractions. They are to be considered 

 rather as collective entities, that is, 

 they are groups of species that stand in 

 certain phylogcnetic relations to each 

 other, corresponding to the positional 

 relations that detennine geographic 

 groups. 



It is true that the boundaries of 

 species often appear less definite in 

 nature than in books, Ijut the same is 

 true of islands where the shore lines 

 change with every tide, to say nothing of 

 the more extensive and permanent 

 changes by erosion of cliffs or ele\'ation 

 of beaches. Yet these incidental limita- 

 tions need not destroy concrete ideas of 

 islands as continuous bodies of land, nor 

 of species as coherent groups of organ- 

 isms. The members of each species are 

 bound together by a network of lines 

 interbreeding into a physiological unity, 

 quite independent of mor]jhological 

 similarities or diversities inside the 

 species. It is organization that con- 

 stitutes the species, not the characters 

 that may be ascribed to it. 



The difiiculties of classifying and dis- 

 covering diagnostic characters for the 

 several millions of species that exist on 

 the earth's surface should not be allowed 

 to confuse the minds of physiological 

 and statistical workers, or to obscure 

 the reality of specific organization. The 

 specific structure or speciety of living 

 matter is as truly a fact as any other 

 biological phenomenon. That there 

 should be so many species, and so diffi- 

 cult to classify, adds annoN'ing com- 

 ])lexities to biological investigation, but 

 the difficulties are not removed by dis- 

 regarding the existence of si)ecies. 

 Sailors encounter similar difficulties in 

 the navigation of arehij^elagoes, but do 

 not find it safe to o\'erlook any of the 

 islands that lie in their cf)urses. 



