NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I49 



for it is seldom found that all characters will fit into a scheme of 

 relationship that attempts to relate every feature in one form with a 

 similar feature in another. It must be recognized that various struc- 

 tural adaptations have been often independently developed in approxi- 

 mately the same way. A successful adaptation will be equally valuable 

 in many groups, and it is, therefore, not surprising that an adaptive 

 structure should independently recur either in distantly related or 

 in closely related groups. To distinguish between such structures 

 and those that have had an identical origin, however, is one of the 

 most uncertain tasks of the phylogeneticist, but the very condition of 

 uncertainty injects into the study of phylogeny the element of per- 

 sonal opinion which gives to phylogeny that controversial status by 

 which it never lacks in interest. Every biologist must have a working 

 creed of phylogeny, but he should not too implicitly believe its tenets. 



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