Pkylttu Parallelism in Metamorphic Species. 499 



rank them with the true Diptera under the sub- 

 order Nonocera. On first finding such a larva we 

 should expect to see emerge from the pupa a small 

 gnat. 



While the imagines of the Ncmocera and 

 Aphaniptera thus show but a very remote form- 

 relationship their larvae are very closely allied. 

 Can any one doubt that in this case it is not the 

 larva but the imago which has diverged to the 

 greatest extent ? Have not the fleas moreover 

 become adapted to conditions of life widely 

 different from those of all other Diptera, whilst 

 their larvae do not differ in this respect from many 

 other Dipterous larvae ? 



We have here, therefore, another case of une- 

 qual phyletic development, which manifests itself in 

 the entirely different form-relationship of the larvae 

 and the imagines. Thus in this case, as in that of 

 the Lepidoptera, it is sometimes the larval and at 

 other times the imaginal stage which has ex- 

 perienced the greatest transformation, and, as in 

 the order mentioned, the objection that a phyletic 

 vital force produces greater and more important 

 differentiations in the higher imaginal stage than in 

 the lower or less developed larval stage, is equally 

 ineffectual. 



If, however, it be asked whether the unequal 

 phyletic development depends in this case upon an 

 unequal number of transforming impulses which 

 the two stages may have experienced during an 



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