Phyletic Parallelism in Metamorphic Species. 5 1 3 



precisely the " typical " parts which become 

 partly suppressed and partly converted into an 

 entirely new structure. From the ancient typical 

 appendages there have here arisen new structures, 

 which again have every right to be considered as 

 typical. This transformation is not to be compared 

 with that experienced by the swimming appen- 

 dages of the Nauplius-\\\iz. ancestor of an Apus 

 or Branchipus which have become mandibulate, 

 nor with the transformation which the anterior 

 limbs must have gone through in the reptilian 

 ancestors of birds. The changes in question 

 (Dipterous larvae) go still further and are more 

 profound. I lay great emphasis upon this because 

 we have here one of the few cases which show 

 that typical parts are quite as dependent upon the 

 environment as untypical structures, and that the 

 former are not only able to become adapted to 

 external conditions by small modifications as 

 shown in a most striking manner by the transfor- 

 mations of the appendages in the Crustacea and 

 Vertebrata but that these parts can become 

 modelled on an entirely new type which, when 

 perfected, gives no means of divining its mode of 

 origin. I may here repeat a former statement : 

 With reference to the causes of their origination 

 we have no grounds for drawing a distinction be- 

 tween typical and untypical structures. 



It may be mentioned in concluding that quite 

 analogous although less sharply defined results are 



L 1 



