Phylctic Parallelism in Metamorphic S/*fies. 501 



In this conclusion there is implied as it appears 

 to me an important theoretical result which tells 

 further against the efficacy of a phyletic force. 



If the so-called "typical parts" of an animal 

 disappear completely through the action of the 

 environment only, and still further, if these parts 

 cun become so entirely modified as to give rise to 

 quite new and again typical structures (suctorial 

 head of the Muscida) without the typical parts of 

 the other stage of the same individual being 

 thereby modified and transformed into a new type 

 of structure, how can we maintain a distinction 

 between typical and non-typical parts with respect 

 to their origin ? But if a difference exists with 

 respect only to the physiological importance of 

 such parts, i.e. their importance for the equilibrium 

 of the whole organization, while, with reference to 

 transformation and suppression, exactly the same 

 influences appear to be effective, as those which 

 bring about a change in or a disappearance of the 

 so-called adventitious parts, where is there left any 

 scope for the operation of the supposed phyletic 

 force ? What right have we to assume that the 

 typical structures arise by the action of a vital 

 force? Nevertheless this is the final refuge of 

 those who are bound to admit that a great num- 

 ber of parts or characters of an animal can become 

 changed, suppressed, or even produced by the 

 action of the environment. 



