Phyletic Parallelism in Metamorphic Species. 391 



in which the different forms that occur are dis- 

 tributed among different individuals alternating 

 with and proceeding from one another (alternation 

 of generation). Nevertheless, it is precisely here 

 that quite distinct form-relationships would be 

 expected according as the development of the 

 organic world depended on a phyletic vital force, 

 or was simply the response of the specific organism 

 to the action of the environment. 



Assuming the first to be the case, there must 

 have occurred, and must still occur, what I desig- 

 nate " phyletic parallelism," i. e. the two stages of 

 metamorphic species must have undergone a pre- 

 cisely parallel development every change in the 

 butterfly must have been accompanied or followed 

 by a change in the caterpillar, and the systematic 

 groups of the butterflies must be also found in a 

 precisely corresponding manner in a systematic 

 grouping of the caterpillars. If species are able 

 to fashion themselves into new forms by an innate 

 power causing periodic change, this re-moulding 

 cannot possibly affect only one single stage of 

 development such as the larva only but would 

 rather extend, either contemporaneously or suc- 

 cessively, to all stages larva, pupa, and imago : 

 each stage would acquire a new form, and it might 

 even be expected that each would change to the 

 same extent. At least, it cannot be perceived 

 why a purely internal force should influence the 

 development of one stage more than that of 



