Phyletic Parallelism in Metamorphic Species. 395 



should always keep in mind that there are two 

 kinds of relationship form- and blood-relation- 

 ship which might possibly not always coincide. 



It has hitherto been tacitly assumed that the 

 degree of relationship between the imagines is 

 always the same as that between the larvae, and if 

 blood-relationship is spoken of this must naturally 

 be the case, since the larva and the imago are the 

 same individual. In all groups of animals we have 

 not always the means of deciding strictly between 

 form- and blood-relationship, and must accordingly 

 frequently content ourselves by taking simply the 

 form-relationship as the basis of our systems, 

 although the latter may not always express the 

 blood-relationship. But it is exactly in the 

 case of metamorphic species that there is no 

 necessity for, nor ought we to remain satisfied with, 

 this mode of procedure, since we have here two 

 kinds of form-relationship, that of the larvae and 

 that of the imagines, and, as I have just attempted 

 to show, it is by no means self-evident that these 

 always agree ; there are indeed already a sufficient 

 number of instances to show that such agreement 

 does not generally exist. 



This want of coincidence is strikingly shown in 

 a group of animals widely remote from the Insecta, 

 viz. the Hydromedusae, the systematic arrange- 

 ment of which is quite different according as this 

 is based on the polypoid or on the medusoid gene- 

 ration. Thus, the medusoid family of the oceanic 



