Phyletic Parallelism in Metamorphic Species. 393 



By this means there would arise an unequal 

 difference between the two stages of two species. 

 Thus, the butterflies, supposing these to have 

 become changed, would bear a more remote form- 

 relationship to each other than the caterpillars, 

 and the differences between the former (imagines) 

 would always be greater than that between the 

 larvae if the butterflies were, at several successive 

 periods, affected by changing influences whilst the 

 larvae continued under the same conditions and 

 accordingly remained unaltered. The two stages 

 would not coincide in their phyletic development - 

 the latter could not be expressed by parallel lines, 

 and we should accordingly expect to find that 

 there was by no means a complete congruity 

 between the systems founded on the larval and 

 imaginal characters respectively, but rather that 

 the caterpillars frequently formed different sys- 

 tematic groups to the butterflies. 2 



Accordingly, the problem to be investigated 

 was whether in those species which develope by 

 means of metamorphosis, and of which the indi- 

 vidual stages exist under very different conditions 

 of life, a complete phyletic parallelism was to be 

 found or not. This cannot be decided directly 

 since we cannot see the phyletic development 

 unfolded under our observation, but it can be 



* [For Fritz Miiller's application of this principle to the case 

 of certain groups of Brazilian butterflies see Appendix II. to this 

 Part. R.M.] 



