Phyletic Parallelism in Metamorphic Species. 399 



it can be asked whether larvae and imagines have 

 undergone a precisely parallel development, we 

 must know whether unequal development is pos- 

 sible whether there does not exist such an 

 intimate structural relationship between the two 

 stages that every change in one of these must 

 bring about a change in the other. Were this the 

 case, every change in the butterfly would cause a 

 correlative change in the caterpillar, and vice versa, 

 so that an inequality of form-relationship between 

 the larvae on one hand and the imagines on 

 the other would be inconceivable systems based 

 on the characters of the caterpillars would com- 

 pletely coincide with those based on the characters 

 of the butterflies and we should arrive at a false 

 conclusion if we attributed the phyletically parallel 

 development of the two stages to the existence 

 of an internal phyletic force, whilst it was only 

 the known factor, correlation, which caused the 

 equality of the course of development. 



For these reasons it must first be established 

 that the larva and imago are not respectively fixed 

 in form, and the whole of the first section will there- 

 fore be devoted to proving that the two stages 

 change independently of one another. Conclusions 

 as to the causes of change will then be drawn, and 

 these will corroborate from another side a subse- 

 quent inquiry as to the presence or absence of com- 

 plete congruence in the two morphological systems. 

 The two questions the answers to which will be 



