Phyiclic Parallelism in Mctamorphic Species. 505 



Hymenoptera differ from one another to a much 

 greater extent than their imagines, since the 

 latter have experienced a complete transformation 

 of typical parts ; whilst in the caterpillar-formed 

 larva; these parts vary only within moderate limits. 

 Similarly in the case of the Diptera, of which the 

 gnat-like larvae diverge more widely from those of 

 the grub type than do the gnats from the true 

 Hies. On the other hand the divergence between 

 the imagines of the fleas and gnats is considerably 

 greater than that between their larvae indeed 

 the larvae of the fleas would have to be ranked as 

 a family of the sub-order of the gnat-like larvae 

 if we wished to carry out a larval classification. 

 By this it is also made evident that these unequal 

 divergences, when they occur in the higher 

 systematic groups, always induce at the same 

 time the second form of incongruence that of the 

 formation of unequal systematic groups. 



In general whenever such unequal divergences 

 occur in the higher groups they run parallel with 

 a strong deviation in the conditions of life. If 

 these differ more strongly on the side of the 

 larva;, we find that the structure of the latter like- 

 wise diverges the more widely, and that their 

 form-relationship is in consequence made more 

 remote (saw-flies and ichneumons, gnats and flies) ; 

 if, on the ether hand, the difference in the con- 

 ditions of life is greater on the side of the 

 imagines, we find among the latter the greater 



