508 S Indies in the Theory of Descent. 



The second kind of unequal group formation 

 arises from the circumstance that groups of equal 

 value can be formed from the two stages, but 

 these groups do not possess the same limits they 

 overlap, and only coincide in part. 



This is most clearly seen in the order Hymen- 

 optera, in which both larvae and imagines form 

 two well-defined morphological sub-orders, but 

 in such a manner that the one larval form not 

 only prevails throughout the whole of the one 

 sub-order of the imagines, but also extends beyond 

 and spreads over a great portion of the other 

 imaginal sub-order. 



Here again the dependence of this phenomenon 

 upon the influence of the environment is very 

 distinct, since it can be demonstrated (by the 

 embryology of bees) that the one form of larva 

 the maggot-type although the structure now 

 diverges so widely, has been developed from the 

 other form, and that it must have arisen by adapta- 

 tion to certain widely divergent conditions of life. 



This form of incongruence is always connected 

 with unequal divergence between the two stages of 

 the one systematic group in this case the Tere- 

 brant ia. The larvae of this imaginal group partly 

 possess caterpillar-like (Phytospheces) and partly 

 maggot-formed (Entomospheces) larvae, and differ 

 from one another to a considerably greater extent 

 than the saw-flies from the ichneumons. r The 



1 [This illustration of course only applies to the old arrange- 



