506 Studies in the Theory of Descent. 



morphological divergence (butterflies and moths, 

 gnats and fleas). 



(2.) The second chief form of incongruence 

 consists in the formation of different systematic 

 groups by the larvse and the imagines, if the latter 

 are grouped simply according to their form- 

 relationship without reference to their genetic 

 affinities. This incongruence again shows itself 

 in two forms in the formation of groups of 

 unequal value, and the formation of groups equal 

 in value but unequal in extent, i. e. of overlapping 

 instead of coinciding groups. 



Of these two forms the first arises as the direct 

 result of a different amount of divergence. Thus 

 the larvse of the fleas, on account of their small 

 divergence from those of the gnats,*could only 

 lay claim to the rank of a family, whilst their 

 imagines are separated from the gnats by such a 

 wide form-divergence that they are correctly 

 ranked as a distinct tribe or sub-order. 



The inequalities in the lowest groups, varieties, 

 can be regarded in a precisely similar manner. If 

 the larva of a species has become split up into 

 two local forms, but not the imago, each of the 

 two larval forms possesses only the rank of a 

 variety, whilst the imaginal form has the value of a 

 species. 



Less simple are the causes of the phenomenon 

 that in the one stage the lower groups can be 

 combined into one of higher rank, whilst the other 



