504 Studies in the Theory of Descent. 



stage (the larva) has actually remained at an older 

 phyletic grade (Deilebhila species). Incongru- 

 ences of this kind depending entirely upon the 

 more frequent action of transforming impulses can 

 only become observable in the smaller systematic 

 groups, in the larger they elude comparative exa- 

 mination. In the higher groups unequal form- 

 divergence may be produced by the transforming 

 impulses affecting parts of unequal physiological 

 and morphological value, or by their influencing 

 parts of equal value in different degrees. All 

 effects of this kind can, however, only become 

 manifest after a long-continued accumulation of 

 single changes, i, e. only in those systematic 

 groups which require a long period of time for 

 their formation. By this means we can completely 

 explain why the incongruences of form-divergence 

 continually diminish from varieties to genera, and 

 then increase again from genera upwards through 

 families, tribes, and sub-orders : the first diminish- 

 ing incongruence depends upon an ^lnequal 

 number of transforming impulses, the latter in- 

 creasing incongruence depends upon the unequal 

 power of these impulses. 



Cases of the second kind are found among 

 the Lepidopterous families, and especially in the 

 higher groups (R/iopalocera and Heteroccra)^ and 

 appear still more striking in the higher groups of 

 the Hymenoptera and Diptera. Thus the cater- 

 pillar shaped and maggot-formed larvae of the 





