496 Studies in the Theory of Descent. 



made a great advance in the process of trans- 

 formation. 



The grub-like larvae of the Hymenoptera and 

 Diptera appear to me especially instructive with 

 reference to the main question of the causes of 

 transformation. The reply to the questions : 

 what gives the impetus to change ? is this 

 impetus internal or external ? can scarcely be 

 given with greater clearness than here. If these 

 larvae have abandoned their ancestral form and 

 have acquired a widely divergent structure, arising 

 not only from suppression but partly also from an 

 essentially new differentiation (suctorial head of 

 the Muscida), and if these structural changes show 

 a close adaptation to the existing conditions of 

 life, from these considerations alone it is difficult 

 to conceive how such transformations can depend 

 upon the action of a phyletic force. The latter 

 must have foreseen that at precisely this or that 

 fixed period of time the ancestors of these larvae 

 would have been placed under conditions of life 

 which would make it desirable for them to be 

 modified into the maggot-type. But if at the 

 same time the imagines are removed in a less 

 degree from those of the caterpillar-like larvae, 

 this divergence being in exact relation with the 

 deviations in the conditions of life, I at least fail 

 to see how we can escape the consequence that it 

 is the external conditions of life which produce 

 the transformations and induce the organism to 



