460 Studies in the Theory of Descent. 



the larvae which, to a certain extent, remain at a 

 lower grade, and which form well defined families ; 

 but these can seldom be associated into groups of 

 a higher order having a common character, as in 

 the case of the imagines (Rhopalocera). 



After having thus collected (so far as I am able) 

 the facts, we have now to attempt their interpreta- 

 tion, and from the observed congruence and incon- 

 gruence of form-relationship of the two stages to 

 endeavour to draw a conclusion as to the under- 

 lying causes of the transformations. 



It is clear at starting that all cases of incongru- 

 ence can only be the expression or the consequence 

 of a phyletic development which has not been 

 exactly parallel in the two stages of larva and 

 imago that one stage must have changed either 

 more rapidly or more slowly than the other. An 

 " unequal phyletic development " is thus the imme- 

 diate cause of incongruence. 



Thus, the occurrence of different larvae in species 

 of which the imagines have remained alike may be 

 simply understood as cases in which the imago 

 only has experienced a change has taken a forward 

 step in phyletic development, whilst the larvae have 

 remained behind. If we conceive this one-sided 

 development to be repeated several times, there 

 would arise two larval forms as widely different as 

 those of Deilephila Nicaa, and Euphorbia, whilst 

 the imagines, as is actually the case io these 

 species, would remain the same. 



