462 Studies in the Theory of Descent. 



ism indeed agrees quite well with the phenomenon 

 of congruence, but not with that of incongruence. 

 Since a large number of cases of the latter depend 

 upon the fact that the larvae are more frequently 

 influenced by causes of change than their imagines, 

 or vice versa, how can this be reconciled with such 

 an internal force ? On this assumption would not 

 each stage of a species be compelled to change, if 

 not contemporaneously at least successively, with 

 the same frequency and intensity, by the action of 

 an innate force ? and how by means of the latter 

 can there ever result a greater form-divergence in 

 the larvae than in the imagines ? 



It is delusive to believe that these unequal 

 deviations can be explained by assuming that the 

 phyletic force acts periodically. Granting that it 

 does so, and that the internal power successively 

 compels the imago, pupa, and finally the larva to 

 change, there would then pass a kind of wave of 

 transformation over the different stages of the 

 species, as was actually shown above to be the case 

 in the single larval stages. The only possible way 

 of explaining the unequal distances between larvae 

 and imagines would therefore be to assume that 

 two allied groups, e. g. species, were not contem- 

 poraneously affected by the wave, so that at a cer- 

 tain period of time the imago alone of one species 

 had become changed, whilst in the other species 

 the wave of transformation had also reached the 

 larva. In this case the imagines of the two 



