Phylettc Parallelism in Metamorphic Species. 469 



a similar amount of divergence between the larva; 

 and between the imagines, although on the one 

 side only *typical parts i. e. those of no impor- 

 tance to the whole organization and on the 

 other side typical parts, were affected. The 

 www&rofthe changes would here alone determine 

 whether congruence or incongruence occurred 

 between the two stages. 



The case would be quite different if, through- 

 out a long period of time, in the one stage only 

 typical and in the other only untypical parts were 

 subjected to change. In the first case a complete 

 transformation of the whole structure would occur, 

 since not only would the typical parts, such as the 

 wings, undergo a much further and increasing 

 transformation in the same direction, but these 

 changes would also lead to secondary altera- 

 tions. 



In this manner, I believe, must be explained 

 the fact that in the higher groups still greater 

 form-divergences of the two stages occur ; and if 

 this explanation is correct, the cause of this strik- 

 ing phenomenon, viz., that incongruence diminishes 

 from varieties to genera, in which latter it occurs 

 but exceptionally, whilst in families and in the 

 higher groups it again continually increases, is 

 likewise revealed. Up to genera the incongru- 

 ence depends entirely upon the one stage having 

 become changed more frequently than the other ; 

 but in families and groups of families, and in the 



