47 2 S (ttdies in tlie Theory of Descent. 



that of the larvae. Such cases actually occur, and 

 an instance of this kind will be mentioned later in 

 connection with the Diptera. But this alone does 

 not explain why, on the side of the imagines, a 

 whole series of families show the same amount of 

 morphological divergence from the families of 

 other ' groups. There are two things, therefore, 

 which must here be explained : First, why is the 

 form-divergence between the imagines of the 

 Rhopalocera and Heterocera greater than that 

 between their larvae ? and, secondly, why can 

 the imagines of the Rhopalocera be formed into 

 one large group by means of common characters 

 whilst the larvae cannot ? 



The answers to both these questions can easily 

 be given from our present standpoint. As far as 

 the first question is concerned, this finds its solu- 

 tion in the fact that the form-divergence always 

 corresponds exactly with the divergence of func- 

 tion, i. e. with the divergence in the mode of life. 



If we compare a butterfly with a moth there 

 can be no doubt chat the difference in the con- 

 ditions of life is far greater on the side of the 

 imagines than on that of the larvae. The differ- 

 ences in the mode of life of the larvae are on the 

 whole but very small. They are all vegetable 

 feeders, requiring large quantities of food, and can 

 only cease feeding during a short time, for which 

 reason they never leave their food-plants fpr long, 

 'and it is of more importance for them to remain 



