Phyletic Parallelism in Mctamorphic Species. 4 1 5 



ations is likewise correct, viz., the supposition that 

 the changes of colour and marking in caterpillars, 

 pupae, and imagines result from external influences 

 only. 



This must not be taken as signifying that the 

 single stages of the larval development are also 

 only able to change through the action of external 

 influences. The larval stages are correlated with 

 each other, as has already been shown (see the 

 previous essay): new characters arise in the adult 

 caterpillar at the last stage and are then gradually 

 transferred back to the younger stages quite in- 

 dependently of external influences, this recession 

 being entirely brought about by the laws of correla- 

 tion. Natural selection here only exerts a secondary 

 action, since it can accelerate or retard this trans- 

 ference, according as the new characters are advan- 

 tageous or disadvantageous to the younger stages. 



Now as considerable individual differences 

 appear in the first acquisition of a new character 

 with respect to the rapidity and completeness with 

 which the individuals acquire such a character, 

 the same must obtain for the transference of an 

 improvement acquired in the last stage to the next 

 younger stage. The new character would be 

 acquired by different individuals in different degrees 

 and at different rates it would have, to a certain 

 extent, to struggle with the older characters of the 

 stage ; in brief, the younger stage would become 

 variable. 



