394 Stztdies in the Theory of Descent. 



established indirectly by examining and comparing 

 with each other the form-relationships of the 

 two separate stages by confronting the larval 

 and imaginal systematic groups. If the phyletic 

 development has been parallel and perfectly equal, 

 so also must its end-results the forms at present 

 existing stand at equal distances from one 

 another ; larval and imaginal systems must coin- 

 cide and be congruent. If the course of the 

 phyletic development has not been parallel, there 

 must appear inequalities incongruences between 

 the two systems. 



I am certain that systematists of the old school 

 will read these lines with dismay. Do we not 

 regard it as a considerable advance in taxonomy 

 that we have generally ceased to classify species 

 simply according to one or to some few characters, 

 and that we now take into consideration not 

 merely the last stage of the development (the 

 imago), but likewise the widely divergent young 

 stages (larva and pupa) ? And now shall it not 

 be investigated whether caterpillars and butterflies 

 do not form quite distinct systems ? In the case 

 of new species of butterflies of doubtful systematic 

 position was not always the first question : what 

 is the nature of the caterpillars ? and did not this 

 frequently throw light upon the relationships of the 

 imago ? Assuredly ; and without any doubt we 

 have been quite correct in taking the larval struc- 

 ture into consideration. But in so doing we 



