398 Studies in the Theory of Descent. 



of life produce changes of structure. Neither in 

 the number of known species of larvae, nor in the 

 intimate knowledge of their mode of life, can any 

 of the remaining orders of insects compete with the 

 Lepidoptera. There is no Dipterous or Hymenop- 

 terous genus in which ten or more species are so 

 intimately known in the larval stage that they can 

 be employed for the purposes of morphological 

 comparison. Who is able to define the distinctions 

 between the life-conditions of the larvae of twenty 

 different species of Culex or of Tipula f The 

 caterpillars of closely allied species of Lepidoptera, 

 on the other hand, frequently live on different 

 plants, from which circumstance alone a certain 

 difference in the life-conditions is brought about. 



The chief question which the research had to 

 reply to was the following : Does there exist a 

 complete phyletic parallelism among Lepidoptera 

 or not ? or, more precisely speaking : Can we 

 infer, from the form-relationships which at pre- 

 sent exist between larvae on the one hand and 

 imagines on the other, an exactly parallel course 

 of phyletic development in both stages ; or do 

 incongruences of form-relationship exist which 

 point to unequal development ? 



Before I proceed to the solution of this question 

 it is indispensable that one point should be cleared 

 up which has not been hitherto touched upon, 

 but which must be settled before the problem 

 can be formally stated in general terms. Before 



