440 Studies in the Theory of Descent. 



If the " natural " system is the expression of the 

 genetic relationship of living forms, the question 

 arises in this and in similar cases as to whether 

 the more credence is to be attached to the larvae 

 or to the imagines or, in more scientific phraseo- 

 logy, which of the two inherited classes of charac- 

 ters have been the most distinctly and completely 

 preserved, and which of these, through its form- 

 relationship, admits of the most distinct recogni- 

 tion of the blood-relationship, or, inversely, which 

 has diverged the most widely from the ancestral 

 form ? The decision in single instances cannot 

 but be difficult, and appears indeed at first sight 

 impossible ; nevertheless this will be arrived at in 

 most cases as soon as the ontogeny of the larvae, 

 and therewith a portion of the phylogeny of this 

 stage, can be accurately ascertained. 



As in the Rhopalocera most of the families show 

 a complete congruence in the form-relationship of 

 the caterpillars and perfect insects, so a similar 

 congruence is also found in the majority of the 

 families belonging to other groups. Thus, the two 

 allied families of the group Sphingina can also be 

 very well characterized by their larvae ; 8 both the 



This group of moths (" Schwarmer ") is regarded as of very 

 different extents by systematists ; when I here comprise under 

 it only the Sphingidie proper and the Sesiidie, I by no means 

 ignore the grounds which favour a greater extension of the 

 group ; the latter is not rigidly limited. [The affinities of the 

 Sesiidce (sEgeriidie) are by no means clearly made out it 



