434 Studies in the Theory of Descent. 



of affairs. Although the larval structure is so 

 characteristic in the individual families of butter- 

 flies, these "larval-families" cannot be united into 

 a larger group by any common characters, and the 

 " Rhopalocera " would never have been estab- 

 lished if only the larvae had been known. It is 

 true that they all have sixteen legs, that they 

 never possess a Sphinx-like horn, and that they 

 are seldom hairy, as is the case with many Bom- 

 bycida? but these common negative characters 

 occur also in quite distinct groups. 



In the butterflies, therefore, a perfect congruence 

 of form-relationship does not exist, inasmuch as 

 the imagines constitute one large group of higher 

 order whilst the larvae can only be formed into 

 families. If it be admitted that the common 

 characters of butterflies depend on their deriva- 

 tion from a common ancestor, the imagines must 

 have retained certain common characters which 

 enable them to be recognized as allies, whilst the 

 larvae have preserved no such characters from the 

 period at which the families diverged. 



Without going at present into the causes of 

 these phenomena I will pass on to the considera- 



Annual Report" on the insects of Missouri, 1876, p. 170. 

 With reference to the antennae as a distinguishing character, 

 see Mr. A. G. Butler's article in "Science for All," 1880, 

 part xxvii. p. 65. R.M.] 



* The genus of Morphituz, Discophora, possesses hairs very 

 similar to those of the genus Cnethocampa belonging to the 

 Geometra. 



