Evolution and Taxonomy 



63 



of the principal vein will be nearer the base of the wing in the 

 more specialized forms than in the more generalized members 

 of the same group. On the other hand when the tendency of 

 natural selection is to narrow one of these areas the branches 

 become consolidated with the main stem to a greater and 

 greater distance from the base in the more and more special- 

 ized forms. This consolidation of a branch with the main 

 stem or of two branches with each other may extend to the 

 margin of the wing, and thus the number of branches be re- 

 duced. This migration of the point of origin of a branch of a 



Fig. \2.—Prionoxystus ; f. frenulum and frenulum brace, enlarged. 



vein often aiTords an excellent clew to the degree of departure 

 from a more generalized type. 



But the most remarkable of the changes which take place 

 in this region of the wing is an abortion of the main trunk of 

 media and a consequent uniting of the branches of this vein 

 either with cubitus or with both cubitus and radius. Ex- 

 cellent illustrations of this occur in the Lepidoptera. In many 

 of the more generalized moths the main trunk of media is well 

 preserved (Fig. 12) ; while in more specialized forms it is en- 



