Evolution and Taxonomy 45 



the group, and in the direction of specialization of the de- 

 scendants of this progenitor. 



Thus, to use again the illustration given above, the Jugatse 

 are essentially characterized as the descendants of those an- 

 cient Lepidoptera in which the wings of each side were united 

 by a jugum ; and they are also characterized hy a tendency 

 towards an equal reduction of the veins of the two pairs of wings. 

 While the Frenatse are essentially characterized as the descend- 

 ants of those ancient lyepidoptera in which the wings of each side 

 were united by a frenulum ; and they are also characterized by 

 a tendencj^ towards a greater reduction of the veins of the hind 

 wings than of the fore wings, or, in other words, by a tendency 

 towards a cephalization of the powers of flight. The fact that 

 in many of the Frenatse the frenulum has been lost, does not 

 invalidate in the least the truth of this characterization. The 

 loss of the frenulum, however, in certain Frenatse renders 

 necessary the use of same other character or characters by the 

 systematists as recognition characters. 



The recognition characters are those usually first observed 

 by the investigator, and are those commonly given in taxo- 

 noniic works. In many cases these recognition characters are 

 also essential characters, especially in the case of groups that 

 have been thoroughly studied. But b}^ the taxonomic 

 methods now commonly used search is chiefly made for recog- 

 nition characters. The more skilled the systematist the more 

 likely is he to discover and use as recognition characters 

 those that are really essential, although the distinction pointed 

 out here may not be recognized by him. 



In the case of those groups where but few or no general- 

 ized forms have persisted till this time, the essential characters 

 must to a greater or less extent be inferred. This is espec- 

 ially true of those characters which refer to the structure of 

 the progenitor of the group. But the direction of specializa- 

 tion may be shown by a single representative of the group, if 

 it be highly specialized, and we have a clear idea of the essen- 

 tial characters of a larger group including the one under inves- 

 tigation. 



It must be borne in mind, however, that the direction of 



