|io.i33i. DRAGON-FLY WING VENATION— NEEDHAM. 759 



(c) Set apart from any group its dominant forms and there will 

 remain those members of it which most closely ally the group with its 

 neighbors. 



:>. What of genealogies based on external characters? 



In this paper we have been dealing almost entirel}^ with external 

 characters — superficial characters, viewed from the standpoint of the 

 physiologist. But hard parts, though dead, are the enduring mold 

 iin which the living being is cast, and represent the outcome of its 

 struggle with environment. Therefore we may make more use of the 

 principle of natural selection than is usually possible in the ordinar}^ 

 iinorphological work, having frequent recourse to the almost axiomatic 

 principle that "useful structures once acquired will not be lost (other 

 things being equal) in a single series, unless replaced by more advan- 

 tageous structures." This is but a partial paraphrase of the more 

 fusual statement of the principle of natural selection, which mav itself 

 fbe condensed into three words — utility determines survival. 

 I In order to apply this principle, we have, therefore, to know that 

 [the structures whose development we are tracing are useful struc- 

 ftures. The proof of their utility may be derived from various sources. 

 ;Take, for illustration, the brace to the stigma, which, we have seen, is 

 [developed from an ordinary cross vein: 



I (a) Its efficiency may be demonstrated mechanically. This I have 

 'not done, though it would not be diflicult. 



(h) It may be demonstrated experimentally. This I have done (on 

 Agrioninas) by cutting out a little piece of the brace in each wing and 

 noting the resulting weakening of flight. 



(c) It is demonstrated biologically by the success in life of those 

 forms which possess the brace. They are the dominant members of 

 their respective groups, being in numbers of species and of individuals 

 vastly in the majority. With creatures absolutely dependent on their 

 wings in mating, in feeding, and in escaping their enemies, this is 

 ample demonstration of the efficiency of the wings as a whole, and, 

 incidentall}^, of each part that is found here better developed than in 

 the less successful members of the series. While this proof is less 

 specific, while one may not learn from it the contribution any one 

 structure has made to the excellence of the wing as a whole, it is the 

 real proof after all. 



(d) I ask no better proof of the efficiency of any structure than is 

 furnished by its repeated independent development in those forms 

 which are acknowledged to be the most specialized members of the 

 several groups. 



By these means we may arrive at some knowledge of the efficiency 

 even of structures about whose use we know so little as we do of the 

 several parts of the insect's wing. 



The application of the principle above stated furnishes the means 



