724 A NEW SPECIES OF NA.\ iXOJ'tiLEBlA, 



matioii of anij suitable bracing or strengtbeniug of tbe wing-area, 

 either on Aeschnine or Libelluline lines. 



I do not contend tbat it is necessary to assume even tbat the 

 present-day " quadrilaterals " in the Libelhdidce, ever possessed 

 ancestors with a fully-formed three-sided triangle. I tbink, rather, 

 that their ancestors were the laggards in this race for the perfect 

 triangle, and that tbey attained only a certain measure of success 

 in that direction, without gaining a position of equilibrium. Need- 

 ham has shown (without emi^hasising the point) by bis excellent 

 diagrams of the gradual formation of the triangle, what an enor- 

 mous stress must be thrown on to those origmally weak cross- 

 veins, which are finally called upon to play the part of strong sides 

 to the triangle. Anything less than complete success, in this diffi- 

 cult piece of evolution, must surely have stood self -condemned, and 

 either exists still as an unstable form (e.g., Synthemis) or em- 

 barked on the backward path of asthenogenesis, in which, since the 

 aim is no longer to produce the ideal flying wing, the reduction 

 back to the weaker form would rapidly proceed. 



The argument for the Single-Development Theory of the Anal 

 Loops may be briefly put as follows : — All present-day hind-wings 

 of the Anisoptera were developed from an original anisopterous 

 type, which had a hind-wing broad enough to contain all the cell- 

 material necessary for jiresent-day developments. By various ar- 

 rangements of the basal cells and the anal and cubital branch- 

 veins traversing them, all the present-day loops (and probably 

 other kinds now lost) were developed. Thus arose, with varying 

 degrees of final success, the Gomphine, Petalurine, Chlorogom- 

 phine, Aeschnine, Macromian, and Libelluline loops, in all their 

 forms and variations. Of these, the most recent and most success- 

 ful is the Libelluline loop, whose origin may probably be sought 

 near to the point from which Chlorogomphus sprang, and from 

 which that genus appears to have diverged but little. 



Besides these menogenetic types, at various points along the 

 evolutionary route, unsuccessful competitors gave up the race, and 

 adopted, as a means for their preservation, the process of asthe- 

 nogenesis, so successfully carried out long before by the main 



