BY R. J. TILLYARD. 441 



ones. From such a comparison, we find at once that the 

 Australian forms are more reduced than the Indo-Malayan, 

 particularly in the complete loss of the itiferior sector of the 

 triangle, in the great reduction in length of the superior 

 sector, in the small head and narrow abdomen, and in the 

 small size of the legs. Take, for example, the species of the 

 genus Alloneura, described by de Selys in 1860. These are 

 divided into three groups, according to the length of the 

 superior sector of the triangle, which is greatest in those 

 species occurring in India, slightly less in the Ceylon species, 

 and considerably less in the Malayan species. In the last 

 group, moreover, we find de Selys making three subgroups, 

 indicating three stages in the reduction of the inferior sec- 

 tor, which is present in A . anal'is Selys, from Mount Ophir, 

 also present, but smaller, in several Singapore and Borneati 

 species, and finally absent in the Bornean species A. dor sails 

 Selys. We see, therefore, a progressive reduction in both 

 the superior and inferior sectors, as we pass from the western 

 to the eastern limit of this genus. It is, I think, evident 

 that the legion Pi'otoneura is not, strictly speaking, an 

 archaic group as it now stands, but an exceedingly astheno- 

 genetic group, derived from ancestors that were possibly 

 archaic in comparison with the immediate ancestors of such 

 forms as Agrion. Indeed, we may say that it was the failure 

 of the Protoneuran ancestors to specialise in the way that 

 Lestes and Agrion did, that left them behind in the struggle, 

 and gave them the choice either of complete extermination 

 or of saving themselves by adopting a strongly asthenogenetic 

 line of development. It was after this development had set in 

 that this group was able to extend its borders, and began 

 to appear in Borneo, New Guinea, and finally in Australia, 

 where, by continuous asthenogenesis, it was able not only to 

 hold its own, but to gain a small footing, even in the more 

 temperate part of the Continent. 



