BY K. J. TILLYARD. 413 



explanation of this is simple, though it has not yet been 

 worked out in detail. Along the coast, with its abundant 

 rainfall and favourable conditions for Odonate life, the older 

 indigenous Odonata of Australia have had to face a continu- 

 ous invasion of new types. The main army of invaders passed 

 into Australia via Torres Straits, and worked down along the 

 Queensland coast into New South Wales, and even to Vic- 

 toria. Besides these, evidence is accumulating that a smaller 

 number crossed via Timor and Port Darwin, thence making 

 eastward to reinforce the main stream of invaders in North 

 Queensland. A few have worked down the North-Western 

 coastline ; about these little is known, but their effect on the 

 autochthonous Odonate fauna of South-Western Australia 

 has been very small indeed, compared with the effect of the 

 main eastern invasion on those of Eastern Australia. Besides 

 a large number of LibellulimH (of which subfamily a very 

 large majority of the recorded Austi'alian species can be 

 shewn to be invaders), we may instance Anax and H eniianax , 

 Macromia, and a number of highly reduced and specialised 

 C ce naff rioni nee. 



Now wherever these invaders have come into competition 

 with the older forms, the latter have either succumbed, or 

 have retreated into the mountain-fastnesses, where the con- 

 ditions are more in their favour, or have remained and com- 

 peted with the invader ; the result being, in the last case, 

 that the invaded, and very often the invader also, have 

 become modified in a direction of greater specialisation, and 

 nearly always by reduction. Many instances of this could be 

 given, but two will suffice. Taking the Synthemina, we have 

 the three genera Syiitlieniis, Metathemis, and Choristhemis, 

 of which the first is the least specialised, the other two show- 

 ing a distinct advance on it. Now, in Western Australia, we 

 have three species of Syntheniis, none of the other two 

 genera. In Eastern Australia (excluding Tasmania), we 

 have only fovir species of Syntheniis, three being confined to 

 the mountains; four species of Metathemis found on the 



