298 



ZOO-GEOGRAPHICAL DISTRIBUTION 



[CH. 



The Oriental Region. This is the least isolated of all the 

 regions of the earth, being penetrable from each of the other 

 three regions of the Old World. Palaearctic genera are excluded 

 on account of the great rise in the mean annual temperature south 

 of the barriers. From Ethiopia, the region has received species 

 of Acisoma, Urothemis, and Libellago. There remains Wallace's 

 Line. Here we have a case closely parallel with that of the 

 Isthmus of Panama. A large number of tropical Oriental genera 

 have penetrated eastwards into the Australian region, but very 

 few of the Australian forms have passed westwards into the 

 Oriental. We note especially Hemicordulia and Diplacodes, which 

 have spread far and wide beyond their Australian zoocentres; 

 the genus Argiolestes, which appears to have passed just across 

 the barrier; Pfotortkemis, Nannophya, and Celebothemis. 



Fig. 155. Wings of Ayrionoptera insignis allogenes Tillyard, $, 

 Queensland. (Hw. 26 mm.) Original. 



The Australian Region. This is only now penetrable across 

 Wallace's Line, by strong-flying species. There is evidence, 

 however, of a separate immigration stream into the Australian 

 continent via Timor, at some not very remote period, as well as 

 some indication of the remains of an original Antarctic fauna 

 driven northward through Tasmania. Australia proper, as dis- 

 tinct from the Papuan sub- region, has a rich Gomphine fauna, 

 which seems certainly to have entered long ago via Timor, since 

 Gomphinae (other than Ictinus) do not occur in Papua. The 

 presence of Diplacodes nebulosa, Orthetrum pruinosum, Zyxomma 

 petiolatum and other species on the mainland points in the same 

 direction. The following genera have penetrated into Papua and 



