BY R. J. TILLYARD. 39 



are : —{ct) the position and extent of the region with reference to 

 the centres of origin of the various groups; {h) barriers. 



ii. Secondary Factors. — Those which determine the form of 

 contour exhibited in a region by a group whose presence has 

 been brought about by the action of i. These are '.~{a) climate 

 (rainfall, temperature); (6) the geology of the region; (c) the 

 strength of the tendency to vary or mutate exhibited by the 

 group in question; (<i) further alteration in the position of barriers, 

 after the arrival of the group within the region. 



In determining the distribution of the Australian Odooiata, 

 the primary causes have been — (a) The proximity of the Austra- 

 lian region to the Oriental, (b) The inefficiency of Wallace's line 

 as a barrier to strong-flying species, (c) The "bridges" across 

 Torres Straits and Timor, allowing of definite streams of immi- 

 gration, (d) Possible lost connections with Antarctica and thence 

 with South America. 



The secondary causes, which have restricted the spread of the 

 group within the Australian continent, have been— (a) The re- 

 striction of the rainfall mainly to the coastal districts, (b) The 

 Desert Barrier between South-East and South-West Australia. 



(c) The Bassian Barrier between Tasmania and the mainland. 



(d) The changes in mean temperature as we pass from north to 

 south. 



Of these, the distribution of the rainfall is, no doubt, the con- 

 trolling factor in determining the narrow form of the Holonotian 

 contours exhibited by Australian Odonate-groups. Entogenic 

 groups of insects of other orders, less dependent upon the rain- 

 fall, exhibit Holonotian contours of very much greater width. 



The subdivision of Holonotian contours into Eonotian and 

 Hesperonotian portions has been brought about by the Desert 

 Barrier. 



The absence of certain forms from Tasmania which occur com- 

 monly on the mainland at the points nearest to the island, can 

 only be explained by the supposition that these forms arrived at 

 their south-eastern limit after the Bassian Istiniius had sunk 

 beneath the sea. This affords valuable evidence of the relative 



