40 ON THE STUDY OF ZOOOEOGRAPHICAL REGIONS, 



archaism of (a) Austrotheynis and Nannophya (present in Tas- 

 mania) as compared with Diplacodes (absent). (6) Procordulia 

 (present) compared with Hemicordulia (absent except for new 

 colonisation by H. tau, a species with strong migratory tenden- 

 cies), (c) uEschna (present) compared with A7iax (absent). 



The lowering of the mean temperature as w^e pass southwards 

 down the eastern coast-line is the chief factor in restricting the 

 ectogenic invasion of groups of Oriental origin. As far as the 

 northern rivei's of Xew South Wales, the mean temperatures are 

 very high, the influence of Antarctic depressions and southerly 

 winds being very little felt. To this limit many essentially 

 tropical groups, sucli as BhT/othemis, have penetrated. Some few 

 reach to Sydney and beyond; but, as we go south, the number 

 diminishes very rapidly, and the ectogenic element soon dis- 

 appears. A similar process, no doubt, affects the composition of 

 the Odonate fauna of the western coast-line, about which very 

 little is known. Around Perth, only Tramea and Pmitala have 

 been noticed as of ectogenic origin. 



In the Plate, the isohyets or lines of equal rainfall are given 

 as supplied by the Federal Meteorological Bureau. In the 

 northern portion of the continent, this rainfall is almost wholly 

 of monsoonal origin, and falls mainly during summer (December 

 to March). In the South- West and South-East, and in Tas- 

 mania, the rainfall is mostly of Antarctic origin, and falls mainly 

 in the winter (May to September). In New South Wales, both 

 monsoonal and Antarctic influences are at work, with the result 

 that both summer and winter may be dry or wet according to the 

 intensities of the two operating factors. It will readily be seen 

 from the map and transparencies that — {a) Ectogenic groups 

 exhibit contours broadly similar to the contours of the monsoonal 

 isohyets. (6) Entogenic groups (Holonotian) exhibit contours 

 more dependent upon the distribution of Antarctic rainfall. 



This correlation between specific contour and rainfall is in no 

 way a complete one. Above a certain amount, rainfall may tend 

 to retard the spread of a group. The west coast of Tasmania, 

 with a rainfall up to 100 inches a year, appears to be very poor 

 in Odonata; doubtless owing to its sunless and cold summer. 



