XV] ZOO-GEOGRAPHICAL DISTRIBUTION 295 



Libellulinae and Agrioninae. The most noticeable forms are 

 Anaciaeschna jaspidea and a few widely spread Trameini. 



The Ectogenic Fauna. 



It is obvious that a group which becomes ectogeriic in any 

 given region must be a fairly vigorous group. The more vigorous 

 it is, the more likely it will be to invade other regions and spread 

 far into them. Thus a very large percentage of the ectogenic 

 fauna is made up of Libellulinae and Agrioninae, which are the 

 dominant and most vigorous groups of to-day. A further point 

 which influences the formation of ectogenic groups is the change 

 of climatic conditions met with in passing the barrier from one 

 region to another. The most obvious case is that of the boundary 

 between a Temperate and a Tropical region, e.g. between Nearctica 

 and Neotropica. In this case, it is clear that change of temperature 

 will be the most important factor. This factor works wholly in 

 favour of the more tropical region. For the groups emigrating 

 from the warmer region to the cooler will hug the coast-line, and 

 will thus have a continuous (or, in the case of a chain of islands, 

 almost continuous) land-area along which they can work their 

 passage onward. On the other hand, the groups emigrating 

 from the colder region to the warmer will tend to seek higher and 

 higher altitudes. As soon as the mean annual temperature of the 

 coastal lands exceeds a certain amount, those lands will be 

 absolutely barred to them. Thence onwards their advance must 

 be confined to the high lands, which usually offer only a discon- 

 tinuous and precarious foothold. 



In the case of the Odonata, it is clear that the nature of the 

 barrier between two regions is of great importance in determining 

 the amount of the ectogenic faunas on either side of it. A desert 

 barrier is impassable to all but the hardiest drought-resisting 

 genera. A high and continuous mountain-range will bar the 

 passage of all but a few alpine forms. On the other hand, a sea- 

 barrier, unless sufficiently wide, will not prove as effective for 

 the strong flying Odonata as it is for most groups of animals. 

 Hence we find that Wallace's Line has not proved a very effective 

 barrier between the Oriental and Australian regions. 



The Neotropical Region. This region is very self-contained, 



