42 ON THE STUDY OF ZOOGEOGRAPHICAL REGIONS, 



The apparent zoocentre in Maine may be, therefore, only due to 

 the completeness of the local records, and the genus may possibly 

 attain as great, or even greater, density in some part of Siberia . 

 Generally speaking, the number of records necessary for drawing 

 an approximate contour in the case of a zonal group will be much 

 higher than in the case of a regional group, since the former will 

 extend into at least two regions. 



Other examples of zonal distribution in Odonata whose dis- 

 tribution might be advantageously studied by this method are : — 



Holarctic — Lihellula, SympetruTn, Leucorrhinia, Gomphus^ 

 Boyeria, Calopteryx. 



Circumtropic — Macromia, IVamea, Gynacantha, Teinobasis. 



In the study of zonal groups, the contour itself will decide in 

 what region or regions a given zonal group may be considered to 

 be entogenic; viz., those regions in which that group can be seen 

 to have established definite zoocentres. For example, the genus 

 Somatochlora may be rightly considered entogenic in the Nearctic 

 Region, and also (though apparently not so definitely) in the 

 Palsearctic Region. Other zonal groups are quite clearly ento- 

 genic in one region but ectogenic in another. Traviea, for 

 instance, appears to be entogenic in the Neotropic Region, with 

 an ectogenic outgrowth into the Nearctic Region and another 

 into the Australian Region. 



Cosmopolitan groups, such as Aiiax. jEschiia, Lestes^ may also 

 be studied by this method; but, of course, the number of records 

 necessary for the complete contouring of such a group will be 

 even greater than in the case of a zonal group. 



The method may also be applied to the study of a harrier, in 

 the following manner : — A map should be taken showing the 

 barrier, with parts of the surrounding regions, and over this map 

 the partial contours of various groups, drawn on transparencies, 

 may be placed in turn. The efficacy of the barrier may be gauged 

 by considering the percentage of contours showing total discon- 

 tinuity across the barrier. In so far as group contours are com- 

 pletely delimited or cut off by the barrier {i.e., the group is pre- 

 vented from passing across the barrier at all), the barrier may be 

 considered a Primary Barrier; but, in so far as group contours 



