296 ZOO-GEOGRAPHICAL DISTRIBUTION [CH. 



being only penetrable along the highlands of the Isthmus of 

 Panama. Hence the ectogenic fauna, received from Nearctica, 

 is exceedingly poor. Several species of Erpetogomphus, three 

 species of Libellula, one of Sympetrum, and one of Anomalagrion, 

 appear to form the only undoubted ectogenic fauna. There 

 are, however, a few groups whose zoocentres seem to lie actually 

 in or near to the Isthmus. I have considered these as dientogenic 

 in both Neotropica and Nearctica. Macromia seems to penetrate 

 occasionally into the region, and the very beautiful Pseudoleon 

 has also a foothold there. 



The Nearctic Region. This region is penetrable from Neotropica 

 (a) along the coast-line northwards from the Isthmus of Panama, 

 (6) along the chain of the Antilles into Florida. It appears also 

 that in fairly recent times Lower California received a considerable 

 influx of Neotropical forms, possibly by a more direct route than 

 any at present existing. Thus we find the fauna of the southern 

 portion of Nearctica, particularly along the coast-line, considerably 

 modified by the influx of a large ectogenic fauna, consisting 

 chiefly of Libellulinae. The following Neotropical genera form 

 this ectogenic fauna of Nearctica : Gomphoides, Negompho'ides, 

 Cyclophylla, Aphylla, Gynacantha, Orthemis, Cannaphila, Micra- 

 thyria, Erythrodiplax, Erythemis, Brachymesia, Dythemis, Palto- 

 themis, Brechmorrhoga, Hyponeura, Telebasis, Telagrion. Most of 

 these do not penetrate beyond the Lower Sonoran. 



Libellula quadrimaculata and Enallagma cyathigerum range 

 through the whole of the Northern Hemisphere, except the tropical 

 parts. There seems to be no evidence to shew whether they 

 originated in Palaearctica or Nearctica. They are best included 

 in the entogenic fauna of both regions. The barriers between 

 the two regions are very effective in the case of the Odonata, 

 chiefly because of the lowness of the mean annual temperature 

 at the point where the two approach most closely together 

 (Behring's Straits). 



The Palaearctic Region. In spite of its very long frontier to 

 the Ethiopian and Oriental regions, this region seems to be very 

 well contained, the nature of the barriers (deserts and high 

 mountain ranges) making the passage of Odonata very difficult. 

 From Ethiopia, Hemianax and Trithemis have penetrated into the 



