ZOOGEOGRAPHY 361 



penetration, limited to coastal zones, river systems and populated places, has 

 served to sample the neotropical fauna, rather than cover the territory. 



Table IV 



Genera common to Genera common to Nearctic, Neotropic 



Nearctic and Neotropic Regions Regions, and distributed outside the 



Western Hemisphere 



Thesium Euplectus 



Rhinoscepsis Thesiastes 



Pseudotrimium (?) Reichenbachia 



Actium Pselaphus 



Melba Fustiger 



Euplectus 



Thesiastes 



Rhexius 



Arthmius 



Euphalepsus (?) 



Scalenarthrus 



Pselaptus 



Reichenbachia 



Decarthron 



Pselaphus 



Pilopius 



Ctenisis 



Hamotus 



Fustiger 



From this table we observe that there are 19 genera of pselaphids com- 

 mon to the two American faunal regions, and only five genera common to 

 these two regions and having in addition species distributed outside of the 

 Western Hemisphere. Of these 19 common genera, practically all have more 

 neotropical than nearctic species. This gives a total of 17 tribes, 187 genera and 

 1279 species for the Western Hemisphere. Of these genera of American 

 pselaphids, 46 are found only in the Nearctic, and 122 are found only in the 

 Neotropic area. In terms of percentages, the Neotropical Region holds 64 

 per cent, and the Nearctic Region 36 per cent of all genera inhabiting the 

 Americas. This shows that the former region is much more endemic, and sug- 

 gests a much greater isolation. This conclusion is confirmed by the geological 

 evidence available on separation of the Americas via the Panamanian Isthmus, 

 and is also supported by the much greater resemblance between the Nearctic 

 and Palaearctic Regions than between the Neotropical and Palaearctic Regions. 

 In terms of species the neotropics are nearly, if not wholly, 100 per cent 

 endemic. The nearctic pselaphid fauna is intermediate in composition, then, 

 between the neotropical areas and the rest of the world. This suggests penetra- 

 tion from one or more northern connections between Asia or Europe, during 

 favorable climate for terrestrial poikilotherms, dispersal southward into South 

 America, either via the Central American or Antillean highway or both, and, 

 under the relatively warm and moist tropical climate, evolution of a diverse 

 and pecuhar neotropical pselaphid fauna, with secondary dispersal northward 

 of some faunal elements. 



