298 J. A. G. REHN 



are clearly expressive of the roles of Bering Sea connections and of 

 Glacial ice in past biogeographic movements or controls. 



CONCLUSIONS 



In the absence of adequate fossil evidence which would integrate 

 with living elements we must draw our relevant evidence from the 

 known centers of existing genera and their allies. Of the 229 genera of 

 Dermaptera and Orthoptera that occur west of the eastern edge of 

 the Great Plains, exclusive of purely Mexican ones, 35% (82 genera) 

 are clearly Sonoran types, using this term in its broad sense or, 

 when limited, to that area of the same east of the Continental 

 Divide. This great evolutionary center has been the outstanding 

 North American center of generic differentiation for Orthoptera. 

 In descending importance 14.8% (36 genera) clearly were derived 

 from the Neotropical Region; 7.3% (17 genera) represent a dis- 

 tinctive and endemic coastal and non-Sierran montane California 

 fauna; 6.5% (15 genera) are endemic there and probably developed 

 in Lower Sonoran Zone deserts; 5.6% (13 genera) are introduced 

 adventives; 4.8% (11 genera) are at present Sierran endemics and 

 most probably autochthons, and 5.2% (12 genera) holds for a 

 group of genera also of Palearctic occurrence or relationship; while 

 3.9% (9 genera) similarly are Cordilleran in their present distribution 

 and probable origin. A number of the genera here regarded as 

 developed in the Sonoran center also moved eastward, and sub- 

 sequently established specific evolutionary centers in the eastern 

 and southeastern United States. I regard 8 genera (3.5%) as 

 derived from purely eastern centers of development. The remaining 

 4% (13) represents basically Sonoran types that developed sub- 

 sidiary radiative centers in eastern North America, Mohavan and 

 Great Basin endemic types, cosmopolitan genera, and others 

 considered purely Nearctic Boreal and restricted Campestran. 

 The occurrence of certain genera in both western North America 

 and eastern Asia leaves unanswered the natural query as to whether 

 their original center was in the one or the other, with much of the 

 weight of evidence in some of the genera in favor of a North Ameri- 

 can, and also definitely pre-Glacial, origin. 



