The relationship between the palaearctic and nearctic faunas 



295 



^J 



Species 42 



DIAGR. 9. Percentage of macropterous (white), dimorphic (striated) and brachy- 

 pterous (black) species of Carabid beetles inhabiting Fennoscandia, arranged into dif- 

 ferent groups of distribution. 



a = exclusively European 

 b = Euro-Caucasian & Euro-Medi- 

 terranean 

 (From Lindroth, 1949, p. 435.) 



c — West-Palaearctic 

 d — generally Palaearctic 

 e = Circumpolar 



entomologists that widespread insects in the northern part of their area or in high 

 mountain regions turn to biennial development. 



(g) Speciation may also be delayed, as an indirect result of climate, by decreased 

 selection within the rather uniform arctic region with its generally low population 

 density (reduced "competition"). This assumption, however, at least as far as 

 insects are concerned, is purely theoretical. 



A question worth raising is why some species reached the outermost limits of 

 the continents, Scandinavia and/or the British Isles in Eurasia, Labrador and/or 

 Newfoundland in America, and acquired a perfectly circumpolar area, and how 

 these were selected from the far more numerous stock of other arctic species. The 

 catch-word "aggressive" species, &c., gives no real explanation. 



An investigation of the means of dispersal of every separate species may do so, 

 at least in part. Circumpolar species of Carabid beetles as a rule have fully devel- 

 oped, functionary hind-wings, as shown in an earlier book (Lindroth, 1949, p. 435) 

 on the Fennoscandian fauna. The diagram here reproduced (diagr. 9) is not quite 

 up to date with respect to later discoveries of several species in North America, 

 but it demonstrates the tendency. 



