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CHAPTER II 



(From Burt & Grossenheider, 

 1952.) 



FIG. 5. Spread of the Muskrat, Ondatra {Fiber) zibethica L., in Central Europe since 

 its introduction 40 km. SSW of Prague, in 1905. 



(After Ulbrich, 1930.) 



On the other hand, introduced species may very well be conservative and remain 

 for a long time restricted to the port of introduction or its immediate vicinity. 

 This, as suggested by Brown (1950, p. 197), is probably due to the fact that these 

 species are often confined to open ground, the surrounding forests forming an 

 efficient barrier against further distribution. The click-beetle Agriotes lineatus L., 

 found in Newfoundland as early as about 1840, is still restricted to the Avalon 

 Peninsula of the southeast where it was first introduced. 



There are also instances, at least in Europe, of animals showing a rapid tem- 

 porary expansion of area not due to human influences, for example certain aquatic 

 birds, the Carabid beetle Amara majuscula Chaud. (Lindroth, 1949, p. 625), and 

 the Click-beetle Corymbites cupreus F. (Holdhaus & Lindroth, 1939, pp. 184, 

 260), in Scandinavia and Finland. In these cases, however, the migration has taken 

 place from a larger continent (Asia and SE Europe) towards the periphery (At- 

 lantic Europe), never in the opposite direction. 



2. THE GEOGRAPHICAL CRITERION. The distribution of an introduced species 

 is often "immature": the area is more or less restricted, usually to some coastal 

 district, and "unnatural", in the sense that its limits can hardly be co-ordinated 

 with any external factor (climate, soil, vegetation, &c.) which could explain this 

 restriction. A disjunct (broken-up) area is especially suspicious; for instance if 

 a species, otherwise not coast-bound, occurs in North America only in the North- 

 east and in the Pacific Northwest. Thus the ground-beetle Bembidioti lampros 

 Hbst. is restricted to British Columbia and the very town of St, John's, New- 



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