The relationship between the palaearctic and nearctic faunas 293 



for a repeated firm connection between Asia and North America by this route 

 are more favourable than between any second pair of continents on the earth. 



The Bering Strait is only 90 km. at its narrowest and quite shallow, a land rise 

 of 60-65 meters would be enough to shape a continuous bridge. This is no high 

 figure. It lies within the limits of what is thought to be the normal world-wide 

 eustatic regression of the sea during a glaciation. Even for the last glaciation ( Wiirm, 

 Wisconsin) this regression is usually calculated as something between 90 and 100 

 meters, somewhat more during at least one of the earlier ones (Zeuner, 1950, p. 

 129; Woldstedt, 1954, p. 289-291. 



For other reasons, too, it is very likely that the Bering land-bridge existed during 

 a period (or several periods) of glaciation. Alaska was little affected by the land-ice, 

 the major part of it remained ice-free throughout the entire Pleistocene period 

 (Flint, 1952) as did the opposite part of eastern Siberia (fig. 52). Therefore, no 

 isostatic sinking of land caused by pressure of an ice-cap could affect the Bering 

 area. The bridge furthermore prevented cold polar water from entering the North 

 Pacific, and the south-coast of the isthmus, as well as the chains of islands off the 

 coast, the present Aleutians, &c., were more under the influence of the warm Kuro 

 Shivo current. 



On the other hand, the Bering Isthmus was a pronounced "filter bridge" 

 (Simpson, 1940, p. 148), certainly in the first place due to climate. It allowed 

 passage to arctic and subarctic organisms but, as we just learned, there are very 

 few circumpolar animals not able to endure at least subarctic conditions. This is 

 well in accordance with the assumption made, that the Bering bridge developed 

 and existed dnring glacial periods of the Pleistocene. Therefore very few organisms 

 demanding a temperate, or warmer, climate are common to North America and 

 Asia. For instance the Tiger-beetles, genus Cicindela, are very abundant in North 

 America, amounting to almost one hundred different species (north of Mexico), 

 but they do not enter the Arctic and none of them has reached countries outside 

 the American continent. 



In Tertiary time conditions may have been different. It is generally accepted 

 that the Bering land-bridge existed throughout the main part of this period 

 (Simpson, 1947) and thus under comparatively favourable climatic conditions. It 

 remains doubtful, however, if they were warm enough to explain the distribution 

 of Pantropical genera or groups (Mayr, 1946, p. 36). But, as we have seen, the 

 actually living species which are supposed to have crossed the Bering bridge are 

 northern in distribution, have low demands on climate and are no doubt Pleisto- 

 cene migrants. Blethisa eschscholtzi Zoubk., among Carabid beetles (p. 280), may 

 be a single surviving representative of Tertiary dispersal. 



The following additional attempts (e-g) to explain the wide east- west distribu- 



