THE LAND GASTROPODA IN THE NORTH ATLANTIC REGION 



163 



Most of the Holarctic genera are characterized by their considerable age; 

 thus in Europe they belong to the oldest persisting element in the gastropod 

 fauna. With some generaHzation, the hypothetical nature of which may be 

 admitted, the recent Holarctic element can be regarded as a relic Mesozoic to 

 early Tertiary gastropod fauna of northern latitudes. It is probably not 

 accidental that this gastropod element is more or less associated with the 

 plants which characterized the corresponding Arcto-Tertiary flora. 



The species which belong to the Holarctic gastropod genera are largely well 

 adapted for passive dispersal. Besides favoring extensive distribution, this 



Table 3 



* The absence of fossil records of Zoogenetes must be seen against the background that 

 its shells, owing to shortage of lime, are not suited for fossilification. 



must have counteracted genetic isolation and thereby taxonomical differentia- 

 tion. Fischer (1960) has argued that the repeated interruption of biotic 

 evolution by climatic disasters in the Circumpolar region tends to keep the 

 biota on an immature, relic level. In fact, this fits very well to the ancient 

 nature of the Holarctic Gastropoda, at the same time as it must be intimately 

 linked with their fitness for passive dispersal, which apparently must be of 

 selective value in regions of violently fluctuating physical conditions. 



The recent distribution of Holarctic gastropod taxa must have reached its 

 pattern to such a great extent during the Quaternary that it hardly gives any 

 indications for, or against, a Tertiary dispersal across the North Atlantic. 

 Reasonably, however, the presence of passive dispersal, a characteristic of the 

 group, may have played a role also in the Tertiary. 



