162 HENRIK W. WALDEN 



ON THE PRE-QUATERNARY WAYS OF DISPERSAL OF LAND 

 GASTROPODA BETWEEN EUROPE AND NORTH AMERICA 



For a correct understanding of the distributional history of the land 

 Gastropoda their means of dispersal must be regarded. Small forms, especially 

 when exposed, e.g. by climbing up into the vegetation, are subject in a con- 

 siderable degree to passive dispersal, by birds and other higher animals, or 

 even as aerial plankton. In all regions it is mainly the minute forms that 

 exhibit wide distributions. This is pertinent on generic as well as specific levels. 



Increasing size limits the chances of passive dispersal, and the animals 

 mainly become dependent on their capacity to actively migrate. It might be 

 superfluous to state that this is rather restricted for land snails. However, 

 the ability to self-fertilize must essentially favor dispersal. Large species may 

 easily be transported passively when juvenile, but if capable of self-fertiliza- 

 tion a single specimen can be the origin of a new population. It is known that 

 different species behave differently in this connection, but unfortunately the 

 knowledge is too Mmited yet to allow a broad apphcation to zoogeographical 

 relationships. 



Hydrochorous dispersal, e.g. by drifts, is of great importance in the case of 

 fresh water. But for transport over the oceans it can almost be discounted, 

 partly because of the marked sensibility to NaCl-exposure of most land 

 Gastropoda, partly due to their specialized ecological requirements. The 

 great majority of species definitely avoid sea-shore habitats. Also when the 

 transport (for instance under the bark of a floating log, cf. Kew 1893), may 

 have been successful, the chance of reaching a proper habitat is very low. 



The marine littoral species constitute exceptions. As a consequence of the 

 possibilities of dispersal which thus are opened for them, they often are 

 very widely distributed. Being irrelevant to the present subject they will be 

 disregarded here. 



All stages between excellent fitness for passive dispersal and an almost 

 absolute dependence upon their own active migration are met among the land 

 Gastropoda. 



The Bering Strait route without a doubt has played an important role in the 

 dispersal of land gastropods. Certainly it has been of essential importance for 

 the development of the Holarctic element. The Holarctic genera, 16 in all, 

 are presented in Table 3, with data for their earliest occurrence in European 

 and North American strata respectively. 



For at least 12 genera earlier, often considerably earlier, records are 

 present from Europe. It seems justified to assume a preponderantly Pale- 

 arctic origin for the recent Holarctic element. Also, the indication that already 

 in the Paleocene those genera reached their relative maximum (15 per cent) 

 in the European gastropod fauna, whereas their North American relative 

 maximum (30 per cent) was reached as late as in the Pleistocene, points in the 

 same direction. 



