244 CHAPTER III 



considered, should belong here!), the rich representation of the Araneae among ani- 

 mals with an Amphiatlantic (or similar) distribution is quite striking. It is difficult 

 to get away from the idea that this is a consequence of their special mode of 

 dispersal. And it is well worth mentioning that one of the species in question, 

 Walckenaera vigilax, was among the spiders included in the famous collection of 

 "aerial plankton", made by Glick (1939) from air-planes in the U.S.A., 3 speci- 

 mens captured at elevations of from 1,000 to 3,000 feet above the ground 

 (Crosby & Bishop, 1936). 



3. Flightless animals living in close connection zvith seawater and therefore liable 

 to become dispersed by active swimming or passive transport by currents and 

 waves. The single instance among the animals above is the Staphylinid beetle 

 Micralymma marinum (fig. 32), inhabiting the tidal zone, on rocky places with 

 a rich supply of Fucus and other seaweeds. Its distribution in the North Atlantic 

 region has already been associated with the Gulfstream (Lindroth, 1931, p. 497- 

 499) and it is difficult to understand it otherwise than as the result of direct 

 transport. 



Two other insects, confined to the seaweed drifts of the shore, the Hydrophilid 

 beetle Cercyon litoralis Gyll. and the Coelopid fly Orygma luctuosum Meig. (Nielsen, 

 Ringdahl & Tuxen, 1954, p. 84), have a similar Amphiatlantic distribution but 

 probably do not endure submersion during any stage of development. The most 

 probable explanation seems to be that they were introduced from Europe into 

 North America. 



It is a well-known fact that pieces of seaweed, seeds of terrestrial plants, &c., 

 are regularly carried with the Gulfstream from North America to the European 

 west-coast and the exclusive seashore plant Mertensia maritima L., unknown 

 in Eurasia E. of the White Sea, is generally regarded as a Gulfstream immigrant, 

 even by Hulten (1937, p. 64) who is otherwise extremely sceptical of all kinds 

 of transatlantic floral exchange. 



The quite isolated area of Ranunculus cymbalaria Pursh on the coast of SW 

 Scandinavia, on both sides of the Swedish-Norwegian border limit, was likewise 

 originally (Nordhagen, 1916; still by Sterner, 1945, p. 40) interpreted as the result 

 of transport with the Gulfstream. Hulten (1937, p. loi) regarded it as an old 

 relict-occurrence, a strained idea, but later (1950, p. 81) changed his mind, 

 apparently in accordance with Tambs-Lyche (1937), who thought of introduction 

 with ballast from North America. This explanation seems to be the most accept- 

 able one. 



The famous flightless Great Auk {Penguinus or Alca impennis L.), now extinct, 

 once had a pronouncedly Amphiatlantic distribution (fig. 33). The limited means 

 of dispersal of this bird refer it to the present group. 



