The relationship between the palaearctic and nearctic faunas 



247 



FIG. 34. The situation of air pressure and winds over the North Atlantic at midday 

 during three days, September 30th to October 2nd, 1953, with considerable influx 

 of North American birds into the British Islands, 



(From Williamson, 1954.) 



ryderi. Others, for instance Scharff (1907, p. 34) and Stephens (1920, p. 247-248), 

 regard this sponge as belonging to a very old faunal element, evidence of a former 

 transatlantic land-connection. Arndt (1928, p. 159-163) tried to select a middle 

 course: he stressed the probability of bird-transport from Ireland to the Faeroes 

 but admits that the Irish population may be old. His view has been misinterpreted 

 by Sparck (1934) who himself ascribed even to the population of the Faeroes a 

 persistence in situ since the last interglacial period. 



It seems advisable to consider the history of the Heteromeyenia in connection 

 with the related American element of the British flora. Phytogeographers have 

 almost unanimously accepted this as an old element too, a "relict" from a period 

 of transatlantic land-connection. Quite recently, however, Heslop-Harrison (1953) 

 has given voice to an opposing view: he is inclined to interprete the occurrence of 

 these American plants in the British Isles as due to transport with migrating birds 

 and especially draws attention to the routes of the Greenland White-fronted 

 Goose (Anser albifrons flavirostris Dalg. & Sc.) which winters partly in Ireland. 

 The weak point of Heslop-Harrison's argument is that, according to him, only 

 one of the species in question, Sisyrinchium angustifolium, grows in Greenland 

 (Bocher, 1948) an this species, it should be added, is the one least likely to be 

 dispersed by birds. ^ He therefore feels compelled to assume that other birds have 

 brought the plant diaspores from NE North America to Greenland, where they 

 "changed birds", without being able, for climatic reasons, to germinate on the spot. 

 This is a most far-fetched hypothesis. 



If we are at all willing to accept bird-transport as a possible agency in the present 

 case, it seems necessary to evaluate the chance of birds arriving in the British 



^ Dr. T. W. Bocher and Dr. J. Iversen have informed me, however, that the Greenlandic 

 and the Irish Sisyrinchium are different. Dr. Iversen has grown both forms together. (Cf. 

 p. 250). 



