The relationship between the palaearctic and nearctic faunas 241 



Potamogeton epihydrus Raf. Outer Hebrides (South Uist). 



Sisyrinchium angustifolium Mill. W. Ireland (widely). 



Spiranthes romanzoffiana Cham. N. E. & S. W. Irel.; Inner Hebr. (Coll 



& Colonsay), W. Argyllshire (Scotl.). 



In North America these species have a wide distribution, all reaching New- 

 foundland in the east, the Spiranthes even central Labrador; the Sisyrinchium 

 goes to Anticosti Island and SE. Labrador. 



The "American Group" has been carefully studied by Praeger (1932, 1934) 

 and, most recently, by Heslop-Harrison (1953). For some unknown reason, he 

 does not mention the LimoseUa but his list contains Juncus tenuis Willd. and J. 

 Dudleyi Wieg., as well as Najas flexilis Willd. Juncus Dudleyi was included tenta- 

 tively only, as probably introduced, and the same, according to other students 

 (Clapham, Tutin & Warburg, 1952, p. 1244), applies to J. tenuis (cf. p. 212). 

 Concerning Najas flexilis, known from several scattered stations on the European 

 mainland and widely distributed there during inter- and early postglacial times, 

 Heslop-Harrison himself admits that it is no more entitled to be listed in the 

 "American" group than for instance Lobelia dortmanna L. 



Five of the six species above (all except Sisyrinchium) are aquatic or grow on 

 very wet places. Water-plants have often been brought by man, purposely (as 

 the water-fern Azolla caroliniana Willd.) or unintentionally (as Elodea canadensis 

 Michx.), from North America to Europe, but there seems to be no reason for 

 a similar explanation of the six American plants in the British Isles. By means 

 of pollen-analysis, it has even been demonstrated by Jessen (1949, p. 202) 

 that Eriocaulon septangulare was growing in Irish pools already in early Atlantic 

 time. 



One animal organism, the fre&h-water Sponge Heteromeyenia ryderi Potts, has 

 a distribution quite similar to that of the above plants (Arndt, 1928). Its main area 

 is in eastern North America, from Florida to Nova Scotia and Newfoundland, 

 it is widely distributed in Ireland (Stephens, 1920) and also found on the isle of 

 Mull in the Inner Hebrides, as well as on the Faeroes (Sparck, 1934). Any idea of 

 human transport of Heteromeyenia ryderi from North America to its outposts in 

 western Europe seems as out of touch with reality as for the corresponding plant 

 species. 



The possible explanation of the presence of an American floral (and faunal) 

 element on the British Isles (and the Faeroes) will be discussed in the next para- 

 graph. 



16 — 565597 Lindroth 



