ScHARFF — Former Lonl- Ilridfjehettvceu X. ICurojif (okI N. Ainen'ca. 25 



interchange between llic fauiui iuid llnru nf iKntli-wcslL'ni Europe and 

 north-eastern America was effected across Ihc iKirlJunii land-bridges. 



Only one otlicr point needs to be commented upon. I have shown that 

 most of the American species occupy the Atlantic coast region in the British 

 Islands. Almost all the southern or Lusitanian species are found in precisely 

 the same area in England, Ireland, and Scotland. This seems to me partly 

 due to tlie fact that the temperature was considerably higher there during 

 the Glacial period than in the more inland localities. Even now the plants 

 are under more favourable climatic conditions on the west coast than further 

 inland, and less exposed there to competition with the stronger eastern 

 rivals. Moreover, almost the whole of Ireland and a large portion of 

 England are thickly swathed in a mantle of Glacial clay. We can only 

 suppose that the forces which controlled the deposition of this clay were less 

 effective on the west coast, which may have extended far to the west of its 

 present boundary, and have thus given rise to the preservation of many 

 species of animals and plants which were destroyed elsewhere. 



LIST OF BOOKS AND PAPERS EEFEEEED TO IN THE TEXT. 



Annandale, N. — Notes on some Freshwater Sponges collected in Scotland. 



Jonrn. Linnean Society London (Zoology), vol. xxx., 1908. 

 Akldt, Th. — Die Euwicklung der Kontinente und ihrer Lebewelt. Leipzig, 



1907. 

 Austen, E. A. C— On the Valley of the English Channel. Quart. .Journ. 



Geological Society London, vol. vi., 1850. 

 Ball, Sir Eobert. — The Cause of an Ice Age. London, 1891. 

 Blytt, a. — Die Theorie der wechselnden kontinentalen und insularen 



Klimate. Engler's Botanische Jahrbiicher, vol. ii., 1881. 

 Born, P. — Zoogeographisch-carabologisclie Studien. EntomologischesAVochen- 



blatt, vol. xxv., 1908. 

 Cakpentee, G. H. — Collembola of Franz-Josef Land. Sci. Proceedings Eoy. 



Dublin Society, vol. ix. (n.s.), 1900. 

 COLGAN, N., and E. W. Scully. — Cybele Hibernica (2nd ed.). Dublin, 1898. 

 Dalgleish, J. J. — List of occurrences of North American Birds in Europe. 



Bullethi Nuttall Ornithol. Club, vol. v., 1880. 

 Dall, W. H. — Land and Freshwater MoUusks. Harriman Alaska Expe- 

 dition, vol. xiii., 1905. 

 Dana, J. D.— Manual of Geology (3rd ed.). 1880. 

 Darwin, C— Origin of Species (6th ed.). London, 1878. 



R.I. A. PROO., VOL. XXVIII., SECT. B. fE] 



