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Show 81 Cottam: Some Bic-Ecoloqicel Factors of Utah' and Britain Contrasted rv1sed Handbook of the British. Flora by' Bentham and. Hooker,l published in 1943 the number of species of flowering plants andferns of Great Britain is given as 1,285,. Not' only does England when compared to Utah show great impoverishment of species of both plants and animals, but it is considerably less rich in species of most biotic forms found on the Continent, to. which the British forms of life show close genetic affinity. - _ These facts of distribution, as well as the g,eneral absence of species peculiar to Britain, may be accounted for only by con sidering the interesting bio-geographical factors of the British Isles. As recently as 25,000 years ago the British Isles were glaciated as far south as the River Thames. The principal sour of this ice advance, were two one moving west across what the North Sea from Scandinavia, and the other advan cing south from the Scottish Highlands. The ice' did not form a solid sheet over the land; small un glaciated pockets here and there apparently served as places of refuge for Arctic vegeta- tion. But the temporate flora and fauna undoubtedly were driven to the Continent, and many species perished. England at this time was not an island, and, when a climatic change brought an end to the glacial period, the temperate flora and fauna re- crossed the narrow neck of land connecting England to the Con tinent and advanced slowly Northward, crowding the Arctic, cold-enduring vegetation steadily back on the alpine summits of the Scottish mountains, where much of it may still be found. ces is ........... now It must be remembered that plants, unlike animals, migrate slowly. The time since glaciation has by no means been suffi cient for many plants by their own powers of dispersal to have migrated from Southern England to Scotland. The evidence for this statement is seen in the remarkable post glacial peat deposits found in many places about England. An excavated peat bed gives< a profile of many layers of vegetation, where, through pollen analysis and through the study of preserved plant organs, a progressive story of plant migration may be read. Thus the post-qlacial history of British vegetation is selatively well-known, ana the varying post-qlacial climates that produced it have been ascertained. These data show that at the close of the glacial period Southern Enqland was dominated by Arctic tundra ve getation. This soon gave ,way to forests of birch and of Scotch pine. 'Then followed the invasion of hazel, English oak, alder and linden. Finally, at about 2,000 B. C., the hornbeam and beech entered '" England., _ : Thus; beginning of the historic period of England; several migratory waves 'of, veqetation had surged northward. By this time the Arctic tundra and the' pine forests were primarily ,» at the -, . . - |
