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Show 818 ISLAND LIFE. [PART II. up of a freshwater shell ( Unio pictorum) off the mouth of the English Channel between the fifty fathom and 100 fathom lines, while in the same locality gravel banks with littoral shells now lie under sixty or seventy fathoms water. 1 More recently Mr. Gwyn Jeffreys has recorded the discovery of eight species of fossil arctic shells off the Shetland Isles in about ninety fathoms water, all being characteristic shallow water species, so that their association at this great depth is a distinct indication of considerable subsidence. 2 Time of last Union with the Oontinent.-The period when this last union with the continent took place was comparatively recent, as shown by the identity of the shells with living species, and the fact that the buried river channels are all covered with clays and gravels of the glacial period, of such a character as to indicate that most of them were deposited above the sealevel. From these and various other indications geologists arc all agreed that the last continental period, as it is called, was subsequent to the greatest development of the ice, but probably before the cold epoch had wholly passed away. But if so recent, we should naturally expect our land still to show an almost perfect community.with the adjacent parts of the continent in its natural productions; and such is found to be the case. All the higher and more perfectly organised animals are, with but few exceptions, identical with those of France and Germany ; while the few species still considered to be peculiar may be accounted for either by an original local distribution, by preservation here owing to favourable insular conditions or by slight modifications having been caused by these conditi~ns resulting in a local race, sub-species, or species. Why Britain is poor in Species.-The former union of our islands with the continent, is not, however, the only recent change they have undergone. There is equally good evjdence that a considerable portion, if not the entire area, has been submerged to a depth of nearly 2,000 feet (see Chap. IX. p. 168), at which time only what are ~ow the h~ghest mountains would remain as groups of rocky Islets. This submersion must have destroyed the ~ Qu~~terly Jo~rn.al of Geolog£qal Society, 1850, p. 96. Bnhsl1- Assocwtzon Repm·t, Dundee, 1867, p. 431. I , ' CIIAI'. X \'I.) TIIE 13Hl'l'IS1I ISLES. 319 o-reater part of the life of our country; and as it certainly ~ccurred during the latter part of the glacial epoch, the subsequent elevation and union with the continent cannot have been of very long duration, and this fact must have had an important bearing on the character. of the existing fau.na and flora of Britain. We know that JUSt before and durmg the glacial period we possessed a fauna al~ost or quite identi?al with that of adjacent parts of the contment and equally nch in species. The submergence destroyed this fauna; and the p~rmanent change of climate on the passing away of the glacial conditions appears to have led to the extinction or migration of many species in the adjacent continental areas, where they were succeeded by the assemblage of animals now occupying Central Europe. When England became continental, these entered our country; but sufficient time does not seem to have elapsed for the migration to have been completed before subsidence again occurred, cutting off the further influx of purely terrestrial animals, and leaving us without tho number of species which our favourable climate and varied surface entitle us to. To this cause we must impute our comparative poverty in mammalia and reptiles-more marked in the latter than the former owinO' to their lower vital activity and smaller powers ' b of dispersal. Germany, for example, possesses nearly ninety species of land mammalia, and even Scandinavia about sixty, • while Britain has only forty, and Ireland only twenty-two. The depth of the Irish Sea being somewhat greater than that of the German Ocean, the connecting land would there probably be of small extent and of less duration, thus offering an additional barrier to migration, whence has arisen the comparative zoological poverty of Ireland. This poverty attains its maximum in the reptiles, as shown by the following figures :- Belgium has 22 species of reptiles and amphibia. Britain , li3 , ,, , Ireland , 4 , , , Where the power of flight existed, and thus the period of migration was prolonged: the difference is less marked; so that Ireland has seven bats to twelve in Britain, and about 110 as against 130 land-birds. .. |