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Show CHAPTER XVI. CONTINENTAL ISLANDS OF RECENT ORIGIN : GREAT BRITAIN. Characteristic Features of Recent Continental Islands-Recent Physical Changes of the British Isles-Proofs of Former Elevation- Submerged Forests-Buried River Channels-Time of Last Union with the Continent- Why Britain is poor in Species-Peculiar British Birds- Freshwater Fishes-Cause of Great Speciality in Fishes--Peculiar British Insects-Lepidoptera confined to the British Isles-Peculiarities of tl1e Isle of Mau-Lepidoptera--Coleoptera confined to the British IslesTrichoptera peculiar to the British Isles-Land and Freshwater Shells-Peculiarities of the British Flora-Peculiarities of the Irish Flora-Peculiar British Mosses and Hepaticre-Concludiug Remarks on the Peculiarities of the British Fauna and Flora. WE now proceed to examine those islands which are the very reverse of the "oceanic" class, being fragments of continents or of larger islands from which they have been separated by subsidence of the intervening land at a period which, geologically, must be considered recent. Such islands are always still connected with their parent land by a shallow sea, usually indeed not exceeding a hundred fathoms deep; they always possess mammalia and reptiles either wholly or in large proportion identical with those of the mainland; while their entire flora and fauna is characterised either by the total absence or comparative scarcity of those endemic or peculiar species and genera which are so striking a feature of all oceanic islands. Such islands will, of course, differ from each other in size, in antiquity, and in the richness of their respective faunas, as well as in their distance from the parent land and the facilities for intercommunication with it; and these diversities of conditions will manifest themselves in the greater or less amount of speciality of their animal productions. CHAP. XVL) THE BRITISH ISLES. 313 This speciality, when it exists, may have been brought about in two ways. A species or even a genus may on a continent have a very limited area of distribution, and this area may be wholly or almost wholly contained in the separated portion or island, to which it will henceforth be peculiar. Even when the area occupied by a species is pretty equally divided at the time of separation between the island and the continent, it may happen that it will become extinct on the latter, while it may survive on the former, because the limited number of individuals after division may be unable to maintain themselves against the severer competition or more contrasted climate of the continent, while they may flourish under the more favourable insular conditions. On the other hand, when a species continues to exist in both areas, it may on the island be subjected to some modifications by the altered conditions, and may thus come to present characters which · differentiate it from its continental allies and constitute it a new species. We shall in the course of our survey meet with cases illustrative of both these processes. The best examples of recent continental islands are Great Britain and Ireland, Japan, Formosa, and the larger Malay Islands, especially Borneo, Java, and Celebes; and as each of these presents special features of interest, we will give a short outline of their zoology and past history in relation to that of the continents from which they have recently been separated, commencing with our own islands, to which the present chapter will be devoted. Recent Physical Changes in the British Isles. -Great Britain is perhaps the most typical example of a large and recent continental island now to be found upon the globe. It is joined to the continent by a shallow bank which extends from Denmark to the Bay of Biscay, the 100 fathom line from these extreme points receding from the coasts so as to include the whole of the British Isles and about fifty miles beyond them to the westward. (See Map.) Beyond this line the sea deepens rapidly to the 500 and 1,000 fathom lines, the distance between 100 and 1,000 fathoms being from twenty to fifty miles, except where there is a great outward curve to include the Porcupine Bank 170 miles west of Galway, and to the north-west of Caithness where a |
