CHAP. XVI.] 



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 tlie 

 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 con- 

 tinental 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 



