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 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 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 
