n 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 {333} coasts so as to include the whole of
the British Isles and about fifty miles beyond them to the westward. (_See_
Map.)

[Illustration: MAP SHOWING THE SHALLOW BANK CONNECTING THE BRITISH ISLES
WITH THE CONTINENT.]

The light tint indicates a depth of less than 100 fathoms.
The figures show the depth in fathoms.
The narrow channel between Norway and Denmark is 2,580 feet deep.

Beyond this line the sea deepens rapidly to the 500 and 1,000 fathom lines,
the distance between 100 and 1,000 {334} 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
narrow ridge less than 500 fathoms below the surface joins the extensive
bank under 300 fathom