318 
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 Continent. — 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 sea- 
level. From these and various other indications geologists are 
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 con- 
tinent 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 conditions 
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 evidence that a consider- 
able 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 now the highest mountains would remain as groups 
of rocky islets. This submersion must have destroyed the 
1 Quarterly Journal of Geological Society , 1850, p. 96. 
2 British Association Report , Dundee, 1867, p. 431. 
