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.[82]

_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 {338} to show an almost perfect community with the
adjacent parts of the continent 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 have been partial submergences to the depth of from one hundred to
perhaps three hundred feet over a large part of our country; while during
the period of maximum glaciation the whole area north of the Thames was
buried in snow and ice. Even the south of England must have suffered the
rigour of an almost arctic climate, since Mr. Clement Reid has shown that
floating ice brought granite blocks from the Channel Islands to the coast
of Sussex. Such conditions must have almost exterminated our preexisting
fauna and flora, and it was only during the subsequent union of Britain
with the continent that the bulk of existing animals and plants could have
entered our islands. We know that just before and during the glacial period
we possessed a fauna almost or quite identical with that of adj