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which began about 240,000 years ago and lasted for some
160,000 years, does not appear to have been one uninterrupted
era of intense arctic climate, but several milder periods inter-
vened, when the great ice-sheet retreated northwards and the
glaciers remained confined to the mountain-tops, till the
recurrence of the glacial climate again caused them to spread
to the lowlands and once more to push their way southwards.
During those mild inter-glacial periods, when the arctic forms
that inhabited this country had retreated with the glaciers,
animals such as the mammoth, rhinoceros, hippopotamus,
lion, tiger, bear, hyaena, &c., became inhabitants of Britain,
which was then connected with the continent of Europe
across what is now the German Ocean. The old inter-glacial
mammals—and probably Palaeolithic man was among them—
would have told of mighty revolutions in physical geography
could they have kept records, as our island, after they had
taken possession of it, became submerged beneath the sea to
an immense depth, till only the high lands appeared above
the waters, forming an archipelago. Then followed a final
return of glacial conditions, when the great ice-covering for
the last time enwrapped our country, which slowly rose from
the frozen ocean and once again became a portion of the
continent. As the more genial climate which has lasted to
our own times came on, the ice-sheet slowly disappeared;
glaciers lingered for some time on our mountains, and finally
vanished, to return no more till the next glacial epoch. It
was during these continental states of Britain, when the
Thames was a tributary of the Rhine, that the country
became the home of those animals and plants the survivors
of which constitute our present fauna and flora, the sea
finally sweeping away the land connection and leaving our
island much as we now find it.
The records of "the great ice age" are more forcibly
impressed upon the mountainous districts of Wales, Scotland,
and the Lake District than in the southern portions of Eng-
land. The great submergence which preceded the last glacial
relapse left only the south of our island above the sea, and a
line drawn across the country from the mouth of the Thames
to the mouth of the Severn represents the southernmost