ve occasion to refer to this again more fully in the next
chapter; meanwhile, it should be remembered that this stage was followed
by a partial retreat of the northern sea, though Scandinavia did not
become joined to the Continent. The date of this retreat of the sea,
represented in Fig. 13, corresponds probably to what is known as the
inter-glacial phase of the Glacial period, and I think it must have been
during this time that the Forest-Bed on the coast of Norfolk was laid
down.[2]

None of the Siberian mammals apparently entered Scandinavia at the time
when they invaded Central Europe and penetrated as far west as England
and Western France. Nor did the great Oriental mammals, like the Mammoth
and others, reach Scandinavia; and Professor Pohlig argued, on the
strength of these facts, that the latter country was either for a very
short time only free from ice, or that it had defective
land-communication with the Continent during inter-glacial times. This
seems to me scarcely to explain the facts of distribution and account
satisfactorily for the absentees. Nor does it, of course, harmonise with
the views that I have announced above. Professor Engler's remark (p.
131), that Scandinavia probably projected above the glacial sea as an
island, is more in accordance with these views, though the term island
is scarcely applicable to that country, since it was always, as I said,
indirectly joined to the Continent (_vide_ Fig. 13, p. 170). The fauna
of Scandinavia, both fossil and recent, points to a direct isolation of
that country from the continent of Europe during a considerable period.

Another proof that Northern Russia and the lowlands of Sweden were
covered by the sea comes to us from a study of the fauna of the relict
lakes--the "Reliktenseen" of Leuckart. This name was first applied by
Leuckart to lakes containing marine organisms, which are supposed to
have been flooded by, or to have been in close communication with the
sea at some former period, like the lakes Ladoga and Onega in Russia.
His views have been worked out subsequently in greater detail by Loven
and O. Peschel, who gave them their strong adherence. Many leading
zoologists, such as Professor Sars and others, have since adopted them,
and though discredited by Professor Credner, the theory still offers the
best explanation for the origin of marine animals in freshwater lakes.

Professor Credner's contention, that marine mollusca are always absent
from