the south
to Belgium and France. The various phases of geographical evolution of
Europe will be studied in the subsequent chapters, and maps will then be
given to show as far as possible in a general way the leading
characteristics of these great changes.

I have now given some reasons for the belief that several different
migrations of animals entered the British Islands in later Tertiary
times. I have also shown why some of them must be looked upon as being
older than others, and in so far we have come to a decision as to their
relative ages. It still remains for us, however, to examine how their
geological ages can be approximately determined. We require for this
purpose palaeontological aid.

In the fifth chapter will be found the history of the Siberian
migration. And since we possess most valuable records of it in the
numerous fossil remains discovered in Central and Western Europe, we are
able to trace their progress from the east to the west in a very
complete and satisfactory manner. In England their first appearance
dates from the Forest-Bed, for here we find remains of the Glutton
(_Gulo luscus_), Musk-Ox (_Ovibos moschatus_), and others (see p. 204).
It seems reasonable to suppose, therefore, that the first entry of these
Siberian mammals into Europe took place at or just before the Forest-Bed
period. But Professor Nehring tells us in his remarkable work on the
Tundra and Steppes (p. 222), that in Germany the remains of the same
mammals occur in deposits which are certainly more recent than the lower
continental boulder clay; and he is inclined to the belief that they
migrated into Europe during the inter-glacial phase which is supposed to
have separated the earlier from the later stage of the Glacial period.
It is evident that in this case the inter-glacial period in Germany
would have corresponded to, and be contemporaneous with, our Forest-Bed
period. The deposits immediately preceding the Forest-Bed would also be
contemporaneous with the lower continental boulder clay. Although this
may seem rather a startling statement to make, from the evidence which
will be brought forward in the fourth and fifth chapters I am inclined
to the belief that such is probably the case.

Having once arrived at a determination of the exact geological period
during which the Siberian mammals invaded our continent, and having also
previously determined the relative ages of the various other migrations,
we have advanced anot