merely claim to give a general idea. There was never any intention to
make them correspond with all the data of which we have geological
evidence. They are what I might call "diagrammatic." In comparing them
with reconstructions of former physical geography such as have been
attempted from time to time, I hope geologists will therefore deal
leniently with the faults I may have committed, and remember that the
maps are "impressions," or "diagrams," and not faithful representations
of all the geographical revolutions witnessed by some of our remote
forefathers at any particular period.

The knowledge we gain from a study of the British Tertiary deposits
enables us to affirm positively that both the eastern and the northern
species arrived in these islands comparatively recently, but that the
southern forms must have migrated northward from the Continent long ages
ago. Since the northern and the eastern migrations--that is to say,
those coming from the north and east--were the last to arrive in
Northern Europe, the remains of the animals contained in the most recent
deposits of that portion of our continent will furnish us with a clue as
to the extent of the area inhabited by them. This is not all, however.
It is also possible to discover from these remains the direction which
the animals that they belonged to came from. As we shall learn later
on, a migration on a vast scale entered Europe during the Pleistocene
epoch--the most recent of the geological epochs, during which great
extensions of glaciers occurred in the mountainous regions of Europe.
The latter period is known to us as the Ice Age or Glacial period. This
will be described more fully in Chapter II., meanwhile I may mention
that we presume that this migration came from the east, because no
remains of the members of that particular fauna are known from Spain,
Southern Italy, Scandinavia, Ireland, or from the Balkan peninsula. The
number of species evidently belonging to this same migration, moreover,
become fewer as we proceed westward, and a large proportion of them
still inhabit Northern Asia, though most of them are now extinct in
Europe. After having thoroughly studied such a recent geological
migration, we learn to understand others better, though the more ancient
they are, the fewer are the traces and the more difficult are they to
follow.

Then again we have to take into consideration the fact, that whilst
mammals, particularly the larger herbivores, ar