nt of these migrations, I have constructed
a map on which the probable course taken across Central Europe is
roughly indicated by dots (Fig. 16).

[Illustration: Fig. 16.--Map of Europe. The dotted portions represent,
approximately, the course of migration of the Siberian mammals. The
principal mountain ranges are roughly indicated in black.]

In the migrations of to-day we perceive the same tendency as in the
older ones of which we have fossil evidence, viz., generally a spreading
of species on a large scale over new territory, and then a gradual
shrinkage towards their original home, with an occasional survival of
small colonies in the invaded part. It must not be supposed that this
observation applies alone to the Siberian migration. In the case of the
Arctic one, precisely the same thing has happened, and we shall see that
the Southern (migration from the south) agrees in this respect with the
others.

As for the immediate cause of these migrations, it is to be looked for
either in the scarcity of food dependent upon a temporary or permanent
change of climate, or in an excessive increase in numbers of a
particular species. I do not propose to trace back migrations beyond the
Pliocene Epoch, or indeed much beyond the beginning of the Glacial
period, which is regarded as a phase of the most recent geological
epoch, viz., the Pleistocene. During the period in question, we have
indirect evidence of one vast migration from Siberia into Europe across
the lowlands lying to the north of the Caspian and to the south of the
Ural Mountains. There is a general consensus of opinion that this
migration took place in Pleistocene times. Professor Nehring thinks that
there can be no doubt (p. 222) that the Siberian migrants arrived in
Northern Germany after the first stage or division of the Glacial
period, and lived there probably during the inter-glacial phase which
occurred between the first and second stages--if indeed we look upon
this period as being divisible into two distinct stages.

Judging from the evidence of distribution of mammals in pleistocene
Europe, Professor Boyd Dawkins came to the conclusion (p. 113) that the
climate of our continent "was severe in the north and warm in the
south, while in the middle zone, comprising France, Germany, and the
greater part of Britain, the winters were cold and the summers warm, as
in Middle Asia and North America." "In the summer time the southern
species would pass northwards