d, which,
according to each particular region, might be divided into several
phases, or into their equivalents--viz., one or more extensions of the
ancient glaciers. But, on the whole, the view that there was at least
one inter-glacial phase in the Glacial period meets with more general
acceptance among geologists, I think, though the other opinion agrees
much better with the nature of the fauna and flora as it has been
revealed to us from the pleistocene deposits.

The occurrence of the remains of such arctic species of mammals as the
Musk-Ox, Arctic Fox, Glutton, Lemming, and many others in these
deposits, is frequently held up to us by geologists as a proof of the
prevalence of an arctic climate while these beds were laid down. And
indeed this appears at first a most satisfactory explanation of the
phenomenon. But we must not judge the climate of Europe by their
presence alone. As I shall explain more fully in Chapter V., these
species invaded Europe owing to two circumstances. Firstly, because the
climate of Siberia was becoming colder, necessitating a southward
movement, with a consequent over-population in a reduced area; secondly,
because a new short route to Europe had been opened up for them about
the same time (see p. 221). An invasion of Europe therefore took place
from east to west. Similar invasions occur even at the present day,
though not caused by a change in our climate, for every now and then
immense flocks of the Siberian Sandgrouse emigrate to our continent. The
mammalian migrants referred to are not to be looked upon as constituting
the whole of our fauna at that time. Europe had a fauna of its own, and
these invaders merely mingled with our animals. There was, no doubt, a
keen struggle for existence, as the result of which the weaker in many
cases succumbed. The hypothesis, however, that these Siberian migrants
occupied an empty continent, forsaken by its pre-glacial inhabitants, is
not supported by any facts.

All those who have investigated the pleistocene fauna have been struck
by the extraordinary mixture of northern and southern types of animals.
Professor Dawkins attempted to explain these facts by the supposition
(p. 113) that "in the summer time the southern species would pass
northwards, and in the winter time the northern would sway southwards,
and thus occupy at different times of the year the same tract of ground,
as is now the case with the elks and reindeer." "In some of the
caverns