explained
how they were prevented from spreading west. An arm of the sea stretched
from the Aralo-Caspian to the Arctic Ocean, thus raising an effectual
barrier between the two continents. There is some evidence for the
belief, as we shall learn presently, that this marine barrier existed
also during the early part of the pleistocene epoch. After having
greatly expanded during pliocene times, the fauna of Siberia gradually
withdrew from the northern regions during the earlier portion of the
succeeding epoch. It was only after the marine connection above referred
to ceased to exist, or became disconnected, that an entry into Europe
was possible.

A fauna, to some extent composed of species now inhabiting the steppes
of Eastern Europe and Siberia, poured into the neighbouring continent.
On p. 95 I have given a list of those which reached as far west as the
British Islands, but, as I mentioned, many other species came from the
east about this time. With regard to the early history of the Siberian
mammals, I favour a view somewhat between that of Tcherski and that of
Brandt. The outpouring of the fauna into Europe seems to me to indicate
that there was a sudden change of climate in Siberia. This was produced,
perhaps, by the rupture of the marine connection between the Arctic
Ocean and the Aralo-Caspian. Such an event would not only have caused
the sudden shrinkage of the area available for food-supply by lowering
the temperature in Siberia, it would have acted also as a means in
assisting the fauna to enter a new continent where an inconsiderable
number of mammals, already established, were mostly dispossessed of
their homes by the advancing eastern host.

Brandt's theory, however, of a marine connection between the Arctic
Ocean and the Aralo-Caspian is by no means generally accepted. That the
Caspian Sea was at that time greatly larger than it is at present, and
joined to the Sea of Aral and the Black Sea, is acknowledged by
everybody. That the deposits laid down by this huge inland sea reach as
far north as the shores of the river Kama, in Central Russia, is also
well known to geologists. But what comes rather as a surprise, is that
Professor Karpinski, whom we must take as one of the highest authorities
on the geology of Russia, asserts that this Aralo-Caspian Sea was
probably joined by a system of lakes or channels to the Arctic Ocean (p.
183). He was by no means the first, though, to put forward such a
theory. We h