imum cold of the Glacial period, that
steppes with a Siberian fauna could exist. Other groups, such as the
Mollusca, however, do not support Professor Nehring's theory, and in
order to arrive at an independent solution of this and the other
problems referred to, a short history is given of the Siberian fauna.
Recent geological ages have witnessed the arrival in Southern Europe of
mammals now almost confined to the arctic and subarctic regions. In
Siberia, on the other hand, many southern species penetrated, apparently
about the same time, to the extreme northern limits of that country. The
greatest authority on the Siberian fossil fauna, Tcherski, believes that
this took place in pliocene times, the gradual retreat occupying the
whole of the Glacial period. If this were correct, the retreat from the
Arctic Regions would have occurred at the same time when, according to
our European authorities, Professors Nehring and Geikie, the much more
southern parts of our continent were already uninhabitable. But Siberia
could not have supported the large mammals at all at a time when Europe
was uninhabitable, as it would be difficult to conceive under what
geographical conditions the climate of the latter was arctic and that of
the former temperate. If the whole fauna was driven into Southern Asia,
how is it that the Siberian invasion of Europe occurred immediately
after the deposition of the lower boulder-clay, that is to say, after
the earlier part of the Glacial period? The difficulty can be met by the
supposition that both Europe and Siberia had a temperate climate at that
time. This view is supported by certain evidences, fully described, of a
connection between the Caspian and the White Sea, which would have had
the effect of influencing the climate. The Siberian fauna would thus
have been prevented from spreading westward in Pliocene and early
Glacial times. But on the disappearance of the marine connection, a way
would have been opened into our continent, which again had an effect on
the climate. The latter would have become sensibly colder and thus have
reduced the habitable area of the Siberian fauna.

Such geographical conditions would have been incompatible with a great
northern _mer de glace_, and the boulder-clay in Northern Europe could
not have represented a ground moraine but is a marine deposit. The sea
is supposed to have covered the Northern Russian and German plains, and
into it icebergs discharged the detritus