as covered by boulder-clay, the
strongly dotted ones those exhibiting Aralo-Caspian and other
post-pliocene deposits.]

The boulder-clay of Northern Continental Europe, as already stated, is
now generally recognised to be the product of a huge ice-sheet which
invaded the lowlands of Continental Europe from the Scandinavian
mountains. Though Alpine glaciers at the present day produce little or
no ground moraine, these ancient larger ice-sheets, or "mers de glace,"
are believed to have deposited immense layers of mud containing
scratched and polished stones. Many of the latter have been carried
great distances from their source of origin. The Scandinavian ice-sheet
is supposed to have advanced as far south as the line indicated on the
map, after which it gradually retreated. On this point, however, as in
almost every detail connected with the Glacial period, geologists are at
variance. Professor James Geikie maintains, that there were no less than
four Glacial periods, separated from one another by milder inter-glacial
phases. On the Continent the view of two Glacial and one inter-glacial
period is, I think, more generally adopted. Professor Geikie's four
periods seem to me to have originated in a desire to correlate the
British pleistocene deposits with the continental ones, and at the same
time to retain the old view of the inter-glacial position of the
Forest-Bed. The two theories agree in so far as that in both the glacial
conditions culminate in a maximum glaciation, followed by a more
temperate phase of climate, with consequent retreat of the ice-sheets,
and finally by a renewed advance of the glaciers.

We are told that there is not the slightest doubt about it that a marked
but gradual decrease of temperature took place all over Europe either
during the beginning of the Pleistocene or towards the end of the
Pliocene Epoch.

We might reasonably suppose, then, that a similar climatic effect was
produced in Siberia, in consequence of which the fauna would have been
obliged to retreat from the extreme northern latitudes southward. No
doubt great efforts would have been made by the members of the Siberian
fauna--at any rate by those possessing strong power of locomotion--to
extend their range in other directions. But we have no evidence that a
migration from Siberia came to Eastern Europe at that time. It seems,
therefore, as if the barrier referred to by Brandt, Koeppen, Boyd
Dawkins, and others (p. 222), had exist