em done by glaciers every day, is the argument commonly used nowadays.
It was not so formerly. But Mr. Mallet and his views are almost
forgotten now; his name does not even appear in our great modern works
on the Ice Age. His argument was that as the land rose out of the
glacial sea, the mud which had accumulated round the shore slipped
downward in a direction determined by the contour of the surrounding
valleys and mountains. The moment the land rose above water-level, the
large mass of gravel and mud lying upon it slipped downward. During a
steady rising of the land there would therefore be produced a continuous
sliding down of this mud-glacier, which would groove and polish the rock
underneath it, in the same manner as the ice-glaciers do in the Alps (p.
47). Professors Sedgwick and Haughton became strong adherents of Mr.
Mallet's theory at the time, but it seems later on to have fallen into
disfavour with geologists, who may not even be thankful to have it
brought to light again.

SUMMARY OF CHAPTER II.

I have endeavoured to show in this chapter how we can determine
approximately the original home of an animal. By this means we are able
to study the component elements of the European fauna, which is found to
consist to a large extent of migrants from the neighbouring continents.
There is a Siberian, an Oriental, and an Arctic element in it. The
remainder of the fauna is derived from local centres of dispersal. What
was formerly believed to have been one great northern migration now
resolves itself, on closer study, into two very distinct ones--the
Siberian and the Arctic. The mammals have received most attention
hitherto, because their remains are so frequently met with, thus
enabling us more easily to investigate their past history; but
butterflies and snails have not been neglected, and at least one very
remarkable work on the latter has been published dealing with their
origin in Europe and in the remainder of the Palaearctic region.

The former distribution of land and water is intimately connected with
the origin of the European fauna, and the changes which have taken place
in this respect may be best traced by the present distribution of
mammals, snails, and earthworms. In this manner the British Islands may
be shown to have been connected with one another and with the Continent;
Spain with Morocco across the Straits of Gibraltar; Greece with Asia
Minor, and so forth.

The British fauna has played such