great many have persisted
in the Alps to the present day and exhibit discontinuous distribution,
having meanwhile disappeared in the intermediate tract between the
latter and their original home in Asia. The lowlands of Eastern and
Central Europe were either occupied by the sea or by large freshwater
lakes, so as effectually to prevent a direct migration northward.

When the newer migrants arrived from Asia not only had the Alps risen to
a lofty mountain chain acting as an effectual barrier, but Southern
Italy and Greece had become disconnected. Some time after, Sicily and
Southern Italy also became separated. Meanwhile the stream of migrants
which consisted less and less of typically southern forms, emigrants
from Central Asia and even Southern Siberia, mingled with the southern
forms on their way to Europe, and these now poured across the newly
opened plain of Central and Northern Europe. But it was not until some
time after this that the Mediterranean Sea broke across the AEgean
region, and that the Northern Sea retired from the plains of Eastern
Russia to admit the typical Siberian fauna and flora into our continent
(_vide_ pp. 189-241).

I cannot close this chapter without referring to the active
distributional centre--or I might say, centre of origin--of species
situated in South-eastern Europe. No group of animals is more
instructive in elucidating the paths of migration from this centre than
the terrestrial mollusca. Wherever the original home of the genus
_Clausilia_ may have been in early Tertiary times, it is certain that
the most active centre of origin is now, and has been for a considerable
time past, in South-eastern Europe. One of the earliest migrants from
that modern centre of this interesting genus is _Clausilia bidentata_,
which is the only species found in Southern Spain, and one of the two
met with in Ireland, and which has been observed in high altitudes in
the Alps and in Scandinavia. As we go eastward from Western Europe the
number of species of _Clausilia_, as we have seen, increases until we
reach a maximum in the Balkan peninsula and the region of the Caucasus.
_Limax_, _Agriolimax_, and _Amalia_, three genera of slugs, likewise
appear to have originated in the same region and spread over Europe from
there. Some species like _Limax maximus_ and _L. marginatus_ are very
ancient, and probably commenced their wanderings in early Tertiary
times. In this manner many animals of European origin hav