e indicates that they
have migrated to Europe from Asia Minor. _Buliminus pupa_ is one of
these. It is known from Asia Minor, Greece, South Italy, Sicily, and
Algeria. _Buliminus detritus_ is perhaps better known, being common in
some parts of Germany. From there its range spreads east as far as Asia
Minor. Many closely allied species inhabit Western Asia, to which they
are confined, while others enter on European territory in some of the
Greek islands. _B. fasciolatus_ occurs on the islands of Crete, Rhodes,
Cyprus, and in Greece and Syria. Most of the species of _Buliminus_ have
a very restricted range, but _Buliminus obscurus_ is found almost all
over Europe, from Ireland in the west to the Crimea and Transcaucasia in
the east.

Whether the sub-genus _Pomatia_ of the genus _Helix_--to which the
so-called Roman Snail belongs--is of Asiatic origin, or whether some of
the species have migrated from Europe to Asia, I am not prepared to say;
but there can be no doubt that _Helix pomatia_ has reached Western
Europe from the east.

On the whole, the number of mollusca which we might point to as having
migrated to Europe is not large, the great majority being indigenous to
our continent. However, some of the other groups of invertebrates differ
very materially in that respect from the mollusca. I cannot leave the
consideration of the mollusca without referring to the fact that there
appears to be a very important centre of distribution in South-eastern
Europe. It is from this centre that many species have spread north and
south, east and west. Take, for example, the genus _Clausilia_, a small
land-shell shaped like a pointed round tower, and abundant on old walls
and tree trunks. In England we have four species of _Clausilia_, in
Ireland only two. In the greater part of Spain only our common _Cl.
bidentata_ occurs. As we go east the number of species rapidly
increases. A maximum is reached in South-eastern Europe, where hundreds
of different kinds are found. Towards Northern Europe a similar decrease
of species takes place. So far the history of the _Clausiliae_ seems
perfectly simple. An active centre of origin appears to exist in
South-eastern Europe, from which the species radiate out in all
directions. But when we come to look more closely into the
extra-European distribution of the genus, and especially when we
examine its past history, we find that its origin is extremely complex,
and dates back to a much more remote