d to South-Western Europe. Only a few species of _Arion_
have a wide range in Europe, one of them, _A. subfuscus_, crossing the
borders of our continent into Siberia. In the British Islands and in
Western Germany, which are about equi-distant from the supposed creative
centre of the genus, there are found five species. In France six or
seven species are met with, and in Spain and Portugal about ten. Towards
the east, _Arions_ diminish in number. This genus, therefore, forms part
of a migration which I have designated as "Lusitanian" from _Lusitania_,
the name applied by the Romans to what we now call Portugal. Another
genus of slugs, _Geomalacus_, is interesting from the fact that one
species occurs in the British Islands, being otherwise confined to the
Lusitanian province. Parmacella, a slug-like animal bearing a tiny shell
at the extremity of its tail, has probably likewise had its origin in
this part of Europe. All this, however, will be more fully referred to
in the seventh chapter, which deals with the Lusitanian fauna.

As regards the Alpine centre of origin, Dr. Kobelt considers three
groups of mollusca as especially characteristic of the Alps, viz., the
sub-genus _Campylaea_ of the great and widely-spread genus _Helix_, and
the genera _Pomatias_ and _Zonites_. The latter, which is not to be
confounded with our British _Hyalinia_ (formerly united with _Zonites_),
does not extend very far south or north of the Alps. There may be others
too, which owe their origin to these mountains, but most of the
terrestrial mollusca are exceedingly ancient, and many genera have
existed long before the Alps had made their appearance above the surface
of the early Tertiary seas. It should be remembered that _Hyalinia_ and
_Pupa_, both British genera, are known from carboniferous deposits in
forms which closely approach those living at the present day, and in
these and a great number of other instances, it is quite impossible to
determine the original home of the genus.

This little digression on centres of dispersion will help us to
understand in what manner the indigenous element of the European fauna
joined in with the alien members as they arrived in our continent. The
species confined to South-Eastern England need not necessarily have come
to us from Eastern Europe or Siberia. Alpine species spread northward
probably at the same time as the Siberian animals went westward. An
Alpine form may therefore have joined a batch of t