well with the facts of distribution. Let us take,
for instance, the genus _Clausilia_, a pretty turret-shaped snail, which
abounds on old ruined walls. Only two species, viz., _Cl. laminata_ and
_Cl. bidentata_, are met with in Ireland. In England we find the same
species with the addition of two others, _Cl. biplicata_ and _Cl.
Rolphii_. Crossing over the Channel to Belgium, these four species occur
again, and also several others not known in England. In Germany the list
of _Clausiliae_ mounts up to twenty-five species, including all those
found in the British Islands. As we proceed eastward the number of
species of this genus increases steadily, and when we reach the Caucasus
or the Balkan Peninsula the conchologist is able to make a collection
of several hundred different kinds, whilst farther east again they
diminish. This clearly indicates there is in South-Eastern Europe a
powerful centre of creation of _Clausiliae_, from which the species have
spread all over Europe. But it is by no means certain that this centre
was always in our continent, for in South-Eastern Asia and the Malay
Archipelago _Clausiliae_ increase once more. It is interesting to note,
however, that almost all these eastern forms belong to the sub-genus
_Phaedusa_ (_vide_ Boettger), which had only been known as a fossil genus
from a few species in the Eocene and Oligocene of Southern Europe. The
first centre of origin, therefore, may possibly have been in Southern
Asia, and in these early Tertiary times a second centre may have become
established in Southern Europe from which the sub-genus _Garnieria_ went
eastward, _Macroptychia_ southward, and _Nenia_ westward across the
Atlantic to South America. Only a few remnants of these primitive
_Clausiliae_ are now left in Europe, such as the interesting _Cl.
(Laminifera) Pauli_.

As an example of a genus which has its centre of distribution in
South-Western Europe we might take that to which our common brown garden
slug belongs, viz., _Arion_. Dr. Simroth, who was the first to point out
that the species of _Arion_ had spread over our continent from
South-Western Europe (p. 5), is inclined to the belief that the
_Arionidae_ had originated on the old land-bridge between Europe and
North America, which is generally known by the name of "Atlantis." From
this a branch went westward to the New World and another eastward as far
as Southern Asia, but _Arion_ and a number of other genera are more or
less confine