o Europe
when--owing to changes of climate perhaps--they forsook their original
homes.

We observe much the same differences of origin in the various groups of
European Invertebrates. The Central European Molluscan fauna, remarks
Dr. Kobelt, had already developed from the pliocene--in almost all its
details, as regards formation of species and distribution--when the
Ice-Age commenced (_b_, i. p. 162). Certain very interesting
dislocations, however, in the range of land mollusca can be proved to
have taken place about that time. Thus, as Dr. Kobelt has pointed out,
the genus _Zonites_, which is now almost confined to the south-east of
Europe, occurs in inter-glacial deposits in the valley of the Neckar,
and even as far west as the Seine. If we might judge from this single
instance, a molluscan migration from the east to the west seems to have
occurred either in early or pre-glacial times. That _Helix pomatia_ has
migrated only comparatively recently from the East to Western Europe is
rendered probable by its general range in northern and western Europe,
but I cannot agree with Dr. Kobelt in the belief that _Helix aspersa_ is
of an equally recent origin in the North. No matter whether it has been
found fossil or no, its range in the British Islands points to its
having penetrated to Ireland when the latter was still connected with
the Continent by way of England. Its migration from the Mediterranean
dates therefore from early pleistocene or late pliocene times.

In referring to the sixty-five species of Land and Freshwater Mollusca
which have been described from the continental "Loess," Dr. Kobelt
states (p. 166) that this fauna has certainly not a steppe-character. It
does not therefore strengthen Professor Nehring's view that Europe
during the deposition of the loess had a climate comparable to that of
the Siberian steppes. The Glacial period had hardly any effect on the
molluscan fauna of Europe. Dr. Kobelt believes in a certain movement of
that fauna from the least favourable areas, with a subsequent
re-immigration; but even that could not have taken place on a large
scale. Nothing like a destruction of the fauna occurred, as far as we
know from fossil evidence.

Not a single species of land or freshwater mollusc can be quoted as
having migrated to Europe from Siberia in recent geological times. The
molluscan fauna of the latter country is so closely connected with that
of Europe, that it is quite impossible to ele