and at any rate. But it is also absent from Scandinavia, from the
Spanish peninsula, from almost the whole of Italy and the Alps, as also
from the Mediterranean Islands, whilst the little mouse occurs
abundantly right across Siberia. We shall learn more about centres of
dispersion later on; meanwhile I should mention that such a distribution
indicates that the Harvest Mouse has most likely originated in the east,
and has spread from there westward in recent geological times.

Conchologists have long ago been acquainted with the fact that many
molluscs, for example the so-called "Stone-cutter" Snail (_Helix
lapicida_) and the "Cheese Snail" (_Helix obvoluta_), have a very
restricted range in the British Islands. Both are entirely absent from
Scotland and Ireland, the Cheese Snail being confined to South-eastern
England. The Stone-cutter has rather a wider range, is even known from a
Welsh locality, and is met with as far north as Yorkshire. Their
distribution would indicate, therefore, that while both are recent
immigrants, the Cheese Snail is probably the last comer. This
supposition is in so far supported by fossil evidence, as the latter is
unknown in the fossil state, whilst the Stone-cutter has been described
by Messrs. Kennard and Woodward (p. 243)[1] as occurring in the cave
deposit known as the Ichtham fissure, and also from several English
pleistocene and holocene deposits. The Stone-cutter can scarcely be
looked upon as a very recent immigrant in the light of this evidence,
though we have no proof of its having ever had a much wider range in the
British Islands than it has to-day.

Among the lichens, which so abundantly cover the rocks and trees in
South-western Ireland, and which impart such a characteristic feature to
the scenery, we find a beautifully spotted slug (_Geomalacus
maculosus_).[2] It is a stranger to the rest of the British Islands, and
indeed occurs nowhere else in Northern Europe. We have to travel as far
as Northern Portugal before we again meet with it, and it is there also
that its nearest relations live.

Many more similar examples might be quoted, but enough, I think, has
been said to show that the British fauna is made up of several elements
whose original homes may lie widely apart and in different directions.
We have fossil evidence that some of the northern species, and also a
few of the southern ones, have become extinct within comparatively
recent times; others are apparently on th