ess difficulty
in deciding this problem. But butterflies are scarcely ever preserved in
a fossil state, and birds very rarely. We know little or nothing,
therefore, of their past history from direct evidence, and are obliged
to trust to indirect methods of research which will be indicated later
on.

Mammals and Snails tell us their story more plainly. The bones of the
former and the shells of snails are easily preserved, and thus furnish
us with the necessary data as to their past history, for we find them
abundantly in most of the recent geological deposits. Among the mammals
of the British Islands there are some instances of distribution which
much resemble those I have quoted. Thus the Arctic Hare (_Lepus
variabilis_) is in the British Islands confined to Ireland and to the
mountains of Scotland; and if it were not for the fact that its bones
have been discovered in a cave in the south-west of England, we should
perhaps never have known that, formerly, it must have inhabited that
country as well. Of other mammals we possess fossil and also historical
evidence of their having once lived in these islands. Such are the Wolf
and the Wild Boar, both of which were abundant in Great Britain and
Ireland. The latter is a distinctly southern species. We assume this,
because its remains have never been found in high northern latitudes;
nor does it now occur in Northern Europe or Northern Asia, whilst all
its nearest relatives live in sub-tropical or tropical climates. The
Arctic Hare, on the contrary, has probably come to us from the north.
Its remains are unknown even in Southern Europe, and the more we
approach the Arctic Regions, the more abundant it becomes. Thus we have
here two instances of British mammals, one of which, the Wild Boar, has
died out--as it were in a southerly direction; whilst the other, the
Arctic Hare, is apparently retreating towards the north.

There are also some British mammals of which we have no fossil history,
at least of which no remains have as yet been found in these islands.
Such a one is the Harvest Mouse (_Mus minutus_). It has a somewhat
restricted range in England, and only just crosses the Scottish border
in the east. From the rest of Scotland and from the whole of Ireland it
is absent. To judge from this distribution, in connection with the fact
of its being unknown as a British fossil species, it is probably a late
immigrant to England, and has not had time to spread, throughout
Scotl