icapillus_, which has been able to reach Ireland, has a wider range
and came earlier with the Orientals.

Messrs. Speyer state (p. 68) that almost all those species of Central
European Butterflies whose northern limit is deflected southward as we
approach the west coast of Europe, inhabit also the Volga country and
the adjoining parts of Asia. Many of them are much commoner there than
in Central Europe, and it appears probable to the authors of the
_Geographical Distribution of Butterflies_ that these species came from
the east. Asia and Central Europe have, according to Messrs. Speyer, no
fewer than 156 species in common. Mr. Petersen estimates that no less
than 91 per cent. of the Arctic-European Butterflies also occur in
Siberia. He made a special study of the Arctic _Macro-lepidoptera_, and
came to the conclusion that Central Asia, not having been glaciated in
the Ice-Age, offered a possibility of existence to both animals and
plants. Here, he thinks, was the principal centre to which Europe owed
its re-population in post-glacial times. Mr. Petersen is of opinion (p.
40) that the Arctic-European _Lepidoptera_ are composed of two
elements--the pliocene relics which persisted in Europe during the
Glacial period, and the new immigrants from Siberia.

No doubt Siberia supplied Europe with a number of species of
Butterflies and Moths in recent geological times, but we need not
necessarily suppose that these arrived only after the Glacial period.
Even the most extreme glacialists admit that large areas on our
continent were free from ice at the height of the Ice-Age, Siberia had
therefore no particular advantage over Europe in giving an asylum to
Butterflies and Moths which were escaping from the rigours of a supposed
arctic climate. But we have already learned (p. 80) that the climate
during the Glacial period probably differed but little from that which
we enjoy at the present day, and we may assume, therefore, that the
_Lepidoptera_ of Siberia migrated during that time or even earlier to
Europe.

Let us for a moment reconsider some instances of mammalian migration
from Siberia, with a view to studying more closely the nature of these
great events. I mentioned the fact that some of the Siberian migrants
have remained in England, that more have settled down permanently on our
continent, but that many others have either become entirely extinct or
do not live any longer in Europe.

Of the mammals which made their appea