emaining seventeen species probably lived only for
a very short time in England, and the rest gradually became extinct one
by one. This process of extinction of the aliens still continues. The
Beaver (_Castor fiber_) has died out within recent historic times. We
possess legends and uncertain historic records pointing to the existence
of the Reindeer in Scotland as recently as about seven centuries ago.
But much the same state of things has happened on the Continent. The
Glutton (_Gulo luscus_), which still lived in Northern Germany last
century, has now entirely vanished from that country, as also the
Reindeer. The Lemmings have found an asylum in Scandinavia. The Musk-Ox
(_Ovibos moschatus_) has disappeared not only from Europe but also from
Asia, and is now confined to Arctic America and Greenland. The Horse no
longer occurs in Europe in the wild state, and the Saiga Antelope
(_Saiga tartarica_) has retreated to the Steppes of Eastern Europe and
Western Siberia.

As we proceed more and more eastward across Central Europe, we find that
a larger and larger percentage of the Siberian migrants have adopted the
new country as their permanent home, though in France and Germany, as
well as in Austria, we have evidence that a great number of Siberian
species, which formerly lived there, have either become entirely
extinct, or have retreated towards the land of their origin. There is a
prevalent belief that these migrants have taken refuge on the higher
European mountain ranges, but this idea is altogether erroneous, as will
be shown in the chapter dealing with the origin of the Alpine fauna.

One of the Jerboas (_Alactaga jaculus_) occurs fossil as far west as
Western Germany, but it is now confined to Russia and Western Siberia.
The Bobak marmot (_Arctomys bobak_), which has a similar range now,
probably inhabited France in former times. A Siberian species which has
retreated but little is the Hamster (_Cricetus vulgaris_). Its fossil
remains have been found in Central France, but it does not now occur
west of the Vosges Mountains.

It appears, therefore, as if a wave of migration had swept over Central
Europe from east to west, that those species which were able to adapt
themselves to the new surroundings had remained, and as if the rest had
died out or were gradually retreating to the east.

Ornithologists are well acquainted with the fact that in some years
there is an unusually large exodus from Eastern Europe and Sib