tive
land. But it might be asked, how is it known that these species did not
originate in Europe, and thence migrate to Siberia? Because if they had
originated on our continent, they would have spread there. They would
have invaded Northern and Southern Europe, and they would probably have
left some remains in Spain, Italy, or Greece. They would also have left
some of their relations in Europe; but all their nearest allies, too,
are Asiatic. Moreover,--and this completes, I think, the proof of their
Siberian origin,--the Pleistocene remains of these animals in Europe
become less abundant, and the number of species likewise decreases, as
we proceed from east to west. With these remains of Steppe animals are
generally associated those of others, which we must also look upon as
Siberian emigrants, such as the Pikas or tailless Hares belonging to the
genus _Lagomys_, the pouched Marmots (_Spermophilus_), and others. Some
of them, as I have mentioned, still inhabit Central and Eastern Europe,
whilst others have a wider distribution on our continent.

[Illustration: Fig. 2.--The Saiga-Antelope (_Saiga tartarica_). (From
Lydekker's _Royal Natural History_, vol. ii. p. 298.)]

This migration must have been an unusually large one. It has been
suggested that the Glacial period had some connection with it, and there
can be little doubt, as we shall see later on, that a change of climate
probably brought about this great Siberian invasion of Europe. But other
causes might tend in the same direction, such as want of sufficient food
after a few years of great increase of any particular species. It is not
known to what we owe the periodic visits of the Central Asiatic
Sandgrouse (_Syrrhaptes paradoxus_), Fig. 3, but certain it is that
immense flocks of these birds invade Europe from time to time at the
present day, just as those mammals may have done in past ages.

[Illustration: Fig. 3.--Central Asiatic Sandgrouse (_Syrrhaptes
paradoxus_).]

The _Siberian_ migrations will be spoken of in the subsequent pages, as
the Siberian element of the European fauna. These migrations, however,
are not the only ones which reached Europe from Asia. The sixth chapter
deals with migrations which have influenced our fauna far more than the
Siberian. The latter did not last long, nor did they affect the whole of
Europe. But what I may call the _Oriental_ migrations spread to every
corner of Europe and certainly lasted throughout the whole of the
T