eastern steppes into Europe, where
they inhabit the arid regions between the Caspian and the River Don in
Southern Russia.

The species of Mammals living in Europe at the present day have, with
few exceptions, migrated to our continent from other parts of the world.
With regard to the Birds, it is possible that a somewhat larger number
proportionally may be of European origin. Still, the great majority are,
I think, to be regarded as immigrants. The autochthones are about equal
to the immigrant reptiles, but many of the European Amphibians and the
majority of the Fishes have probably originated on our continent. Some
of the European Amphibia--especially among the tailless forms--appear to
be immigrants from Asia. Thus the distribution of _Rana arvalis_ in
Europe is remarkably like that of a Siberian migrant. This frog occurs
in Siberia, ranging southward as far as Persia and parts of Asia Minor.
Crossing the European border, we find it in Russia, Upper Hungary, North
and Central Germany,--being rarer in the south,--Denmark, and
Scandinavia. According to Bedriaga, it crosses the Rhine only in Alsace,
but occurs no farther west. It only just enters Holland. If we suppose
the species to have originated in Central Europe, we should expect to
find it in Switzerland, France, and perhaps England. If it had its
ancestral home in Eastern Europe, we might expect it to occur on the
Balkan peninsula. It seems to me more probable, therefore, that _Rana
arvalis_ came with the Siberian migration. This need not cause surprise,
as the genus _Rana_ is certainly not European. Out of about 110 species,
only four are peculiar to Europe, the rest are scattered over all parts
of the globe. Moreover, the fact that these four species are confined to
Southern Europe would seem to indicate that the first species entered
from the south, and there either became modified or spread over nearly
the whole continent, as did, for instance, _Rana esculenta_ and _R.
temporaria_. Neither of these is by any means confined to Europe. _R.
esculenta_ ranges right across the Asiatic continent to Japan, and also
enters North Africa, while the other has a wide distribution in northern
and temperate Asia.

The various groups of Vertebrates are not dependent on each other in
their migrations. Mammals and Birds extend their range with so much
greater facility than Reptiles and Amphibians, that the surplus
population of our neighbouring continents readily poured int