Physical geography. By Mary Somerville ...

450 PHYSICAL GEOGRAPHY. CHAP. XXXII
Western Ocean, which they enter and are drowned; other bands go
through Swedish Lapland and perish in the Gulf of Bothnia. Thus
nature provides a remedy against the over-increase of any one species, and maintains the balance of the whole. A temporary migration for food is not uncommon in animals. The wild ass, a native
of the deserts of Great Tartary, in summer feeds to the east and
north of the lake of Aral, and in autumn they migrate in thousands
to the north of India, and even to Persia.' The ruminating animals
that dwell in the inaccessible parts of the Himalaya descend to their
lower declivities in search of food in winter; and for the'same reason the reindeer and musk-ox leave the Arctic snows for a more
southern latitude.
The Arctic regions form a district common to Europe, Asia, and
America. On this account, the animals inhabiting the northern
parts of these continents are sometimes identical, often very similar;
in fact, there is no genus of quadrupeds in the Arctic regions that
is not found in the three continents, though there are only 27 species common to all, and these are mostly fur-bearing animals. In
the temperate zone of Europe and Asia, which forms an uninterrupted region, identity of species is occasionally met with, but for
the most part marked by such varieties in size and colour as might
be expected to arise from difference of food and climate. The same
genera are sometimes found in the intertropical parts of Asia, Africa, and America, but the same species never; much less in the
south temperate zones of these continents, where all the animals are
different, whether birds, beasts, insects, or reptiles; but in similar
climates analogous tribes replace one another.
Europe has no family and no order peculiarly its own, and many
of its species are common to other countries; consequently the great
zoological districts, where the subject is viewed on a broad scale, are
Asia, Africa, Oceaniea, America, and Australia; but in each of
these there are smaller districts, to which particular genera and
families are confined. Yet when the regions are not separated by
lofty mountain-chains, acting as barriers, the races are in most cases
blended together on the confines between the two districts, so that
there is not a sudden change.
EUROPEAN QUADRUPEDS.
The character of the animals of temperate Europe has been more
changed by the progress of civilization than that of any other quar-' Perhaps no quadruped in the wild state will be found to have so wide
a vertical range of habitat as this animal. It is found in the plains of Tartary, in the valley of the Tigris, at a very few feet above the sea-level, and
in the most elevated valleys of the Himalaya and in Tibet, at elevations
exceeding 15,200 feet.