er, when, after an unusually
prolific season, or upon a sudden scarcity of provisions, great
multitudes are threatened by famine. It may be useful to enumerate some
examples of these migrations, because they may put us upon our guard
against attributing a high antiquity to a particular species merely
because it is diffused over a great space; they show clearly how soon,
in a state of nature, a newly created species might spread itself, in
every direction, from a single point.

In very severe winters, great numbers of the black bears of America
migrate from Canada into the United States; but in milder seasons, when
they have been well fed, they remain and hybernate in the north.[881]
The rein-deer, which, in Scandinavia, can scarcely exist to the south of
the sixty-fifth parallel, descends, in consequence of the greater
coldness of the climate, to the fiftieth degree in Chinese Tartary, and
often roves into a country of more southern latitude than any part of
England.

In Lapland, and other high latitudes, the common squirrels, whenever
they are compelled, by want of provisions, to quit their usual abodes,
migrate in amazing numbers, and travel directly forwards, allowing
neither rocks nor forests, nor the broadest waters, to turn them from
their course. Great numbers are often drowned in attempting to pass
friths and rivers. In like manner the small Norway rat sometimes pursues
its migrations in a straight line across rivers and lakes; and Pennant
informs us, that when the rats, in Kamtschatka, become too numerous,
they gather together in the spring, and proceed in great bodies
westward, swimming over rivers, lakes, and arms of the sea. Many are
drowned or destroyed by water-fowl or fish. As soon as they have crossed
the river Penginsk, at the head of the gulf of the same name, they turn
southward, and reach the rivers Judoma and Okotsk by the middle of July;
a district more than 800 miles distant from their point of departure.

The lemings, also, a small kind of rat, are described as natives of the
mountains of Kolen, in Lapland; and once or twice in a quarter of a
century they appear in vast numbers, advancing along the ground, and
"devouring every green thing." Innumerable bands march from the Kolen,
through Nordland and Finmark, to the Western Ocean, which they
immediately enter; and after swimming about for some time, perish. Other
bands take their route through Swedish Lapland, to the Bothnian Gulf,
where they