circumstances. It is obvious that powerful tides, winds, and
currents may sometimes carry along quadrupeds capable, in like manner,
of preserving themselves for hours in the sea, to very considerable
distances; and in this way, perhaps, the tapir (_Tapir Indicus_) may
have become common to Sumatra and the Malayan peninsula.

To the elephant, in particular, the power of crossing rivers is
essential in a wild state, for the quantity of food which a herd of
these animals consumes renders it necessary that they should be
constantly moving from place to place. The elephant crosses the stream
in two ways. If the bed of the river be hard, and the water not of too
great a depth, he fords it. But when he crosses great rivers, such as
the Ganges and the Niger, the elephant swims deep, so deep, that the end
of his trunk only is out of the water; for it is a matter of
indifference to him whether his body be completely immersed, provided he
can bring the tip of his trunk to the surface, so as to breathe the
external air.

Animals of the deer kind frequently take to the water, especially in the
rutting season, when the stags are seen, swimming for several leagues at
a time, from island to island, in search of the does, especially in the
Canadian lakes; and in some countries where there are islands near the
sea-shore, they fearlessly enter the sea and swim to them. In hunting
excursions, in North America, the elk of that country is frequently
pursued for great distances through the water.

The large herbivorous animals, which are gregarious, can never remain
long in a confined region, as they consume so much vegetable food. The
immense herds of bisons (_Bos Americanus_) which often, in the great
valleys of the Mississippi and its tributaries blacken the surface of
the prairie lands, are continually shifting their quarters, followed by
wolves, which prowl about in their rear. "It is no exaggeration," says
Mr. James, "to assert, that in one place, on the banks of the Platte, at
least ten thousand bisons burst on our sight in an instant. In the
morning, we again sought the living picture; but upon all the plain,
which last evening was so teeming with noble animals, not one
remained."[880]

_Migratory instincts._--Besides the disposition common to the
individuals of every species slowly to extend their range in search of
food, in proportion as their numbers augment, a migratory instinct often
developes itself in an extraordinary mann