ting masses of wood, with
trees growing upon them, and ships have sometimes been in imminent
peril, as often as these islands were mistaken for terra firma, when, in
fact, they were in rapid motion.

It is highly interesting to trace, in imagination, the effects of the
passage of these rafts from the mouth of a large river to some
archipelago, such as those in the South Pacific, raised from the deep,
in comparatively modern times, by the operations of the volcano and the
earthquake, and the joint labours of coral animals and testacea. If a
storm arise, and the frail vessel be wrecked, still many a bird and
insect may succeed in gaining, by flight, some island of the newly
formed group, while the seeds and berries of herbs and shrubs, which
fall into the waves, may be thrown upon the strand. But if the surface
of the deep be calm, and the rafts are carried along by a current, or
wafted by some slight breath of air fanning the foliage of the green
trees, it may arrive, after a passage of several weeks, at the bay of an
island, into which its plants and animals may be poured out as from an
ark, and thus a colony of several hundred new species may at once be
naturalized.

The reader should be reminded, that I merely advert to the
transportation of these rafts as of extremely rare and accidental
occurrence; but it may account, in tropical countries, for some of the
rare exceptions to the general law of the confined range of mammiferous
species.

_Migrations of the Cetacea._--Many of the Cetacea, the whales of the
northern seas for example, are found to desert one tract of the sea, and
to visit another very distant, when they are urged by want of food, or
danger. The seals also retire from the coast of Greenland in July,
return again in September, and depart again in March, to return in June.
They proceed in great droves northwards, directing their course where
the sea is most free from ice, and are observed to be extremely fat when
they set out on this expedition, and very lean when they come home
again.[897]

_Species of the Mediterranean, Black Sea, and Caspian identical._--Some
naturalists have wondered that the sea-calves, dolphins, and other
marine mammalia of the Mediterranean and Black Sea, should be identical
with those found in the Caspian: and among other fanciful theories, they
have suggested that they may dive through subterranean conduits, and
thus pass from one sea into the other. But as the occurrence of