re exposed to the same conditions of temperature. In this
state of affairs the influences which now make for change in organic
species would be far less than they are. Journeying in the great
whirlpools which the continental barriers make out of the westward
setting tropical currents, these organic species are ever being
exposed to alterations in their temperature conditions which we know
to be favourable to the creation of those variations on which the
advance of organic life so intimately depends. Thus the ocean currents
not only help to vary the earth by producing changes in the climate of
both sea and land, breaking up the uniformity which would otherwise
characterize regions at the same distance from the equator, but they
induce, by the consequences of the migrations which they enforce,
changes in the organic tenants of the sea.

Another immediate effect of ocean streams arises where their currents
of warm water come against shores or shallows of the sea. At these
points, if the water have a tropical temperature, we invariably find a
vast and rapid development of marine animals and plants, of which the
coral-making polyps are the most important. In such positions the
growth of forms which secrete solid skeletons is so rapid that great
walls of their remains accumulate next the shore, the mass being built
outwardly by successive growths until the realm of the land may be
extended for scores of miles into the deep. In other cases vast mounds
of this organic _debris_ may be accumulated in mid ocean until its
surface is interspersed with myriads of islands, all of which mark the
work due to the combined action of currents and the marine life which
they nourish. Probably more than four fifths of all the islands in the
tropical belt are due in this way to the life-sustaining action of the
currents which the trade winds create.

There are many secondary influences of a less important nature which
are due to the ocean streams. The reader will find on most wall-maps
of the world certain areas in the central part of the oceans which are
noted as Sargassum seas, of which that of the North Atlantic, west and
south of the Azore Islands, is one of the most conspicuous. In these
tracts, which in extent may almost be compared with the continents, we
find great quantities of floating seaweed, the entangled fronds of
which often form a mass sufficiently dense to slightly restrain the
speed of ships. When the men on the caravels of