Columbus entered this
tangle, they were alarmed lest they should be unable to escape from
its toils. It is a curious fact that these weeds of the sea while
floating do not reproduce by spores the structures which answer to the
seeds of higher plants, but grow only by budding. It seems certain
that they could not maintain their place in the ocean but for the
action of the currents which convey the bits rent off from the shores
where the plant is truly at home. This vast growth of plant life in
the Sargassum basins doubtless contributed considerable and important
deposits of sediment to the sea floors beneath the waters which it
inhabits. Certain ancient strata, known as the Devonian black shale,
occupying the Ohio valley and the neighbouring parts of North America
to the east and north of that basin, appear to be accumulations which
were made beneath an ancient Sargassum sea.

The ocean currents have greatly favoured and in many instances
determined the migrations not only of marine forms, but of land
creatures as well. Floating timber may bear the eggs and seeds of many
forms of life to great distances until the rafts are cast ashore in a
realm where, if the conditions favour, the creatures may find a new
seat for their life. Seeds of plants incased in their often dense
envelopes may, because they float, be independently carried great
distances. So it comes about that no sooner does a coral or other
island rise above the waters of the sea than it becomes occupied by a
varied array of plants. The migrations of people, even down to the
time of the voyages which discovered America, have in large measure
been controlled by the run of the ocean streams. The tropical set of
the waters to the westward helped Columbus on his way, and enabled him
to make a journey which but for their assistance could hardly have
been accomplished. This same current in the northern part of the Gulf
Stream opposed the passage of ships from northern Europe to the
westward, and to this day affects the speed with which their voyages
are made.

THE CIRCUIT OF THE RAIN.

We have now to consider those movements of the water which depend upon
the fact that at ordinary temperatures the sea yields to the air a
continued and large supply of vapour, a contribution which is made in
lessened proportion by water in all stages of coldness, and even by
ice when it is exposed to dry air. This evaporation of the sea water
is proportional t