near to the poles, while shortly
afterward an ice sheet, such as now covers the greater part of
Greenland, extended down to the line of the Ohio River at Cincinnati.
Although these changes of climate are, as we shall hereafter note,
probably due to entangled causes, we must look upon the modifications
of the ocean streams as one of the most important elements in the
causation. We can the more readily imagine such changes to be due to
the alterations in the course and volume of the ocean current when we
note how trifling peculiarities in the geography of the
shores--features which are likely to be altered by the endless changes
which occur in the form of a continent--affect the run of these
currents. Thus the growth of coral reefs in southern Florida, and, in
general, the formation of that peninsula, by narrowing the exit of the
great current from the Gulf of Mexico, has probably increased its
velocity. If Florida should again sink down, that current would go
forth into the North Atlantic with the speed of about a mile an hour,
and would not have momentum enough to carry its waters over half the
vast region which they now traverse. If the lands about the western
border of the Caribbean Sea, particularly the Isthmus of Darien,
should be depressed to a considerable depth below the ocean level,
the tropical current would enter the Pacific Ocean, adding to the
temperature of its waters all the precious heat which now vitalizes
the North Atlantic region. Such a geographic accident would not only
profoundly alter the life conditions of that part of the world, but it
would make an end of European civilization.

In the chapter on climatal changes further attention will be given to
the action of ocean currents from the point of view of their influence
on the heat and moisture of different parts of the world. We now have
to consider the last important influence of ocean currents--that which
they directly exercise on the development of organic life. The most
striking effect of this nature which the sea streams bring about is
caused by the ceaseless transportation to which they subject the eggs
and seeds of animals and plants, as well as the bodies of the mature
form which are moved about by the flowing waters. But for the
existence of these north and south flowing currents, due to the
presence of the continental barriers, the living tenants of the seas
would be borne along around the earth, always in the same latitude,
and therefo