re such
cold currents exist the effect is to chill the air without adding much
to the rainfall; while the currents setting northward not only warm
the regions near which they flow, but by so doing send from the water
surfaces large quantities of moisture which fall as snow or rain. Thus
the Gulf Stream, directly and indirectly, probably contributes more
than half the rainfall about the Atlantic basin. The lack of this
influence on the northern part of North America and Asia causes those
lands to be sterilized by cold, although destitute of permanent ice
and snow upon their surfaces.

We readily perceive that the effect of the oceanic circulation upon
the temperatures of different regions is not only great but widely
contrasted. By taking from the equatorial belt a large part of the
heat which falls within that realm, it lowers the temperature to the
point which makes the district fit for the occupancy of man, perhaps,
indeed, tenable to all the higher forms of life. This same heat
removed to high latitudes tempers the winter's cold, and thus makes a
vast realm inhabitable which otherwise would be locked in almost
enduring frosts. Furthermore, this distribution of temperatures tends
to reduce the total wind energy by diminishing the trades and counter
trades which are due to the variations of heat which are encountered
in passing polarward from the equator. Still further, but for this
circulation of water in the sea, the oceans about the poles would be
frozen to their very bottom, and this vast sheet of ice might be
extended southward to within the parallels of fifty degrees north and
south latitude, although the waters under the equator might at the
same time be unendurably hot and unfit for the occupancy of living
beings.

A large part of the difficulties which geologists encounter in
endeavouring to account for the changes of the past arise from the
evidences of great climatal revolutions which the earth has undergone.
In some chapters of the great stone book, whose leaves are the strata
of the earth, we find it plainly written in the impressions made by
fossils that all the lands beyond the equatorial belt have undergone
changes which can only be explained by the supposition that the heat
and moisture of the countries have been subjected to sudden and
remarkable changes. Thus in relatively recent times thick-leaved
plants which retained their vegetation in a rather tender state
throughout the year have flourished