of the isothermal lines in a given zone,
and at a given period, must always be circumscribed within narrow
limits, the climate of each place in that zone being controlled by the
combined influence of the geographical peculiarities of all other parts
of the earth. Whereas, if we compare the state of things at two distinct
and somewhat distant epochs, a particular zone may at one time be under
the influence of one class of disturbing causes, and at another time may
be affected by an opposite combination. The lands, for example, to the
north of Greenland cause the present climate of North America to be
colder than that of Europe in the same latitudes; but the excess of cold
is not so great as it would have been if the western hemisphere had been
entirely isolated, or separated from the eastern like a distinct planet.
For not only does the refrigeration produced by Greenland chill to a
certain extent the atmosphere of northern and western Europe, but the
mild climate of Europe reacts also upon North America, and moderates the
chilling influence of the adjoining polar lands.

To return to the state of the earth after the changes above supposed, we
must not omit to dwell on the important effects to which a wide expanse
of perpetual snow would give rise. It is probable that nearly the whole
sea, from the poles to the parallels of 45 degrees, would be frozen
over; for it is well known that the immediate proximity of land is not
essential to the formation and increase of field ice, provided there be
in some part of the same zone a sufficient quantity of glaciers
generated on or near the land, to cool down the sea. Captain Scoresby,
in his account of the arctic regions, observes, that when the sun's rays
"fall upon the snow-clad surface of the ice or land, they are in a great
measure reflected, without producing any material elevation of
temperature; but when they impinge on the black exterior of a ship, the
pitch on one side occasionally becomes fluid while ice is rapidly
generated at the other."[190]

Now field ice is almost always covered with snow;[191] and thus not only
land as extensive as our existing continents, but immense tracts of sea
in the frigid and temperate zones, might present a solid surface covered
with snow, and reflecting the sun's rays for the greater part of the
year. Within the tropics, moreover, where the ocean now predominates,
the sky would no longer be serene and clear, as in the present era; but
mass