ture in every zone, it accords with this theory
that the general climate should not experience any sensible change in
the course of a few thousand years; because that period is insufficient
to affect the leading features of the physical geography of the globe.

Notwithstanding the apparent uncertainty of the seasons, it is found
that the mean temperature of particular localities is very constant,
when observations made for a sufficient series of years are compared.

Yet there must be exceptions to this rule; and even the labors of man
have, by the drainage of lakes and marshes, and the felling of extensive
forests, caused such changes in the atmosphere as greatly to raise our
conception of the more important influence of those forces to which, in
certain latitudes, even the existence of land or water, hill or valley,
lake or sea, must be ascribed. If we possessed accurate information of
the amount of _local_ fluctuation in climate in the course of twenty
centuries, it would often, undoubtedly, be considerable. Certain tracts,
for example, on the coast of Holland and of England consisted of
cultivated land in the time of the Romans, which the sea, by gradual
encroachments, has at length occupied. Here, at least, a slight
alteration has been effected; for neither the distribution of heat in
the different seasons, nor the mean annual temperature of the atmosphere
investing the sea, is precisely the same as that which rests upon the
land.

In those countries, also, where earthquakes and volcanoes are in full
activity, a much shorter period may produce a sensible variation. The
climate of the great table-land of Malpais in Mexico, must differ
materially from that which prevailed before the middle of the last
century; for, since that time, six mountains, the highest of them rising
sixteen hundred feet above the plateau, have been thrown up by volcanic
eruptions. It is by the repetition of an indefinite number of such local
revolutions, and by slow movements extending simultaneously over wider
areas, as will be afterwards shown, that a general change of climate may
finally be brought about.

CHAPTER VIII.

ON FORMER CHANGES IN PHYSICAL GEOGRAPHY AND CLIMATE.

Geographical features of the northern hemisphere, at the period of
the oldest fossiliferous strata--State of the surface when the
mountain limestone and coal were deposited--Changes in physical
geography, between the carboniferous period and th