of the air, and the uniformity of climate, both
in the different seasons of the year, and in different latitudes,
appears to have been most remarkable when some of the oldest of the
fossiliferous strata were formed. The approximation to a climate similar
to that now enjoyed in these latitudes does not commence till the era of
the formations termed tertiary; and while the different tertiary rocks
were deposited in succession, from the eocene to the pliocene, the
temperature seems to have been lowered, and to have continued to
diminish even after the appearance upon the earth of a considerable
number of the existing species, the cold reaching its maximum of
intensity in European latitudes during the glacial epoch, or the epoch
immediately antecedent to that in which all the species now contemporary
with man were in being.

CHAPTER VII.

FARTHER EXAMINATION OF THE QUESTION AS TO THE ASSUMED DISCORDANCE OF THE
ANCIENT AND MODERN CAUSES OF CHANGE.

On the causes of vicissitudes in climate--Remarks on the present
diffusion of heat over the globe--On the dependence of the mean
temperature on the relative position of land and sea--Isothermal
Lines--Currents from equatorial regions--Drifting of
icebergs--Different temperature of Northern and Southern
hemispheres--Combination of causes which might produce the extreme
cold of which the earth's surface is susceptible--Conditions
necessary for the production of the extreme of heat, and its
probable effects on organic life.

_Causes of Vicissitudes in Climate._[165]--As the proofs enumerated in
the last chapter indicate that the earth's surface has experienced great
changes of climate since the deposition of the older sedimentary strata,
we have next to inquire how such vicissitudes can be reconciled with the
existing order of nature. The cosmogonist has availed himself of this,
as of every obscure problem in geology, to confirm his views concerning
a period when the planet was in a nascent or half-formed state, or when
the laws of the animate and inanimate world differed essentially from
those now established; and he has in this, as in many other cases,
succeeded so far, as to divert attention from that class of facts which,
if fully understood, might probably lead to an explanation of the
phenomena. At first it was imagined that the earth's axis had been for
ages perpendicular to the plane of the ecliptic, so that there was a
perp