e quantity of equatorial land was
always decreasing and the land in regions nearer the poles augmenting in
height and area, until at length it attained its present excess in high
latitudes. There is nothing improbable in supposing that the
geographical revolutions of which we have hitherto obtained proofs had
this general tendency; and in that case the refrigeration must have been
constant, although, for reasons before explained, the rate of cooling
may not have been uniform.

It may, however, be as well to recall the reader's attention to what was
before said of the indication brought to light of late years, of a
considerable oscillation of temperature, in the period immediately
preceding the human era. We have seen that on examining some of the most
northern deposits, those commonly called the northern drift in Scotland,
Ireland, and Canada, in which nearly all, in some cases, perhaps all,
the fossil shells are of recent species, we discover the signs of a
climate colder than that now prevailing in corresponding latitudes on
both sides the Atlantic. It appears that an arctic fauna specifically
resembling that of the present seas, extended farther to the south than
now. This opinion is derived partly from the known habitations of the
corresponding living species, and partly from the abundance of certain
genera of shells and the absence of others.[204] The date of the
refrigeration thus inferred appears to coincide very nearly with the era
of the dispersion of erratic blocks over Europe and North America, a
phenomenon which will be ascribed in the sequel (ch. 16) to the cold
then prevailing in the northern hemisphere. The force, moreover, of the
German critic's objection has been since in a great measure destroyed,
by the larger and more profound knowledge acquired in the last few years
of the ancient carboniferous flora, which has led the ablest botanists
to adopt the opinion, that the climate of the coal period was remarkable
for its warmth, moisture, equability, and freedom from cold, rather than
the intensity of its _tropical heat_. We are therefore no longer
entitled to assume that there has been a constant and gradual decline in
the absolute amount of heat formerly contained in the atmosphere and
waters of the ocean, such as it was conjectured might have emanated from
the incandescent central nucleus of a new and nearly fluid planet,
before the interior had lost, by radiation into surrounding space, a
great part o