then, of all the facts above enumerated, respecting the
ancient geography of the globe as attested by geological monuments,
there appear good grounds for inferring that changes of climate
coincided with remarkable revolutions in the former position of sea and
land. A wide expanse of ocean, interspersed with islands, seems to have
pervaded the northern hemisphere at the periods when the Silurian and
carboniferous rocks were formed, and a warm and very uniform temperature
then prevailed. Subsequent modifications in climate accompanied the
deposition of the secondary formations, when repeated changes were
effected in the physical geography of our northern latitudes. Lastly,
the refrigeration became most decided, and the climate most nearly
assimilated to that now enjoyed, when the lands in Europe and northern
Asia had attained their full extension, and the mountain chains their
actual height.

Soon after the first publication of this theory of climate, an objection
was made by an anonymous German critic in 1833 that there are no
geological proofs of the prevalence at any former period of a
temperature _lower_ than that now enjoyed; whereas, if the causes above
assigned were the true ones, it might reasonably have been expected that
fossil remains would sometimes indicate colder as well as hotter
climates than those now established.[202] In answer to this objection, I
may suggest, that our present climates are probably far more distant
from the extreme of possible heat than from its opposite extreme of
cold. A glance at the map (fig. 6, p. 111) will show that all the
existing lands might be placed between the 30th parallels of latitude on
each side of the equator, and that even then they would by no means fill
that space. In no other position would they give rise to so high a
temperature. But the present geographical condition of the earth is so
far removed from such a state of things, that the land lying between the
poles and the parallels of 30, is in great excess; so much so that,
instead of being to the sea in the proportion of 1 to 3, which is as
near as possible the average general ratio throughout the globe, it is 9
to 23.[203] Hence it ought not to surprise us if, in our geological
retrospect, embracing perhaps a small part only of a complete cycle of
change in the terrestrial climates, we should happen to discover
everywhere the signs of a higher temperature. The strata hitherto
examined may have originated when th