ing of the height of the land from which
they started.

For our main purpose, however, it is fortunately not necessary to clear
up these mysteries. It is enough for us that the Carboniferous land
rises high above the surface of the ocean over the earth generally.
The shallow seas are drained off its surface; its swamps and lagoons
generally disappear; its waters run in falling rivers to the ocean. The
dense, moist, warm atmosphere that had so long enveloped it is changed
into a thinner mantle of gas, through which, night by night, the
sun-soaked ground can discharge its heat into space. Cold winds blow
over it from the new mountains; probably vast regions of it are swept by
icy blasts from the glaciated lands. As these conditions advance in the
Permian period, the forests wither and shrink. Of the extraordinarily
mixed vegetation which we found in the Coal-forests some few types are
fitted to meet the severe conditions. The seed-bearing trees, the thin,
needle-leafed trees, the trees with stronger texture of the wood, are
slowly singled out by the deepening cold. The golden age of Cryptogams
is over. The age of the Cycad and the Conifers is opening. Survivors
of the old order linger in the warmer valleys, as one may see to-day
tree-ferns lingering in nooks of southern regions while an Antarctic
wind is whistling on the hills above them; but over the broad earth
the luscious pasturage of the Coal-forest has changed into what is
comparatively a cold desert. We must not, of course, imagine too abrupt
a change. The earth had been by no means all swamp in the Carboniferous
age. The new types were even then developing in the cooler and drier
localities. But their hour has come, and there is great devastation
among the lower plant population of the earth.

It follows at once that there would be, on land, an equal devastation
and a similar selection in the animal world. The vegetarians suffered an
appalling reduction of their food; the carnivores would dwindle in
the same proportion. Both types, again, would suffer from the enormous
changes in their physical surroundings. Vast stretches of marsh, with
teeming populations, were drained, and turned into firm, arid plains or
bleak hill-sides. The area of the Amphibia, for instance, was no less
reduced than their food. The cold, in turn, would exercise a most
formidable selection. Before the Permian period there was not on the
whole earth an animal with a warm-blooded (four-chambe