pulation is laid waste. The seed-ferns
die out, and a new and hardy type of fern, with compact leaves, the
Glossopteris, spreads victoriously over the globe; from Australia it
travels northward to Russia, which it reaches in the early Permian, and
westward, across the southern continent, to South America. A profoundly
destructive influence has fallen on the earth, and converted its rich
green forests, in which the mighty Club-mosses had reared their crowns
above a sea of waving ferns, into severe and poverty-stricken deserts.

No botanist hesitates to say that it is the coming of a cold, dry
climate that has thus changed the face of the earth. The geologist finds
more direct evidence. In the Werribee Gorge in Victoria I have seen the
marks which Australian geologists have discovered of the ice-age which
put an end to their Coal-forests. From Tasmania to Queensland they
find traces of the rivers and fields of ice which mark the close of the
Carboniferous and beginning of the Permian on the southern continent.
In South Africa similar indications are found from the Cape to the
Transvaal. Stranger still, the geologists of India have discovered
extensive areas of glaciation, belonging to this period, running down
into the actual tropics. And the strangest feature of all is that the
glaciers of India and Australia flowed, not from the temperate zones
toward the tropics, but in the opposite direction. Two great zones of
ice-covered land lay north and south of the equator. The total area was
probably greater than the enormous area covered with ice in Europe and
America during the familiar ice-age of the latest geological period.

Thus the central idea of this chapter, the destructive inroad of a
colder climate upon the genial Carboniferous world, is an accepted fact.
Critical geologists may suggest that the temperature of the Coal-forest
has been exaggerated, and the temperature of the Permian put too low.
We are not concerned with the dispute. Whatever the exact change of
temperature was, in degrees of the thermometer, it was admittedly
sufficient to transform the face of the earth, and bring a mantle of ice
over millions of square miles of our tropical and subtropical regions.
It remains for us to inquire into the causes of this transformation.

It at once occurs to us that these facts seem to confirm the prevalent
idea, that the Coal-forests stripped the air of its carbon-dioxide until
the earth shivered in an atmosphere th