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 thinner than that of to-day. On reflection, however, it will be seen that, if this were all that happened, we might indeed expect to find enormous ice-fields extending from the poles--which we do not find--but not glaciation in the tropics. Others may think of astronomical theories, and imagine a shrinking or clouding of the sun, or a change in the direction of the earth's axis. But these astronomical theories are now little favoured, either by astronomers or geologists. Professor Lowell bluntly calls them "astrocomic" theories. Geologists think them superfluous. There is another set of facts to be considered in connection with the Permian cold.