CHANGES IN THE WORLD'S CLIMATE
THE Record of the Rocks is like a great book that has been carelessly misused. All its pages are torn, worn, and defaced, and many are altogether missing. The outline of the story that we sketch here has been pieced together slowly and painfully in an investigation that is still incomplete and still in progress. The Carboniferous Rocks, the "coal-measures," give us a vision of the first great expansion of life over the wet low-lands. Then come the torn pages known as the Permian Rocks (which count as the last of the Palæozoic), that preserve very little for us of the land vestiges of their age. Only after a long interval of time does the history spread out generously again.
It must be borne in mind that great changes of climate have always been in progress, that have sometimes stimulated and sometimes checked life. Every species of living thing is always adapting itself more and more closely to its conditions. And conditions are always changing. There is no finality in adaptation. There is a continuing urgency towards fresh change.
About these changes of climate some explanations are necessary here. They are not regular changes; they are slow fluctuations between heat and cold. The reader must not think that because the sun and earth were once incandescent, the climatic history of the world is a simple story of cooling down. The centre of the earth is certainly very hot to this day, but we feel nothing of that internal heat at the surface; the internal heat, except