_Alternate Cold and Warm Periods._—If the theory developed in the foregoing chapters in reference to the cause of secular changes of climate be correct, it follows that that long age known as the glacial epoch did not, as has hitherto been generally supposed, consist of one long unbroken period of cold and ice. Neither did it consist, as some have concluded, of two long periods of ice with an intervening mild period, but it must have consisted of a long succession of cold and warm periods; the warm periods of the one hemisphere corresponding in time with the cold periods of the other and _vice versâ_. It follows also from theory that as the cold periods became more and more severe, the warm intervening periods would become more and more warm and equable. As the ice began to accumulate during the cold periods in subarctic and temperate regions in places where it previously did not exist, so in like manner during the corresponding warm periods it would begin to disappear in arctic regions where it had held enduring sway throughout the now closing cycle. As the cold periods in the southern hemisphere became more and more severe, the ice would continue to advance northwards in the temperate regions; but at that very same time the intervening warm periods in the northern hemisphere would become warmer and warmer and more equable, and the ice of the arctic regions would continue to disappear farther and farther to the north, till by the time that the ice had reached a maximum during the cold antarctic periods, Greenland and the arctic regions would, during the warm intervening periods, be probably free of ice and enjoying a mild and equable climate. Or we may say that as the one hemisphere became cold the other became warm, and when the cold reached a maximum in the one hemisphere, the warmth would reach a maximum in the other. The time when the ice had reached its greatest extension on the one hemisphere would be the time when it had disappeared from the other.