Over against this marvel of a mild climate, luxuriant vegetation, and nearly equal days and nights within the Polar Circle, we may set that other wonder of the glacial or ice age--the time when the polar cold wandered beyond its bounds and came down into the temperate zone to stay, for how long we cannot tell; for very long indeed, no doubt, though not for as long as a world-building epoch. The Glacial age was only a portion of the age that came just before the human period. This Glacial period is also one of the problems which continue to puzzle the geologist. Why was the Northern hemisphere then so cold? What occasioned the change from tropic warmth to nipping frost? How suddenly did this change come on? How far did it extend, and how long did it last? All these queries cannot be answered, but some of the wonders of that strange, forbidding age we can unfold. We know that the northern half of the world grew colder and colder, and that in this chilling climate the flora and fauna greatly changed. The plants could not accommodate themselves to the cold and most of them perished, and their places were taken by flowers and trees which belonged to colder climates. Instead of palms, figs, cinnamon trees, and magnolias, grew dwarf birch, the willow, and pines, while the less hardy vegetation left representatives only in the tropics. The animals also changed, but more slowly than the plants. Some migrated to the south, others lessened in size and put on thicker coats of fur or feathers; many species perished entirely. Still the cold increased. Vast snow fields loaded the Alps and the Pyrenees, and all the plains and valleys of northern and middle Europe and North America began to be filled with glaciers. Here let us pause to consider that three-fourths of the surface of our globe is water. As we have seen from the study of the moon, a waterless globe is also a lifeless globe, bereft of plant or animal, and we owe in a large part the habitable character of the earth to its abundant water. This water has many forms, as lakes, seas, ponds, springs, rivers. It is salt and fresh, and varies in temperature from the boiling springs or geysers, to the icy seas of the poles. Water has changes of state; from the liquid form it changes by loss of heat to solid ice; moisture, frozen as it falls through the air, becomes crystals of snow; water diffused in the air is mist and dew; partially condensed in the air it forms clouds; and dew in a frozen state is frost.