The Ice period has occasioned much dispute and wonderment among men of science. Its remaining traces are chiefly drift, ice-markings such as ice-worn rocks and boulders, with the bones of animals and the traces of Arctic plants found far south of the regions which such organisms occupy at the present time, or did occupy in an age earlier than the Glacial. They indicate a period when ice and cold had sway in lands now temperate. In a future chapter we shall consider for a little the work of icebergs and glaciers in carving the world. At present we can only say that geologists consider that the ice-cap came down to the 40° parallel of latitude north, while others believe that the frequent ice-markings and boulders found throughout the temperate zone are the work of icebergs drifting in Arctic currents, and swept much further south than they are now seen. Up to this period we have considered red-hot worlds, melted worlds, drowned worlds; the Glacial age gives us a frozen world. But the one great fact written over the face of the globe is change. Earth-building sweeps through great circles or periods of change. That a certain state exists is, it seems, merely proof that presently it will not be, but will have made room for some new and different state. Even the Ice age was not to last forever. Frost was king for a time, as fire had been king and water had been king. But the Ice age began at last to loosen its cold clasp upon the world. The atmosphere grew warmer; the land rose up above the icy seas; the bergs sailed into equatorial regions and were dissolved in the warm currents of the Gulf Stream. The Northern hemisphere never returned to the warmth of climate it had enjoyed before the Ice age came on, but it gained the mild temperature of to-day. As the ice-sheet slowly retreated plants sprang up on the earth, the flora following the withdrawing edges of the ice. There was an age when in England and Ireland and France flowers bloomed which now are found only on the high Alps. After a time the flora of the earth became what it is now.