The history of plains, as well as that of mountains, will have further light thrown upon it when in the next chapter we come to consider the effect of rain water on the land. We may here note the fact that the level surfaces which are above the seashores are divisible into two main groups--those which have been recently lifted above the sea level, composed of materials laid down in the shallows next the shore, and which have not yet shared in mountain-building disturbances, and those which have been slightly tilted in the manner before indicated in the case of the plains which border the Rocky Mountains on the east. The great southern plain of eastern and southern United States, extending from near New York to Mexico, is a good specimen of the level lands common on all the continents which have recently emerged from the sea. The table-lands on either side of the Mississippi Valley, sloping from the Alleghanies and the Cordilleras, represent the more ancient type of plain which has already shared in the elevation which mountain-building brings about. In rarer cases plains of small area are formed where mountains formerly existed by the complete moving down of the original ridges. There is a common opinion that the continents are liable in the course of the geologic ages to very great changes of position; that what is now sea may give place to new great lands, and that those already existing may utterly disappear. This opinion was indeed generally held by geologists not more than thirty years ago. Further study of the problem has shown us that while parts of each continent may at any time be depressed beneath the sea, the whole of its surface rarely if ever goes below the water level. Thus, in the case of North America, we can readily note very great changes in its form since the land began to rise above the water. But always, from that ancient day to our own, some portion of the area has been above the level of the sea, thus providing an ark of refuge for the land life when it was disturbed by inundations. The strongest evidence in favour of the opinion that the existing continents have endured for many million years is found in the fact that each of the great lands preserves many distinct groups of animals and plants which have descended from ancient forms dwelling upon the same territory. If at any time the relatively small continent of Australia had gone beneath the sea, all of the curious pouched animals akin to the opossum and kangaroo which abound in that country--creatures belonging in the ancient life of the world--would have been overwhelmed.