Equally true is this of other natural forces that shape the rocks; such as the daily and seasonal changes of temperature that chip away the mountain peaks and the faces of the cliffs, and the character and number of plants that grow on rocks where they can get a foothold and dying and decaying generate acids which help to etch the rocks away. Trees growing on rocks search out the cracks with their roots and, pushing in and prying them apart, help to change their form. And there is sure to be variety in the arrangement of the wild trees growing on rocks in the mountains and by the sea, since the seeds, being carried by the winds or by running water or by birds or four-footed creatures, fall in an endless variety of groupings. So of the shadows cast by the trees. These shadow masses, so different in shape, owing in part to the irregular arrangement of the trees and in part to the differences in shape of the trees themselves, protect portions of the rock, to a certain extent, against changes in temperature, while the bare rocks are fully exposed to it, so there results a corresponding variety in the result of the sun's work upon the rock. At the same time they help on the acid etching process, because in these shadowed spots there is more moisture and therefore more rapid decay. The form of whole continents follows the same law. Take, for example, Europe. "The geological history of Europe," says Geikie,[49] "is largely the history of its mountain chains"; and the mountain chains, for all their picturesque variety, have also, and necessarily, a certain uniformity, because in the wrinkling of the rocks which made them the vast areas over which they now extend were all subjected to the same force--a big push from one side which crumpled up the earth's outer crust as a table-cloth is crumpled up when pushed forward against a book lying on it. [Footnote 49: Encyclopædia Britannica: article on Geology.] HOW THE VERY SCENERY PLAYS MANY PARTS The ancient history written in the rocks, in the present relative positions of the strata, shows that four times a great mountain system has thus been raised across the face of what is now Europe; that three times large portions of these mountain ranges have been sunk under the sea and new rocks deposited over them; and that the mountains of to-day--the Alps, the Carpathians, and the rest--are the survivors of the fourth time up. Here we have another striking example of the fact that on the great stage of life the very scenery has its exits and its entrances!