[Illustration] [Illustration: FIG. 8.—CAMBRIAN FOSSILS (above); UPPER SILURIAN FOSSILS (below).] The rock pillar (Fig. 6) represents the rock formation of the crust of the earth. It gives us an idea of what a cross-section of the earth’s crust would look like, if all the rock deposits could be found superimposed one above the other in any one place. These rocks, built up layer upon layer, by sedimentary deposits in the ocean, are believed by geologists to reach a depth of more than forty miles; and it is held that it must have required at least a hundred million years—perhaps a thousand million years—to lay down all the strata, all the series of layers of rock, that form the crust of the globe. That these immense depths of rock have been formed by a slow process of growth, of gradual up-building, is certain. The world was not made in a day. It has grown through innumerable ages. It is still growing. At the present time, the rivers of England are carrying away thousands of tons of land every year and depositing it on the floor of the sea. Every year the Mississippi river carries four hundred million tons of solid material into the Gulf of Mexico. This one stream alone displaces more than a million tons of solid matter every day. Similar things are occurring, in greater or less degree, in every part of the world. Everywhere rivers are widening and deepening their channels, cutting their way into mountains, depositing sediment for rock formations, or overflowing and fertilizing soil; everywhere mountains and hills are being worn down by the action of the elements; continents are crumbling into the ocean; island surfaces are rising above or falling beneath the waves. Nature is ever flowing. Throughout her infinite domain there is everlasting movement. Her trademark is eternal change. Nowhere is she at rest. Her labors never cease. To-day, as in the past, she builds and destroys. In her endless process of evolution, every day is a day of creation. [Illustration: FIG. 9.—THE AMPHIOXUS. This illustration and several others used in this book are reprinted from J. A. S. Watson’s “Evolution.” Courtesy of The Frederick A. Stokes Co.] These rocks contain fossils, the skeletons of creatures that lived during the periods when the rocks were being formed. This fact is one of the keys to the temple of evolution; for when we know the kind of fossils the different rocks contain, as we rise from the lower to the higher, we know the kind of creatures that lived in the various periods when those rocks were formed. Let myth-makers say what they will, Nature does not lie.