This book treats of the Evolution and the Distribution of Animals

“No one with good eyes and brains behind them has ever looked forth on the varied life of the world—on forest or field or brook or sea—without at least once asking himself this question: "What is the cause of nature's endless variety?" We see many kinds of beasts and birds and trees and flowers and insects and blades of grass, yet when we look closely we find not one grass-blade in the meadow quite like another blade. Not one worm is like its fellow-worm, and not one organism in body or soul is the measure of its neighbor. You may search all day to match one clover-leaf, and, should you succeed, even then you have failed; for, if the two leaves agree in all physical respects, they may still be unlike in that which we cannot see, their ancestries, their potentialities. Again, with each change of conditions, of temperature, of moisture, of space, of time, with each shifting of environment, the range in variety increases. "Dauer in Wechsel" (persistence in change); "this phrase of Goethe," says Amiel, "is a summing up of nature." And the naturalist will tell you that the real variety is far greater than that which appears. He will tell you that, where commonness seems to prevail, it is the cover of variety..."