Page:Popular Science Monthly Volume 37.djvu/327

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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 can not 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. The green cloak which covers the brown earth is the shelter under which millions of organisms, brown or green, carry on their life-work.

Each recognizable kind of animal or plant is known in biology as a species. The number of forms now considered as distinct species is far beyond the usual conception of those who have not made a special study of such matters. I have an old book in my library, the tenth edition of the Systema Naturæ, published by Linnæus in 1758. This book treats of all the species of animals known a little more than a century ago. In its eight hundred and twenty-three pages some four thousand different kinds of animals are named and briefly described. But for every one of these enumerated by Linnæus, more than one hundred kinds are known to the modern naturalist, and the number of species still unknown doubtless exceeds the number of those already recorded. Every year for the last quarter of a century there has been published in London a plump octavo volume known as the Zoölogical Record. Each of these volumes, larger than the whole Systema

  1. An address delivered before the Chicago Institute, in a course on the Testimony of Science in regard to Evolution.