Page:Popular Science Monthly Volume 4.djvu/546

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As now understood, the following titles may express the characteristic features of all the great ages of the world, from the birth of matter to the advent of man:

Matter converted into Vapors.
Nebula composed of the entire Solar System.
The Earth-Nebula.
Period of Igneous Fluidity.
Age of Chemical Changes.
Beginning of the Sedimentary Period.
Introduction of Vegetation; or, Eophytic Period.
Introduction of Animal Life; or, Eozoic Period.
Paleozoic Era.
Mesozoic Era.
Cenozoic Era, completed by the advent of Man.

By E. LEWIS, Jr.

THROUGHOUT the realms of Nature motion is indispensable to physical stability and organic existence. It is everywhere present, and equally among molecules and masses the mind searches in vain for evidence of absolute rest. It has been declared that "organic life is a result of motion;" certain it is that motion is a condition of life. It appears in the endless manifestations of beauty and utility, in the world of living creatures of which ourselves are a part. The heavens are more beautiful when clouds are drifting, and the motions of animals give a charm to a landscape which disappears in the solitude of a desert. Stillness to the eye, like silence to the ear, becomes at last painfully oppressive. We scarcely realize, perhaps we seldom consider, how much of the joy and value of existence depends upon the movement of beings, and the marvelous perfection of the means by which it is effected. Walking, swimming, and flying, are the means by which we traverse the three great highways of Nature—the land, the water, and the air. If we change our position, it is in one or other of these. There is no more fascinating chapter of science than this. The mere fact of animal locomotion is felt to be an expression of beneficence, and of adaptation of means to ends which surpasses human ingenuity.

What laws of motion are revealed, what principles of mechanics are brought into action, when animals walk, swim, or fly, has been discussed by many writers, but by none in a more able or interesting manner than by Dr. Pettigrew, who, in a volume soon to appear in