5 z8 THE POPULAR SCIENCE MONTHLY. 



As now understood, the following titles may express the character- 

 istic features of all the great ages of the world, from the birth of mat- 

 ter 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. 



WALKING, SWIMMING, AND FLYING. 



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 " or- 

 ganic life is a result of motion ; " certain it is that motion is a con- 

 dition 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 soli- 

 tude 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 ex- 

 pression 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 



