With the progress of science we become more and more aware of the undiscovered, and of our feebleness to visualize or express what is dimly known to us. Geologists estimate that man evolved some 1,000,000 years ago on an earth which astronomers say is some 2,000,000,000 years old. Caution is required in accepting such figures, for we must remember how far out Lord Kelvin was in estimating the age of the earth—before the discovery of radium. Man has been civilized for some 5,000 years, and Galileo, with his telescope and revolutionary ideas, lived some 300 years ago. There may be, we are told, a million million years before the sun grows weak and the earth becomes farther from the sun so that all life freezes. Long before this man may use up the metals available for his machines, and those ready sources of energy now so wastefully used, by which he at present multiplies and swarms in cities. In such case he will return through birth control or famine, or both, to a simple, uncrowded, pastoral existence. There is little likelihood of his being able to use atomic energy, other than that which reaches the earth as sun radiation and is available directly or as energy stored in water-power, wood, coal, and oil.
Lord Kelvin and the Age of the Earth