Does not a man imply a cooler planet and a greater depth and refinement of soil than a dinosaur? Only after a certain housecleaning and purification of the elements do higher forms appear; the vast accumulation of Silurian limestone must have hastened the age of fishes. The age of reptiles waited for the clearing of the air of the burden of carbon dioxide. The age of mammals awaited the deepening and the enrichment of the soil and the stability of the earth's crust. Who knows upon what physical conditions of the earth's elements the brain of man was dependent? Its highest development has certainly taken place in a temperate climate. There can be little doubt that beyond a certain point the running-down of the earth-temperature will result in a running-down of life till it finally goes out. Life is confined to a very narrow range of temperature. If we were to translate degrees into miles and represent the temperature of the hottest stars, which is put at 30,000 degrees, by a line 30,000 miles long, then the part of the line marking the limits of life would be approximately three hundred miles. Life does not appear in a hard, immobile, utterly inert world, but in a world thrilling with energy and activity, a world of ceaseless transformations of energy, of radio-activity, of electro-magnetic currents, of perpetual motion in its ultimate particles, a world whose heavens are at times hung with rainbows, curtained with tremulous shifting auroras, and veined and illumined with forked lightnings, a world of rolling rivers and heaving seas, activity, physical and chemical, everywhere. On such a world life appeared, bringing no new element or force, but setting up a new activity in matter, an activity that tends to check and control the natural tendency to the dissipation and degradation of energy. The question is, Did it arise through some transformation of the existing energy, or out of the preexisting conditions, or was it supplementary to them, an addition from some unknown source? Was it a miraculous or a natural event? We shall answer according to our temperaments. One sees with his mind's eye this stream of energy, which we name the material universe, flowing down the endless cycles of time; at a certain point in its course, a change comes over its surface; what we call life appears, and assumes many forms; at a point farther along in its course, life disappears, and the eternal river flows on regardless, till, at some other point, the same changes take place again. Life is inseparable from this river of energy, but it is not coextensive with it, either in time or in space.