inappreciable to our eyes, and suns seem no bigger than motes in the air, that nebula is more insignificant than the faintest cloud.  Galileo, in his description of the constellation of Orion, did not think it worth while so much as to mention it.  The most rigorous theologian of those days would have seen nothing to blame in imputing its origin to secondary causes, nothing irreligious in failing to invoke the arbitrary interference of God in its metamorphoses.  If such be the conclusion to which we come respecting it, what would be the conclusion to which an Intelligence seated in it might come respecting us?  It occupies an extent of space millions of times greater than that of our solar system; we are invisible from it, and therefore absolutely insignificant.  Would such an Intelligence think it necessary to require for our origin and maintenance the immediate intervention of God?

From the solar system let us descend to what is still more insignificant—­a little portion of it; let us descend to our own earth.  In the lapse of time it has experienced great changes.  Have these been due to incessant divine interventions, or to the continuous operation of unfailing law?  The aspect of Nature perpetually varies under our eyes, still more grandly and strikingly has it altered in geological times.  But the laws guiding those changes never exhibit the slightest variation.  In the midst of immense vicissitudes they are immutable.  The present order of things is only a link in a vast connected chain reaching back to an incalculable past, and forward to an infinite future.

There is evidence, geological and astronomical, that the temperature of the earth and her satellite was in the remote past very much higher than it is now.  A decline so slow as to be imperceptible at short intervals, but manifest enough in the course of many ages, has occurred.  The heat has been lost by radiation into space.

The cooling of a mass of any kind, no matter whether large or small, is not discontinuous; it does not go on by fits and starts; it takes place under the operation of a mathematical law, though for such mighty changes as are here contemplated neither the formula of Newton, nor that of Dulong and Petit, may apply.  It signifies nothing that periods of partial decline, glacial periods, or others of temporary elevation, have been intercalated; it signifies nothing whether these variations may have arisen from topographical variations, as those of level, or from periodicities in the radiation of the sun.  A periodical sun would act as a mere perturbation in the gradual decline of heat.  The perturbations of the planetary motions are a confirmation, not a disproof, of gravity.

Now, such a decline of temperature must have been attended by innumerable changes of a physical character in our globe.  Her dimensions must have diminished through contraction, the length of her day must have lessened, her surface must have collapsed, and fractures taken place along the lines of least resistance; the density of the sea must have increased, its volume must have become less; the constitution of the atmosphere must have varied, especially in the amount of water-vapor and carbonic acid that it contained; the barometric pressure must have declined.