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.
These changes, and very many more that might be mentioned, must have taken place not in a discontinuous but in an orderly manner, since the master-fact, the decline of heat, that was causing them, was itself following a mathematical law.
But not alone did lifeless Nature submit to these inevitable mutations; living Nature was also simultaneously affected.
An organic form of any kind, vegetable or animal, will remain unchanged only so long as the environment in which it is placed remains unchanged.
Should an alteration in the environment occur, the organism will either be modified or destroyed.
Destruction is more likely to happen as the change in the environment is more sudden; modification or transformation is more possible as that change is more gradual.
Since it is demonstrably certain that lifeless Nature has in the lapse of ages undergone vast modifications; since the crust of the earth, and the sea, and the atmosphere, are no longer such as they once were; since the distribution of the land and the ocean and all manner of physical conditions have varied; since there have been such grand changes in the environment of living things on the surface of our planet--it necessarily follows that organic Nature must have passed through destructions and transformations in correspondence thereto.
That such extinctions, such modifications, have taken place, how copious, how convincing, is the evidence! Here, again, we must observe that, since the disturbing agency was itself following a mathematical law, these its results must be considered as following that law too.
Such considerations, then, plainly force upon us the conclusion that the organic progress of the world has been guided by the operation of immutable law--not determined by discontinuous, disconnected, arbitrary interventions of God.