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 which now grows. Time after time have these races of organized beings passed away and been replaced by others entirely different. There were times when this globe was inhabited by reptiles far larger than any terrestrial animals now living. All the zoological gardens in the world at present would not be nearly large enough to contain representative specimens of all the varieties of animals which have from time to time found a home on this earth. But these animals have now passed away, and the only means we have of learning that they ever existed is afforded by the occasional skeletons which in the form of fossils are now and then extracted from the rocks. No one has succeeded in making any reliable estimate of the number of years which have run their course since first this globe assumed its present shape. But no one can doubt that these years are to be reckoned in their millions, though whether these millions are to be expressed in units or tens or hundreds, or in periods even greater still, is a matter beyond our knowledge.

During all these ages the earth must gradually have been growing colder, and therefore at the beginning it must have been hotter than it is to-day. As we look back earlier and still earlier the earth ever seems hotter and hotter through the ages. At least the argument points to a time when the earth must have been hot even to its surface, so hot that you could not stand on it, and then earlier still it was red hot, white hot, and molten. Even here the argument does not fail; we find the heat must have been greater and greater the further we look back, until at last we come to a time when the now solid materials of our earth must have been in a widely different form. For we know that the most infusible materials