28. How far that which astronomy thus asserts as possible, is probable:--what is the value of these possibilities of life in distant regions of the universe, we shall hereafter consider. But in what geology asserts, the case is clear. It is no possibility, but a certainty. No one will now doubt that shells and skeletons, trunks and leaves, prove animal and vegetable life to have existed. Even, therefore, if Astronomy could demonstrate all that her most fanciful disciples assume, Geology would still have a complete right to claim an equal hearing;--to insist upon having her analogies regarded. She would have a right to answer the questions of Astronomy, when she says, How can we believe this? and to have her answers accepted.
29. Astronomy claims a sort of dignity over all other sciences, from her _antiquity_, her _certainty_, and the _vastness_ of her discoveries. But the antiquity of astronomy as a science had no share in such speculations as we are discussing; and if it had had, new truths are better than old conjectures; new discoveries must rectify old errors; new answers must remove old difficulties. The vigorous youth of Geology makes her fearless of the age of Astronomy. And as to the certainty of Astronomy, it has just as little to do with these speculations. The certainty stops, just when these speculations begin. There may, indeed, be some danger of delusion on this subject. Men have been so long accustomed to look upon astronomical science as the mother of certainty, that they may confound astronomical discoveries with cosmological conjectures; though these be slightly and illogically connected with those. And then, as to the vastness of astronomical discoveries,--granting that character, inasmuch as it is to a certain degree, a matter of measurement,--we must observe, that the discoveries of geology are no less vast: they extend through time, as those of astronomy do through space. They carry us through millions of years, that is, of the earth's revolutions, as those of astronomy do through millions of the earth's diameters, or of diameters of the earth's orbit. Geology fills the regions of duration with events, as astronomy fills the regions of the universe with objects. She carries us backwards by the relation of cause and effect, as astronomy carries us upwards by the relations of geometry. As astronomy steps on from point to point of the universe by a chain of triangles, so geology steps from epoch to epoch of the earth's history by a chain of mechanical and organical laws. If the one depends on the axioms of geometry, the other depends on the axioms of causation.