Page:Popular Science Monthly Volume 31.djvu/523

Rh putting dynamical geology on a firm basis of ascertained fact. The heated interior has been shown almost with certainty to consist of a rigid and solid mass, incandescent, but reduced to solidity under the enormous pressure of superincumbent rocks and oceans. The age of the earth has been approximately measured, at least by plausible guess-work; and the history of its component parts has been largely reconstructed. Structural and stratigraphical geology have reached a high pitch of accuracy. It is beginning to be possible, by convergence of evidences, as the American geologists have shown, and as Geikie has exemplified, to rewrite in part the history of continents and oceans, and to realize each great land-mass as an organic whole, gradually evolved in a definite direction and growing from age to age by regular accretions. Where the old school saw cataclysms and miracles, vast submergences and sudden elevations, the new school sees slow development and substantial continuity throughout enormous periods of similar activity.

It would be impossible to pass over in silence, in however brief a résumé, the special history of the glacial epoch theory—a theory referring indeed only to a single episode in the life of our planet, but fraught with such immense consequences to plants and animals, and to man in particular, that it rises into very high importance among the scientific discoveries of our own era. Demonstration of the fact that the recent period was preceded by a long reign of ice and snow, in the northern and southern hemispheres alike, we owe mainly to the fiery and magnetic genius of Agassiz; and the proof that this glacial period had many phases of hotter and colder minor spells has been worked up in marvelous detail by James Geikie and other able coadjutors. Its theoretic explanation, its probable causes, and its alternation in the northern and southern hemispheres by turns, have been adequately set forth by Croll in a profoundly learned and plausible hypothesis. Upon the glacial epoch depend so many peculiarities in the distribution of plant and animal forms at the present day that it has come to assume a quite exceptional importance among late geological and biological theories. Standing at the very threshold of the recent period, the great ice age forms the fixed date from which everything in modern Europe and America begins—it is the real flood which stands to the true story of our continent and our race in the same relation as the Noachian deluge stood to the imagined or traditional world of our pre-scientific ancestors. Modern history begins with the glacial epoch.

The science of life has been even more profoundly affected by the evolutionary impulse than the concrete sciences of inorganic totals. In 1837 biology as such hardly existed; zoölogy and botany, its separate components, were still almost wholly concerned with minute questions of classification; "vital force" and other unimaginable metaphysical entities were the sole explanations currently offered of all the