THE PROGRESS OF SCIENCE FROM 1836 TO 1886. 507 



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 conver- 

 gence 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 

 resume, 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 coad- 

 jutors. Its theoretic explanation, its probable causes, and its alterna- 

 tion 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 geo- 

 logical and biological theories. Standing at the very threshold of the 

 recent period, the great ice age forms the fixed date from which every- 

 thing 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 ; zoology and botany, its sepa- 

 rate components, were still almost wholly concerned with minute ques- 

 tions of classification ; " vital force " and other unimaginable meta- 

 physical entities were the sole explanations currently offered of all the 



