Marine shells, found on mountain-tops far in the interior of continents, were regarded by theological writers as an indisputable illustration of the Deluge.  But when, as geological studies became more exact, it was proved that in the crust of the earth vast fresh-water formations are repeatedly intercalated with vast marine ones, like the leaves of a book, it became evident that no single cataclysm was sufficient to account for such results; that the same region, through gradual variations of its level and changes in its topographical surroundings, had sometimes been dry land, sometimes covered with fresh and sometimes with sea water.  It became evident also that, for the completion of these changes, tens of thousands of years were required.

To this evidence of a remote origin of the earth, derived from the vast superficial extent, the enormous thickness, and the varied characters of its strata, was added an imposing body of proof depending on its fossil remains.  The relative ages of formations having been ascertained, it was shown that there has been an advancing physiological progression of organic forms, both vegetable and animal, from the oldest to the most recent; that those which inhabit the surface in our times are but an insignificant fraction of the prodigious multitude that have inhabited it heretofore; that for each species now living there are thousands that have become extinct.  Though special formations are so strikingly characterized by some predominating type of life as to justify such expressions as the age of mollusks, the age of reptiles, the age of mammals, the introduction of the new-comers did not take place abruptly. as by sudden creation.  They gradually emerged in an antecedent age, reached their culmination in the one which they characterize, and then gradually died out in a succeeding.  There is no such thing as a sudden creation, a sudden strange appearance—­but there is a slow metamorphosis, a slow development from a preexisting form.  Here again we encounter the necessity of admitting for such results long periods of time.  Within the range of history no well-marked instance of such development has been witnessed, and we speak with hesitation of doubtful instances of extinction.  Yet in geological times myriads of evolutions and extinctions have occurred.

Since thus, within the experience of man, no case of metamorphosis or development has been observed, some have been disposed to deny its possibility altogether, affirming that all the different species have come into existence by separate creative acts.  But surely it is less unphilosophical to suppose that each species has been evolved from a predecessor by a modification of its parts, than that it has suddenly started into existence out of nothing.  Nor is there much weight in the remark that no man has ever witnessed such a transformation taking place.  Let it be remembered that no man has ever witnessed an act of creation, the sudden appearance of an organic form, without any progenitor.