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.