ecame altogether out of
the question to refer their entombment to the confusion of a single
catastrophe, for every thing indicated an orderly and deliberate
proceeding. Still more cogent did this evidence become when, in a more
critical manner, the fossils were studied, and some strata were
demonstrated to be of a fresh-water and others of a marine origin, the
one intercalated with the other like leaves in a book. To this fact may
be imputed the final overthrow of the doctrine of a single catastrophe,
and its replacement by a doctrine of periodical changes.

[Sidenote: The orderly progression of organization.] From these
statements it will therefore be understood that, commencing with the
first appearance of organization, an orderly process was demonstrated
from forms altogether unlike those with which we are familiar, up to
those at present existing, a procedure conducted so slowly that it was
impossible to assign for it a shorter duration than thousands of
centuries. Moreover, it seemed that the guiding condition which had
controlled this secular march of organization was the same which still
determines the possibility of existence and the distribution of life.
The succession of organic forms indicates a clear relation to a
descending temperature. The plants of the earliest times are plants of
an ultratropical climate, and that primitive vegetation seemed to
demonstrate that there had been a uniform climate--a climate of high
temperature--all over the globe. The coal-beds of Nova Scotia exhibited
the same genera and species as those of Europe, and so well marked was
the botanical connexion with the declining temperature in successive
ages that attempts were made to express eras by their prevailing
organisms; thus Brongniart's division is, for the Primary strata, the
Age of Acrogens; the Secondary, exclusive of the Cretaceous, the Age of
Gymnogens; the third, including the Cretaceous and Tertiary, the Age of
Angiosperms. It is to be particularly remarked that the Cretaceous
flora, in the aggregate, combines the antecedent and succeeding periods,
proving that the change was not by crisis or sudden catastrophe, but
that the new forms rose gently among the old ones. After the Eocene
period, dicotyledonous angiosperms became the prevalent form, and from
that date to the Pleistocene the evidences of a continued refrigeration
are absolute.

[Sidenote: Climates in time and in place.] As thus an examination was
made from the