members of the
Tertiary system we meet with many chasms, but none which separate
entirely, by a broad line of demarcation, one state of the organic world
from another. There are no signs of an abrupt termination of one fauna
and flora, and the starting into life of new and wholly distinct forms.
Although we are far from being able to demonstrate geologically an
insensible transition from the Eocene to the Miocene, or even from the
latter to the recent fauna, yet the more we enlarge and perfect our
general survey the more nearly do we approximate to such a continuous
series, and the more gradually are we conducted from times when many of
the genera and nearly all the species were extinct to those in which
scarcely a single species flourished which we do not know to exist at
present. We must remember, too, that many gaps in animal and floral life
were due to ordinary climatic and geological factors. We could, under no
circumstances, expect to meet with a complete ascending series.

The great vicissitudes in climate which the earth undoubtedly
experienced, as shown by geological records, have been held to be
themselves proof of sudden violent revolutions in the life-history of
the world. But all the great climatic vicissitudes can be accounted for
by the action of factors still, in operation--subsidences and elevations
of land, alterations in the relative proportions and position of land
and water, variations in the relative position of our planet to the sun
and other heavenly bodies.

Altogether, the conclusion is inevitable that from the remotest period
there has been one uniform and continuous system of change in the
animate and inanimate world, and accordingly every fact collected
respecting the factors at present at work in forming and changing the
world, affords a key to the interpretation of its part. And thus,
although we are mere sojourners on the surface of the planet, chained to
a mere point in space, enduring but for a moment of time, the human mind
is enabled not only to number worlds beyond the unassisted ken of mortal
eye, but to trace the events of indefinite ages before the creation of
our race, and to penetrate into the dark secrets of the ocean and the
heart of the solid globe.

_II.--Changes in the Inorganic World now in Progress_

The great agents of change in the inorganic world may be divided into
two principal classes--the aqueous and the igneous. To the aqueous
belong rain, rivers, springs, c