vern this part of our terrestrial system may be
derived; but, in the present deficiency of historical records, we have
traced up the subject to that point where geological monuments alone are
capable of leading us on to the discovery of ulterior truths. To these,
therefore, we must appeal, carefully examining the strata of recent
formation wherein the remains of _living_ species, both animal and
vegetable, are known to occur. We must study these strata in strict
reference to their chronological order, as deduced from their
superposition, and other relations. From these sources we may learn
which of the species, now our contemporaries, have survived the greatest
revolutions of the earth's surface; which of them have co-existed with
the greatest number of animals and plants now extinct; and which have
made their appearance only when the animate world had nearly attained
its present condition.

From such data we may be enabled to infer, whether species have been
called into existence in succession, or all at one period; whether
singly, or by groups simultaneously; whether the antiquity of man be as
high as that of any of the inferior beings which now share the planet
with him, or whether the human species is one of the most recent of the
whole.

To some of these questions we can even now return a satisfactory answer;
and with regard to the rest, we have some data to guide conjecture, and
to enable us to speculate with advantage: but in order to be fully
qualified to enter upon such discussions the reader must study the ample
body of materials amassed by the industry of modern geologists.

CHAPTER XLIV.

EFFECTS PRODUCED BY THE POWERS OF VITALITY ON THE STATE OF THE EARTH'S
SURFACE.

Modifications in physical geography caused by organic beings--Why
the vegetable soil does not augment in thickness--The theory, that
vegetation is an antagonist power counterbalancing the degradation
caused by running water untenable--Conservative influence of
vegetation--Rain diminished by felling of forests--Distribution of
American forests dependent on direction of predominant
winds--Influence of man in modifying the physical geography of the
globe.

The second branch of our inquiry, respecting changes of the organic
world, relates to the processes by which the remains of animals and
plants become fossil, or, to speak still more generally, to all the
effects produced by the powers of vitality o