ses, geology is to be studied
comprehensively as the evolution of the earth and its inhabitants. The
earth in itself is to be regarded as an organism and as the
foster-parent of a great series of organisms that sprang into being
and pursued their careers in the contact zones between its rigid body
and its fluidal envelopes. These contact zones are, in a special
sense, the province of geography in both its physical and its biotic
aspects. The evolution of the biotic and the psychic worlds in these
horizons is an essential part of the history of the whole, for each
factor has reacted powerfully on the others. An appreciative grasp of
these great evolutions, and of their relations to one another, is
essential to a really broad view of the world of which we are a part;
it is scarcely less than an essential factor in a modern liberal
education.

=Geology embraces all the great evolutions=

Let us agree, then, at the outset, that a true study of the career of
the earth is not adequately compassed by a mere tracing of its
inorganic history or an elucidation of its physical structure and
mineral content, but that it embraces as well all the great evolutions
fostered within the earth's mantles in the course of its career.

Greatest among these fostered evolutions, from the homocentric point
of view, are the living, the sentient, and the thinking kingdoms that
have grown up with the later phases of the physical evolution. It does
not militate against this view that each of these kingdoms is, in
itself, the subject of special sciences, and that these, in turn,
envelop a multitude of sub-sciences, for that is true of every
comprehensive unit. Nor is it inconsistent with this larger view of
the scope of geology that it is, itself, often given a much narrower
definition, as already implied. In its broader sense, geology is an
enveloping science, surveying, in a broad historical way, many
subjects that call for intensive study under more special sciences,
just as human history sweeps comprehensively over a broad field
cultivated more intensively by special humanistic sciences. In a
comprehensive study of the earth as an organism, it is essential that
there be embraced a sufficient consideration of all the vital factors
that entered into its history to give these their due place and their
true value among the agencies that contributed to its evolution. A
true biography of the earth can no more be regarded as complete
without the bi