ve elevations and
subsidences of different portions of the Earth's crust, tending as they
have done to the present irregular distribution of land and sea, have
entailed various modifications of climate beyond those dependent on
latitude; while a yet further series of such modifications have been
produced by increasing differences of elevation in the land, which have
in sundry places brought arctic, temperate, and tropical climates to
within a few miles of each other. And the general result of these
changes is, that not only has every extensive region its own
meteorologic conditions, but that every locality in each region differs
more or less from others in those conditions, as in its structure, its
contour, its soil. Thus, between our existing Earth, the phenomena of
whose varied crust neither geographers, geologists, mineralogists, nor
meteorologists have yet enumerated, and the molten globe out of which it
was evolved, the contrast in heterogeneity is sufficiently striking.

When from the Earth itself we turn to the plants and animals that have
lived, or still live, upon its surface, we find ourselves in some
difficulty from lack of facts. That every existing organism has been
developed out of the simple into the complex, is indeed the first
established truth of all; and that every organism that has existed was
similarly developed, is an inference which no physiologist will hesitate
to draw. But when we pass from individual forms of life to Life in
general, and inquire whether the same law is seen in the _ensemble_ of
its manifestations,--whether modern plants and animals are of more
heterogeneous structure than ancient ones, and whether the earth's
present Flora and Fauna are more heterogeneous than the Flora and Fauna
of the past,--we find the evidence so fragmentary, that every conclusion
is open to dispute. Two-thirds of the Earth's surface being covered by
water; a great part of the exposed land being inaccessible to, or
untravelled by, the geologist; the greater part of the remainder having
been scarcely more than glanced at; and even the most familiar portions,
as England, having been so imperfectly explored that a new series of
strata has been added within these four years,--it is manifestly
impossible for us to say with any certainty what creatures have, and
what have not, existed at any particular period. Considering the
perishable nature of many of the lower organic forms, the metamorphosis
of many sediment