ile in all probability there
have been corresponding changes in the bed of the ocean. As a
consequence of these ceaseless differentiations, we now find that no
considerable portion of the Earth's exposed surface is like any other
portion, either in contour, in geologic structure, or in chemical
composition; and that in most parts it changes from mile to mile in all
these characters. Moreover, there has been simultaneously going on a
differentiation of climates. As fast as the Earth cooled and its crust
solidified, there arose appreciable differences in temperature between
those parts of its surface more exposed to the sun and those less
exposed. As the cooling progressed, these differences became more
pronounced; until there finally resulted those marked contrasts between
regions of perpetual ice and snow, regions where winter and summer
alternately reign for periods varying according to the latitude, and
regions where summer follows summer with scarcely an appreciable
variation. At the same time the many and varied elevations and
subsidences of portions of the Earth's crust, bringing about the present
irregular distribution of land and sea, have entailed 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 one another.
And the general outcome 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 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 extreme.

When from the Earth itself we turn to the plants and animals which 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 which existed in past
times was similarly developed, is an inference no physiologist will
hesitate to draw. But when we pass from individual forms of life to Life
in gen