ust thus have arisen: and as
the condensation must have taken place on the coolest parts of the
surface--namely, about the poles--there must thus have resulted the
first geographical distinction of parts. To these illustrations of
growing heterogeneity, which, though deduced from the known laws of
matter, may be regarded as more or less hypothetical, Geology adds an
extensive series that have been inductively established. Its
investigations show that the Earth has been continually becoming more
heterogeneous in virtue of the multiplication of the strata which form
its crust; further, that it has been becoming more heterogeneous in
respect of the composition of these strata, the latter of which, being
made from the detritus of the older ones, are many of them rendered
highly complex by the mixture of materials they contain; and that this
heterogeneity has been vastly increased by the action of the Earth's
still molten nucleus upon its envelope, whence have resulted not only a
great variety of igneous rocks, but the tilting up of sedimentary strata
at all angles, the formation of faults and metallic veins, the
production of endless dislocations and irregularities. Yet again,
geologists teach us that the Earth's surface has been growing more
varied in elevation--that the most ancient mountain systems are the
smallest, and the Andes and Himalayas the most modern; while 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 characteristics.

Moreover, it must not be forgotten that there has been simultaneously
going on a gradual 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 most exposed to the sun
and those less exposed. Gradually, 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 successi