scent state existed as an indefinitely extended and nearly
homogeneous medium--a medium almost homogeneous in density, in
temperature, and in other physical attributes. The first change in the
direction of increased aggregation, brought a contrast in density and a
contrast in temperature, between the interior and the exterior of this
mass. Simultaneously the drawing in of outer parts caused motions ending
in rotation round a centre with various angular velocities. These
differentiations increased in number and degree until there was evolved
the organized group of sun, planets, and satellites, which we now
know--a group which presents numerous contrasts of structure and action
among its members. There are the immense contrasts between the sun and
the planets, in bulk and in weight; as well as the subordinate contrasts
between one planet and another, and between the planets and their
satellites. There is the similarly-marked contrast between the sun as
almost stationary (relatively to the other members of the Solar System),
and the planets as moving round him with great velocity: while there are
the secondary contrasts between the velocities and periods of the
several planets, and between their simple revolutions and the double
ones of their satellites, which have to move round their primaries while
moving round the sun. There is the yet further strong contrast between
the sun and the planets in respect of temperature; and there is good
reason to suppose that the planets and satellites differ from each other
in their proper heats, as well as in the amounts of heat they receive
from the sun. When we bear in mind that, in addition to these various
contrasts, the planets and satellites also differ in respect to their
distances from each other and their primary; in respect to the
inclinations of their orbits, the inclinations of their axes, their
times of rotation on their axes, their specific gravities, and their
physical constitutions; we see what a high degree of heterogeneity the
solar system exhibits, when compared with the almost complete
homogeneity of the nebulous mass out of which it is supposed to have
originated.

Passing from this hypothetical illustration, which must be taken for
what it is worth, without prejudice to the general argument, let us
descend to a more certain order of evidence. It is now generally agreed
among geologists and physicists that the Earth was at one time a mass
of molten matter. If so, it