or had
he revolved on his axis in a direction opposite to that in which the
planets go round him; or in a direction at right angles to the average
plane of their orbits. With equal safety the motion of the Moon round
the Earth might have been the reverse of the Earth's motion round its
axis; or the motions of Jupiter's satellites might similarly have been
at variance with his axial motion; or those of Saturn's satellites with
his. As, however, none of these alternatives have been followed, the
uniformity must be considered, in this case as in all others, evidence
of subordination to some general law--implies what we call natural
causation, as distinguished from arbitrary arrangement.

Hence the hypothesis of evolution would be the only probable one, even
in the absence of any clue to the particular mode of evolution. But when
we have, propounded by a mathematician of the highest authority, a
theory of this evolution based on established mechanical principles,
which accounts for these various peculiarities, as well as for many
minor ones, the conclusion that the Solar System _was_ evolved becomes
almost irresistible.

The general nature of Laplace's theory scarcely needs stating. Books of
popular astronomy have familiarized most readers with his
conceptions;--namely, that the matter now condensed into the Solar
System, once formed a vast rotating spheroid of extreme rarity extending
beyond the orbit of the outermost planet; that as this spheroid
contracted, its rate of rotation necessarily increased; that by
augmenting centrifugal force its equatorial zone was from time to time
prevented from following any further the concentrating mass, and so
remained behind as a revolving ring; that each of the revolving rings
thus periodically detached, eventually became ruptured at its weakest
point, and, contracting on itself, gradually aggregated into a rotating
mass; that this, like the parent mass, increased in rapidity of rotation
as it decreased in size, and, where the centrifugal force was
sufficient, similarly left behind rings, which finally collapsed into
rotating spheroids; and that thus, out of these primary and secondary
rings, there arose planets and their satellites, while from the central
mass there resulted the Sun. Moreover, it is tolerably well known that
this _a priori_ reasoning harmonizes with the results of experiment. Dr.
Plateau has shown that when a mass of fluid is, as far may be, protected
from the act