in every part. We find life under the most varied
conditions that can be conceived. It is met with under the burning heat
of the tropics and in the everlasting frost at the poles. We find life
in caves where not a ray of light ever penetrates. Nor is it wanting in
the depths of the ocean, at the pressure of tons on the square inch.
Whatever may be the external circumstances, Nature generally provides
some form of life to which those circumstances are congenial.

It is not at all probable that among the million spheres of the universe
there is a single one exactly like our earth--like it in the possession
of air and of water, like it in size and in composition. It does not
seem probable that a man could live for one hour on any body in the
universe except the earth, or that an oak-tree could live in any other
sphere for a single season. Men can dwell on the earth, and oak-trees
can thrive therein, because the constitutions of the man and of the oak
are specially adapted to the particular circumstances of the earth.

Could we obtain a closer view of some of the celestial bodies, we should
probably find that they, too, teem with life, but with life specially
adapted to the environment--life in forms strange and weird; life far
stranger to us than Columbus found it to be in the New World when he
first landed there. Life, it may be, stranger than ever Dante described
or Dore sketched. Intelligence may also have a home among those spheres
no less than on the earth. There are globes greater and globes
less--atmospheres greater and atmospheres less. The truest philosophy on
this subject is crystallised in the language of Tennyson:--

"This truth within thy mind rehearse,
That in a boundless universe
Is boundless better, boundless worse.

"Think you this mould of hopes and fears
Could find no statelier than his peers
In yonder hundred million spheres?"

[Illustration: PLATE X.

TYCHO AND ITS SURROUNDINGS.

(AFTER NASMYTH.)]

CHAPTER IV.

THE SOLAR SYSTEM.

Exceptional Importance of the Sun and Moon--The Course to be
pursued--The Order of Distance--The Neighbouring Orbs--How are they
to be discriminated?--The Planets Venus and Jupiter attract Notice
by their Brilliancy--Sirius not a Neighbour--The Planets Saturn and
Mercury--Telescopic Planets--The Criterion as to whether a Body is
to be ranked as a Neighbour--Meaning of the word _Planet_--Uranus
and Neptune--C