Although our sole knowledge of animated nature is confined to what we
learn by experience on the earth, yet it is a most entertaining, and by
no means unedifying, occupation, to seek to apply to the exceedingly
diversified conditions prevailing in the other planets, as astronomical
observations reveal them to us, the principles, types, and limitations
that govern the living creatures of our world, and to judge, as best we
can, how far those types and limits may be modified or extended so that
those other planets may reasonably be included among the probable abodes
of life.
In order to form such judgments each planet must be examined by itself,
but first it is desirable to glance at the planetary system as a whole.
To do this we may throw off, in imagination, the dominance of the sun,
and suppose ourselves to be in the midst of open space, far removed both
from the sun and the other stars. In this situation it is only by
chance, or through foreknowledge, that we can distinguish our sun at
all, for it is lost among the stars; and when we discover it we find
that it is only one of the smaller and less conspicuous members of the
sparkling host.
We rapidly approach, and when we have arrived within a distance
comparable with that of its planets, we see that the sun has increased
in apparent magnitude, until now it enormously outshines all the other
stars, and its rays begin to produce the effect of daylight upon the
orbs that they reach.
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