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PREFACE.
WHEN children are shown the wonders of the heavens, for the first time, in the telescope, their natural exclamation usually is, "Are there any people up there in those planets?"
It is an old question, and the affirmative answer is rather supported than overturned by the discoveries of modern science. When we are told that everything almost that we can see on Earth — yea, every particle of dust — has once lived, we are inclined to think that the same law which seems to be the dominant law of Earth, i,e., that nearly all things on Earth's surface either have lived or are now living, may perchance be the general law of the universe.
Our Earth is singular in nothing. In size, the giant planets Jupiter, Saturn, Uranus, and
