e, is based upon the probable similarity in
composition of the earth and the moon. This similarity results almost
equally whether we regard the moon as having originated in a ring of
matter left off from the contracting mass that became the earth, or
whether we accept the suggestion of Prof. G.H. Darwin, that the moon is
the veritable offspring of the earth, brought into being by the
assistance of the tidal influence of the sun. The latter hypothesis is
the more picturesque of the two, and, at present, is probably the more
generally favored. It depends upon the theory of tidal friction, which
was referred to in Chapter III, as offering an explanation of the manner
in which the rotation of the planet Mercury has been slowed down until
its rotary period coincides with that of its revolution.

The gist of the hypothesis in question is that at a very early period in
its history, when the earth was probably yet in a fluid condition, it
rotated with extreme rapidity on its axis, and was, at the same time,
greatly agitated by the tidal attraction of the sun, and finally huge
masses were detached from the earth which, ultimately uniting, became
the moon.[18]

[Footnote 18: The Tides, by G.H. Darwin, chapter xvi.]

Born in this manner from the very substance of the earth, the moon would
necessarily be composed, in the main, of the same elements as the globe
on which we dwell, and is it conceivable that it should not have carried
with it both air and water, or the gases from which they were to be
formed? If the moon ever had enough of these prime requisites to enable
it to support forms of life comparable with those of the earth, the
disappearance of that life must have been a direct consequence of the
gradual vanishing of the lunar air and water. The secular drying up of
the oceans and wasting away of the atmosphere on our little neighbor
world involved a vast, all-embracing tragedy, some of the earlier scenes
of which, if theories be correct, are now reenacted on the
half-desiccated planet Mars--a planet, by the way, which in size, mass,
and ability to retain vital gases stands about half-way between the
earth and the moon.

One of the most interesting facts about the moon is that its surface
affords evidence of a cataclysm which has wiped out many, and perhaps
nearly all, of the records of its earlier history, that were once
written upon its face. Even on the earth there have been geological
catastrophes destroying or bury