but a number of phenomena show that it must be very rare indeed. Some of these have been already considered, along with other lunar phenomena, in an article which appeared in the Cornhill Magazine for August, 1873; and for this reason (especially as that article has since been republished) we do not here enter into this portion of the evidence, our object being to discuss here certain relations which were not dealt with in that earlier paper.
But now that astronomers have almost by unanimous consent, accepted the doctrine of the development of our system, which involves the belief that the whole mass of each member of the system was formerly gaseous with intensity of heat, they can no longer doubt that the moon once had seas and an atmosphere of considerable density. The moon has, in fact, passed through the same changes as our own earth, though not necessarily in the same exact way. She was once vaporous, as was our earth, though not at the same time nor for so long a time. She was once glowing with intensity of heat, though this stage also must have continued for a much shorter time than the corresponding stage of our earth's history. Must we not conclude that after passing through that stage the moon was for a time a habitable world as our earth is now? The great masses of vapor and of cloud which had girt our moon's whole globe, even as in the youth of our earth her seas enwrapped her in cloud form, must at length have taken their place as seas upon her surface. The atmosphere which had supported those waters must at first have been dense by comparison with the present lunar atmosphere, perhaps even by comparison with the present atmosphere of our earth. Then the glowing surface of the moon gradually cooled, until at length the moon must have been a fit abode for life. But whether, when thus swept and garnished into fitness for habitation, the moon actually became an inhabited world, is a question which will be variously answered according to our views respecting the economy of nature in this respect. Those who hold that nature makes nothing in vain, will need only to ask whether the support of life is the one sole purpose which a planet can subserve; if that should appear probable, they would at once decide that the moon must during its habitable stage have been inhabited. Others who, looking around at the workings of nature as known to us, perceive, or think they perceive, that there is much which resembles waste in nature, will be less confident on this point. They may reason that as of many seeds which fall upon the ground, scarce one subserves the one purpose for which seeds can he supposed to have been primarily intended, as many younglings among animals perish untimely, as even many races and types fail of their apparent primary purpose, so our moon, and possibly many such worlds, may never have subserved and never come to subserve that one chief purpose for which the orbs peopling space can be supposed to have been formed, if purpose indeed reigns throughout the universe.
But we are not here concerned to inquire carefully whether the moon ever was inhabited; we care only to show the probability, the all but certainty, that the moon during one stage of her existence was a habitable body, leaving the questions whether she ever actually had inhabitants, and what (if she had) their nature may have been, to the imagination of the reader. Most certainly there is little reason for believing that on this point men will ever have any real information for their guidance.
Combining together several considerations, viz., first that the moon must have been fashioned as a planet many millions of years before the earth, that her original heat must have been greatly less than that of the earth (corresponding to a reduction of many millions of years in the time required for cooling down to the habitable condition), that each stage of the moon's cooling must have lasted less by many millions of years than the corresponding stage for the earth's cooling, and that lunar gravity being so much less than terrestrial gravity the moon's vulcanian vitality must have lasted for a much shorter time than the earth's, we perceive that the moon must have passed that stage of her history which corresponded to that through which our earth is now passing, many many millions of years ago. It would probably be no exaggeration whatever of the truth to say that more than a thousand millions of years have passed since the moon was a habitable world. But we may quite confidently assert that fully a hundred millions of years have passed since that era of her history. And as the changes which she has undergone since then have occurred at a much more rapid rate than those by which the earth is now passing on and will continue to pass on, for ages yet to come, towards planetary decrepitude, we may assert with equal confidence that the moon is passing through a stage of planetary existence which the earth will not reach for
