But the planets are probably not all in the same stage of what may be termed geological history. Some may very possibly be in the same state in which the Earth was a few millions of years ago, long before it was fitted for the reception of man on its surface, or, indeed, for that of any of the higher mammalia. The Earth had had a long history, and had undergone vast changes, ranging perhaps over many millions of years, before man appeared on the scene; and the period that has elapsed since that event, whatever the date of it may be, is simply nothing in comparison of the ages that had previously rolled by since the first moment when the darkness gave way, and the light appeared. It is, then, far from unlikely that our own Earth is the only planet in the solar system which at the present time is suitable for the habitation of man, or creatures resembling him.[22]
Passing then from our own system, we come to the myriads of suns, some, we may well believe, far greater than our Sun, which are spread through the realms of space.[23] Many of these we may reasonably suppose are surrounded by planets, and in one or two cases there are special reasons for thinking that some opaque body intervenes occasionally between the star and ourselves. But the conditions under which several of the stars (we know not how many) exist, is very different from that to which we are accustomed here with our own Sun. There are double stars which appear to revolve round a common centre of gravity, a system of two suns. Have each of them, or have both of them in common, a set of planets moving round them? Who can tell? And where there are stars with planets accompanying them, does any one know in what state those planets are? The whole subject, however interesting as a speculation, is shrouded in impenetrable mystery.
From all this it follows that although there certainly may be rational and intellectual inhabitants on some or other of these distant worlds, yet, on the other hand, there _may not_ be. And it is perfectly possible that our Earth, minute little object as it is, comparatively speaking, may still be the great and favoured life-house of the universe, the _moral_, though not _material_, centre. That the Earth is not the physical centre of the universe we now are well aware; nor is the Sun the centre; nor, indeed, do we know whether there is any such centre at all. There is good reason for thinking that the Sun, with his attendant planets, is in motion in a certain direction in space; and I may observe that this direction is not in the plane of the Earth’s orbit, or anything near it; so that though the Earth describes an elliptical orbit with regard to the Sun, its path in space is some kind of spiral curve, that is as it would appear to a being poised for a time in some point of space far away outside our orbit, having the necessary powers of vision, and having a plane of reference from which he could take his observations.