Supposing for a moment that we were fully assured that Jupiter and Saturn had separate existence, hundreds of millions of years before our earth had been separated from the great glowing mass of vapour formerly constituting the solar system, and that having this enormous start, so to speak, they need not necessarily be regarded as now very greatly in arrear as respects development, or might even be in advance of the earth, it is altogether improbable that either of them, and far more improbable that both of them, are passing through precisely the same stage of development. If we knew only of two ships, that one had to travel from New York to London, and another from Canton to Liverpool, some time during the year, and that the one which had to make the longer journey was likely to start several weeks before the other, would it not be rather unsafe to conclude, when the former had entered the mouth of the Thames, that in all probability the other was sailing up the Mersey? Yet something like this, or in reality much wilder than this, is the reasoning which permits the student of science to believe, independently of the evidence, or altogether against all evidence, that Jupiter and Saturn are necessarily passing through the very stage of planetary existence through which the one planet we know much about is passing. It seems to me that the student of science should be prepared to widen his conceptions of time even as he has been compelled to widen his conceptions of space. As he knows that the planets are not, as was once supposed, mere attendants upon our earth and belonging to her special domain in space, so should he understand that neither do the other planets appertain of necessity to the domain of time in which our earth's existence has been cast, or only do so in the same sense that like her they occupy a certain domain in space, not her domain, but the sun's. Their history in time, like hers, doubtless belongs to the history of the solar system, but the duration of that system enormously surpasses the duration of the earth as a planet, and immeasurably surpasses the duration of that particular stage of life through which she is now passing. Prepared thus to view the other planets independently of preconceived ideas as to their resemblance to our own earth, we shall not find much occasion to hesitate, I think, in accepting the conclusion that Jupiter is a very much younger planet.