I think it very likely that the recent discovery of two Martian satellites will lead many to look with more disfavour than ever on the idea that Mars may not at present be the abode of life. For moons seem so manifestly convenient additions to a planet's surroundings, as light-givers, time-measurers, and tide-rulers, that many will regard the mere fact that these conveniences exist as proof positive that they are at this present time subserving the purposes which they are capable of subserving. I would point out, however, that our own moon must have existed for ages before any living creatures, far less any reasoning beings, could profit by her light, or by the regularity of her motions, or by her action in swaying the waters of ocean. And doubtless she will continue to exist for ages after all life shall have passed away from the earth. Again, there can be no question that our earth would present a most attractive scene if she were viewed from the moon, and would be a most useful ornament of the lunar skies. Yet we have every reason to believe that there is not a living creature on the moon at present to profit by her light. The case may well be the same (apart from the actual evidence that it _is_ the same) with Mars. His satellites may long since have served most useful purposes to his inhabitants; but it by no means follows that because if there were inhabitants on Mars now the same purposes would still be subserved, therefore there are inhabitants there. Let us, however, without considering the question whether the satellites of Mars serve such special purposes for creatures living on the planet, consider briefly the history of their discovery, their nature, and the laws of their motion around the planet.