We well know what are the requisite conditions of life on the earth; and we can go no further for grounds of inference; for if we were to start by assuming forms of life capable of existence under conditions widely and essentially different from those pertaining to our planet, there would be no need for discussing our subject further: we could revel in conjectures, without a thought as to their extravagance. The only legitimate phase of the question we can entertain is this:—can there be on the moon any kind of living things analogous to any kind of living things upon the earth? And this question, we think, admits only of a negative answer. The lowest forms of vitality cannot exist without air, moisture, and a moderate range of temperature. It may be true, as recent experiments seem to show, that organic germs will retain their vitality without either of the first, and with exposure to intense cold and to a considerable degree of heat; and it is conceivable that the mere germs of life may be present on the moon.[16] But this is not the case with living organisms themselves. We have, in Chapter V., specially devoted to the subject, cited the evidence from which we know that there can be at the most, no more air on the moon than is left in the receiver of an air-pump after the ordinary process of exhaustion. And with regard to moisture, it could not exist in any but the vaporous state, and we know that no appreciable amount of vapour can be discovered by any observation (and some of them are crucial enough) that we are capable of making. We may suppose it just within the verge of possibility that some low forms of vegetation might exist upon the moon with a paucity of air and moisture such as would be beyond even our most severe powers of detection: but granting even this, we are met by the temperature difficulty; for it is inconceivable that any plant-life could survive exposure first to a degree of cold vastly surpassing that of our arctic regions, and then in a short time (14 days) to a degree of heat capable of melting the more fusible metals—the total range being equal, as we have elsewhere shown, to perhaps 600 or 700 degrees of our thermometric scale. The higher forms of vegetation could not reasonably be expected to exist under conditions which the lower forms could not survive. And as regards the possibility of the existence of animal life in any form or condition on the lunar surface, the reasons we have adduced in reference to the non-existence of vegetable life bear still more strongly against the possibility of the existence of the former. We know of no animal that could live in what may be considered a vacuum and under such thermal conditions as we have indicated. [Illustration: PLATE XXI. NORMAL LUNAR CRATER.]