substance led Whewell to pronounce the planet to be a fluid mass; and Brewster was at some pains to deal with the peculiarity. He endeavoured to show that Jupiter might be formed of solid substances, because there are such substances on our earth of even less specific gravity than Jupiter's. However, the possibility that Jupiter's sphere may be smaller than we might infer from the apparent size of his disc—an extensive cloud-laden atmosphere bounding the disc we measure—is sufficient to remove any objection to the habitability of the planet founded on this peculiarity alone.

In many respects the physical relations of the planet Saturn correspond closely with those of Jupiter. There are, however, two points of difference. In the first place, gravitation at his surface is far less than at Jupiter's, and differs so little from terrestrial gravitation that we may look on this relation as one with respect to which Saturn is well fitted to support terrestrial races. On the other hand, the influence of the Saturnian ring-system would be so unfavourable to most terrestrial races that one can hardly suppose but that Saturnian races are constituted very differently from those which subsist on our earth. It results from a careful examination of the effects of the two gigantic rings which surround Saturn that the sun is totally eclipsed by them for years together in the temperate and sub-tropical zones of Saturn; and that in Saturnian latitudes corresponding to that of Madrid total eclipse lasts for more than eight years.

It appears to me that a careful consideration of all the evidence leads to two conclusions:-First, there is an obvious adaptation of the physical constitution of the planets we have been considering to fit them to be the abodes of living creatures; and secondly, there are obvious reasons for doubting whether these living creatures can very closely resemble terrestrial races.1

To some minds it may appear that to discuss the fitness of the planets to be the abode of living creatures different from those which subsist on the earth is altogether beside the question we are dealing with. The habitability of the planets, many argue, means their fitness to support terrestrial forms of life. But this view appears to me a mistaken one. If indeed it can be shown that in any planet not one of the physical relations subsists which we hold to be essential to the existence of terrestrial races, then indeed it seems idle to speculate upon the general question of the habitability of that planet. For instance, when we consider the case of the moon-without air or water, subjected to a scorching heat during its long day of half a mor th and to a corresponding intensity of cold during its equally long night, and that it is in other important respects utterly unfit for habitation by terrestrial races. -we seem little encouraged to discuss how far the moon may be fitted to support other forms of life, since nothing in our experience enables us to conceive what

This was written in 1868. At present (1874) I hold somewhat different views.

forms of life could possibly exist in so sterile an abode. But when we find in certain planets an obvious provision made for the support of forms of life corresponding to the forms existing on the earth, we seem to be justified in recognising and discussing the habitability of these bodies.

And this leads me to point out a mistake which is commonly made in the application of that argument from the analogy of our own earth which those who believe in the habitability of other worlds justly use. We cannot reason from the fact of the earth's habitability to the habitability of the other planets. We might as reasonably argue from the presumed unfitness of the moon for habitation that the other celestial bodies are also uninhabited. But we can derive a powerful argument from the analogy of our planet when we consider the economy of life upon its surface. When we see the scorched regions of the tropics and the solid ice within the arctic circle freely supporting terrestrial races, while not only the continents but the depths of the ocean and the realms of air are crowded with living creatures; when we find that in long past ages, during which different physical relations from the present have subsisted, the same abundance of life has existed on the earth's surface, we may fairly assume that the planets which present so many physical relations resembling those of our earth are not untenanted by living creatures.

The St. Paul's Magazine, October 1868.

43

OTHER INHABITED WORLDS.

IN the preceding essay we considered the conditions of habitability of the four planets-Venus, Mars, Jupiter, and Saturn. We selected these as being the planets which, so far as we know, present the closest features of general resemblance to our own earth. Yet we could not fail to perceive that although each of them exhibited a striking resemblance to the earth as respects one or more features which we are in the habit of associating with its habitability, there was also, in each case, some distinctive peculiarity which prevented us from pronouncing any of the four planets to be a suitable dwelling-place for man, or probably for any of the principal races now subsisting upon the earth.

And if the four planets which had been selected as affording the strongest evidence in favour of our thesis that there are other inhabited worlds must be pronounced not to be habitable by terrestrial races, it will be readily conceived that the orbs which urge their stately course elsewhere through space, under a thousand different conditions of heat, of illumination, of seasonal changes, or the like, would for the most part be altogether unfit abodes for the present inhabitants of the earth.

Yet it will be noticed that in adopting a different title for the present paper-in which I propose to deal with worlds which seem thus wholly unfit for habitation I have assumed one which, in reality, asserts more than the former one. The title 'Other Habitable Worlds' simply inferred the belief that there are regions throughout space which are more or less fit for habitation, without implying any opinion as to the fact of those worlds or any others being actually inhabited. At present I am about to deal with worlds many of which may at once be admitted to be uninhabitable by most, if not all, of the races living upon the earth, yet I adopt a title which implies the belief that those worlds are certainly the abodes of living creatures.

The fact is, that although when contemplating our solar system we recognise evidence of adaptation to the wants of living creatures, it is when we attempt to conceive the immensity of that space, thronged with suns, which lies beyond the solar system, that we are most powerfully impressed with the conviction that there must be other inhabited worlds. Insignificant as our earth undoubtedly is when her dimensions are compared with the magnificent proportions of many of the other planets, and still more when considered with reference to the grandeur of the solar system itself, we have a far more startling contrast to contemplate when we compare the solar system with the dimensions of the sidereal scheme. From the nearest of the fixed stars the orbit of Neptune would