discoveries of Kepler and the revelations of the telescope demonstrated that our earth was not specially distinguished from the other planets by any superiority of size or position. The idea at once arose that the other planets might be inhabited; and when the rapidly increasing power of the telescope, and of astronomical instruments generally, revealed the wonders of the solar system and the ever-increasing numbers of the fixed stars, the belief in other inhabited worlds became as general as the opposite belief had been in all preceding ages, and it is still held in modified forms to the present day.

But it may be truly said that the later like the earlier belief is founded more upon religious ideas than upon a scientific and careful examination of the whole of the facts both astronomical, physical, and biological, and we must agree with the late Dr. Whewell, that the belief that other planets are inhabited has been generally entertained, not in consequence of physical reasons but in spite of them. And he adds:—'It was held that Venus, or that Saturn was inhabited, not because anyone could devise, with any degree of probability, any organised structure which would be suitable to animal existence on the surfaces of those planets; but because it was conceived that the greatness or goodness of the Creator, or His wisdom, or some other of His attributes, would be manifestly imperfect, if these planets were not tenanted by living creatures.' Those persons who have only heard that many eminent astronomers down to our own day have upheld the belief in a 'Plurality of Worlds' will naturally suppose that there must be some very cogent arguments in its favour, and that it must be supported by a considerable body of more or less conclusive facts. They will therefore probably be surprised to hear that any direct evidence which may be held to support the view is almost wholly wanting, and that the greater part of the arguments are weak and flimsy in the extreme.

Of late years, it is true, some few writers have ventured to point out how many difficulties there are in the way of accepting the belief, but even these have never examined the question from the various points of view which are essential to a proper consideration of it; while, so far as it is still upheld, it is thought sufficient to show, that in the case of some of the planets, there seem to be such conditions as to render life possible. In the millions of planetary systems supposed to exist it is held to be incredible that there are not great numbers as well fitted to be inhabited by animals of all grades, including some as high as man or even higher, and that we must, therefore, believe that they are so inhabited. As in the present work I propose to show, that the probabilities and the weight of direct evidence tend to an exactly opposite conclusion, it will be well to pass briefly in review the various writers on the subject, and to give some indication of the arguments they have used and the facts they have set forth. For the earlier upholders of the theory I am indebted to Dr. Whewell, who, in his Dialogue on the Plurality of Worlds—a Supplement to his well-known volume on the subject—refers to all writers of importance known to him.

The earliest are the great astronomers Kepler and Huygens, and the learned Bishop Wilkins, who all believed that the moon was or might probably be inhabited; and of these Whewell considers Wilkins to have been by far the most thoughtful and earnest in supporting his views. Then we have Sir Isaac Newton himself who, at considerable length, argued that the sun was probably inhabited. But the first regular work devoted to the subject appears to have been written by M. Fontenelle, Secretary to the Academy of Sciences in Paris, who in 1686 published his Conversations on the Plurality of Worlds. The book consisted of five chapters, the first explaining the Copernican Theory; the second maintaining that the moon is a habitable world; the third gives particulars as to the moon, and argues that the other planets are also inhabited; the fourth gives details as to the worlds of the five planets; while the fifth declares that the fixed stars are suns, and that each illuminates a world. This work was so well written, and the subject proved so attractive, that it was translated into all the chief European languages, while the astronomer Lalande edited one of the French editions. Three English translations were published, and one of these went through six editions down to the year 1737. The influence of this work was very great and no doubt led to that general acceptance of the theory by such men as Sir William Herschel, Sir John Herschel, Dr. Chalmers, Dr. Dick, Dr. Isaac Taylor, and M. Arago, although it was wholly founded on pure speculation, and there was nothing that could be called evidence on one side or the other.

This was the state of public opinion when an anonymous work appeared (in 1853) under the somewhat misleading title of The Plurality of Worlds: An Essay. This was written, as already stated, by Dr. Whewell, who, for the first time, ventured to doubt the generally accepted theory, and showed that all the evidence at our command led to the conclusion that some of the planets were certainly not habitable, that others were probably not so, while in none was there that close correspondence with terrestrial conditions which seemed essential for their habitability by the higher animals or by man. The book was ably written and showed considerable knowledge of the science of the time, but it was very diffuse, and the larger part of it was devoted to showing that his views were not in any way opposed to religion. One of his best arguments was founded on the proposition that 'the Earth's Orbit is the Temperate Zone of the Solar System,' that there only is it possible to have those moderate variations of heat and cold, dryness and moisture, which are suitable for animal life. He suggested that the outer planets of the system consisted mainly of water, gases, and vapour, as indicated by their low specific gravity, and were therefore quite unsuitable for terrestrial life; while those near the sun were equally unsuited, because, owing to the great amount of solar heat, water could not exist on their surfaces. He devotes a great deal of space to the evidence that there is no animal life on the moon, and taking this as proved, he uses it as a counter argument against the other side. They always urge that, the earth being inhabited, we must suppose the other planets to be so too; to which he replies:—We know that the moon is not inhabited though it has all the advantage of proximity to the sun that the earth has; why then should not other planets be equally uninhabited?

He then comes to Mars and admits that this planet is very like the earth so far as we can judge, and that it may therefore be inhabited, or as the author expresses it, 'may have been judged worthy of inhabitants by its Maker.' But he urges the small size of Mars, its coldness owing to distance from the sun, and that the annual melting of its polar ice-caps will keep it cold all through the summer. If there are animals they are probably of a low type like the saurians and iguanodons of our seas during the Wealden epoch; but, he argues, as even on our earth the long process of preparation for man was carried on for countless millions of years, we need not discuss whether there are intelligent beings on Mars till we have some better evidence that there are any living creatures at all.

Several of the early chapters are devoted to an attempt to minimise the difficulties of those religious persons who feel oppressed by the immensity and complexity of the material universe as revealed by modern astronomy; and by the almost infinite