diameter, two hundred and fifteen feet away. Mars, a much smaller pea, would circle around the two- foot globe three hundred and twenty-five feet from its surface; Jupiter, an orange, at a distance of a fifth of a mile; Saturn, a small orange, at two fifths of a mile; and Uranus and Neptune, good-sized plums, three quarters of a mile and a mile and a quarter away, respectively. On this same scale the nearest star would lie eight thousand miles off, and an average third-magnitude star at about the present distance of our Moon; that is, on a scale upon which the Moon should be but seven inches off, the average star would still be as far from us as the Moon is now. Now when we think that each of these stars is probably the centre of a solar system grander than our own, we cannot seriously take ourselves to be the only minds in it all.

Probable, however, as extra-terrestrial life in general is, it is another matter to predicate it in any particular case. Nevertheless, if it exist it must exist somewhere, and the first place to scan is the place we can scan best. Now the Moon appears to be hopelessly dead. Mars, therefore, becomes of peculiar interest, and it was in hope of learning something on the subject that the observations about to be described in this book were made. Before proceeding, however, to an account of what in consequence we have learned about our neighbor, a couple of misapprehensions upon the subject,-- not confined, I am sorry to say, wholly to the lay mind,--must first be corrected. One of these is that extra-terrestrial life means extra-terrestrial human life. Such an inference recalls to my mind the exclamation of an innocent globetrotter to a friend of mine in Japan once, a connoisseur of Japanese painting, upon being told that the Japanese pictures were exceedingly fine. " What! " the globe-trotter exclaimed in surprise, "do the Japanese have pictures,--real pictures, I mean, in gilt frames?" The existence of extra-terrestrial life does not involve "real life in trousers," or any other particular form of it with which we are locally conversant. Under changed conditions, life itself must take on other forms.

The next point is as to what constitutes proof. Now, between the truths we take for granted because of their age, and those we question because of their youth, we are apt to forget that in both proof is nothing but preponderance of probability. The law of gravitation; for example, than which we believe nothing to be more true, depends eventually, as recognized by us, upon a question of probability; and so do the thousand and one problems of daily life upon so many of which we act unhesitatingly and should be philosophic fools if we did not. All deduction rests ultimately upon the data derived from experience. This is the tortoise that supports our conception of the cosmos. For us, therefore, the point at issue in any theory is not whether there be a possibility of its being false, but whether there be a probability of its being true. This, which is evident enough when squarely envisaged, is too often lost sight of in discussing theories on their road to recognition. Negative evidence is no evidence at all, and the possibility that a thing might be otherwise, no proof whatever that it is not so. The test of a theory is, first, that it shall not be directly contradicted by any facts, and secondly, that the probabilities in its favor shall be sufficiently great.

As to what constitutes sufficiency it is important to bear in mind one point, namely, that the odds that a thing is true from the fact that two or more witnesses agree on the same statement is not the sum of the odds that each tells the truth, but the product of those odds. Note (See Lacroix, Traite Elementarire des Probabilites, p. 220) Therefore, if the chances for the truth of a theory, in consequence of its explaining a certain set of details, be three to one, and because of its explaining another set,--for the purposes of argument unrelated to the first,--four to one, then the chances in its favor from its explaining both sets are not seven to one but twelve to one. If it explains a third set whose independently resulting odds are of five to one, the chances in its favor, from its explaining all three sets, not twelve to one but sixty to one; if a fourth set be added, with further odds of five to one, the sum total from the four becomes not seventeen to one but three hundred to one in favor of its being true. It will be seen how rapidly the probability of the truth of a theory mounts up from the amount of detail it explains. This law is to be remembered throughout the coming exposition, for whatever the cogency of each detail of the argument in itself, the concurrence of all renders them not simply additionally but multiplicitly effective. That different lines of induction all converge to one point proves that point to be the radiant point of the result.