The same set of conditions, in exaggerated degree, exist in the minor superior planets, Ceres, Pallas, Juno, etc., while the asteroids are as much out of the question as the comets and meteors. In regard to the Jovian and Saturnian satellites, only probable conjecture can be indulged. We do not know with sufficient accuracy the degree of heat and light received from their primaries, to judge of those conditions, but all the obstacles flowing out of deficient gravitation predicated of Mars exist in equal degree in these satellites, the largest of which is inferior to Mars in dimensions.
In regard to our own moon much more definite information is accessible, though little need be said of its present life-conditions. Its bi-monthly axial revolution, its long, more than torrid day and antarctic night render it unnecessary to consider the question. But the mass of this satellite being about a third less than even that of Mars, interposes, and has ever interposed, the same everlasting mechanical obstacles to life there as in Mars. Atmosphere the moon may once have possessed, but it must always have been insufficient for life, insufficient to secure the stability of water, which, even if it continued in a liquid condition, would be swept—so light was it—in vast tides over the highest mountains. For the rest, if there be a man in the moon, it is interesting to know that he weighs less than two pounds, and can jump a mile, more or less.
Mercury, with a temperature of boiling water in the frigid zones and red-hot iron at the equator, may be a good place for a Calvinist to send his wicked neighbor, in imagination, but it can not be placed among the list of inhabited worlds. At last, then, out of all the vast, the countless myriads of circling orbs that do homage to our sun, only two remain to be considered—the Earth and Venus.
Venus, although too near the sun to render it likely that her tropical regions are habitable by man, is, so far as can be judged from her general physical condition, by no means destitute of life. If there be truth in the nebular hypothesis, Venus is younger than the Earth, and is therefore perhaps not evolutionized as to the highest forms. As to this, speculation would be little better than conjecture. Enough, that of Venus it can not be said, as of the other planets, with a certitude derived from the exact sciences, that there is no life in her.
The insignificant little globe called the Earth furnishes the only assurance of the higher forms of life, and, with the one exception of a globe even less than ours, of life in any stage of evolution. The Earth is not the millionth part of the known matter of our system, and, compared with the space occupied by that system, is far more insignificant than the smallest fleck of foam in the ocean. This tiny island in space does indeed teem with life; but, if this life were distributed equally through the space given to its production, thousands of miles would intervene between every individual form.
So much for the space and energy expended in the evolution of