advance from the known to the unknown. "We must keep one foot on this globe while we place the other on the unknown body. To argue that there may possibly be inconceivably greater variations of the conditions of life on the other planets, is to change entirely the line of argument. Speculations regarding the possible far transcend the region of the astronomical or analogical argument, which deals only with the probable.
Let us now see what are the points of similarity on which an analogical argument for a plurality of worlds is founded. What conditions of life have the planets in common with the earth?
1. They derive their light and heat from the same source—the sun.
2. They have an annual revolution and change of seasons.
3. They have a diurnal revolution, and, consequently, the alternation of day and night.
4. Some have atmosphere and clouds.
5. Some have diversities of surface similar to mountains and valleys.
6. Many of the primary planets are provided with satellites.
7. They are probably composed of the same chemical elements.
8. Their orbits have all the elements of stability.
The above are the chief points of similarity; but to
