For two especial reasons Mars has generally been regarded as an older or more advanced planet than the earth. The first reason is that, accepting Laplace's theory of the origin of the planetary system from a series of rings left off at the periphery of the contracting solar nebula, Mars must have come into existence earlier than the earth, because, being more distant from the center of the system, the ring from which it was formed would have been separated sooner than the terrestrial ring. The second reason is that Mars being smaller and less massive than the earth has run through its developments a cooling globe more rapidly. The bearing of these things upon the problems of life on Mars will be considered hereafter. And now, once more, Schiaparelli appears as the discoverer of surprising facts about one of the most interesting worlds of the solar system. During the exceptionally favorable opposition of Mars in 1877, when an American astronomer, Asaph Hall, discovered the planet's two minute satellites, and again during the opposition of 1879, the Italian observer caught sight of an astonishing network of narrow dark lines intersecting the so-called continental regions of the planet and crossing one another in every direction. Schiaparelli did not see the little moons that Hall discovered, and Hall did not perceive the enigmatical lines that Schiaparelli detected. Hall had by far the larger and more powerful telescope; Schiaparelli had much the more steady and favorable atmosphere for astronomical observation. Yet these differences in equipment and circumstances do not clearly explain why each observer should have seen what the other did not. There may be a partial explanation in the fact that an observer having made a remarkable discovery is naturally inclined to confine his attention to it, to the neglect of other things. But it was soon found that Schiaparelli's lines--to which he gave the name "canals," merely on account of their shape and appearance, and without any intention to define their real nature--were excessively difficult telescopic objects. Eight or nine years elapsed before any other observer corroborated Schiaparelli's observations, and notwithstanding the "sensation" which the discovery of the canals produced they were for many years regarded by the majority of astronomers as an illusion. But they were no illusion, and in 1881 Schiaparelli added to the astonishment created by his original discovery, and furnished additional grounds for skepticism, by announcing that, at certain times, many of the canals geminated, or became double! He continued his observations at each subsequent opposition, adding to the number of the canals observed, and charting them with classical names upon a detailed map of the planet's surface.