Allied in appearance to such stars which suddenly flame out in space, but totally different in reality, are comets. These strangers to our own system have excited the wonder and astonishment of mankind from the earliest ages. They seem to defy all rules and all explanation; but, when properly examined, they will fall inevitably into the general scheme of the source and mode of solar energy which we have endeavored to present. These bodies enter our solar system from without. Appleton's Cyclopædia says, "Schiaparelli, to whom the discovery is in part due, considers the meteors to be dispersed portions of the comet's original substance,--that is, of the substance with which the comet entered the solar domain." Professor Proctor, "Meteoric Astronomy," says, "A word or two may be permitted on the question of the condition of comets freshly arriving on the scene of the solar system. It is assumed sometimes that the train of meteors already exists when the comet first comes within the solar domain." In the "Romance of Astronomy" (R. Kalley Miller, M.A.) it is said, "In a sort of debatable territory between our own solar system and the infinite stellar universe around we come upon these erratic and anomalous bodies--the comets; some of which have accidentally become permanent attendants upon our sun; others have only paid it a single casual visit in the course of their wanderings through space, and are not likely again to come within the range of its attracting influence; while countless millions are doubtless scattered throughout the realms of the infinite, whose existence will never be revealed to human ken at all." Professor Helmholtz, in fact (see addendum to his lecture on the origin of the planetary system), advanced the idea in a speculative way, that our terrestrial life might have had its origin in one of these meteoric bodies by the "transmission of organisms through space." In Professor Proctor's article on comets ("Mysteries of Time and Space") he says, "The paths followed by comets show no resemblance either to the planetary orbits or to each other. Here we see a comet travelling in a path of moderate extent and not very eccentric; then another which rushes from a distance of two or three thousand millions of miles, approaches the sun with ever-increasing velocity until nearer to him than parts of his own corona (as seen in eclipses), sweeps around him with inconceivable rapidity, and makes off again to where the aphelion of its orbit lies far out in space beyond the most distant known planet,--Neptune. Some comets travel in a direct, some in a retrograde path; a few near the plane of the earth's orbit, many in planes showing every variety of inclination. Some comets regularly return after intervals of a few years; some after hundreds of years; others are only seen once or twice, and then unaccountably vanish; and not a few show by the paths they follow that they have come from interstellar space to pay our