It now seems probable that it was after this manner that the sun and its family of planets were formed. The matter which is contained in them may have been in the form of a dark, solid body pursuing some sort of course in space. In its journeying it came near another body and was awakened into a life of activity in the form of a flat, spiral nebula which was left spinning around in a pyrotechnic manner, the matter composing it much diffused at the outer edges and densest in the center. Scattered through it were the more or less condensed spots which were the embryonic forms destined to come forth from the parent body as the individual planets. When the separation was completed, each planet fed and grew upon all the matter that it had the force to draw to it, and it swept clean the space that lay within the limits of its power. If the particles thus gathered in were small and slow of motion, they became a part of the body of the planet. If they were large and swift, they became members of the planet’s family as satellites. In whatever area of the nebula each planet came into a separate existence, it fed upon the matter which that area afforded. In the case of Neptune, at the outer edge of the system, it was very diffuse matter; in Mercury’s region, nearer the center, it was more dense. Thus in our family of planets, though its members were born of the same parent and developed under the same guiding laws, each has a distinct individuality arising from its inherent qualities and its environment during the early stages of its existence. The spiral-nebula theory seems to offer a better explanation of these individual qualities than any other that has been advanced thus far, and in its main features it is pretty generally accepted. But one must keep in mind that the details of any theory of the beginning and growth of the planets are more or less speculative, or, at least, have not yet been proved with finality. V THE SEVEN GREAT PLANETS So far as we know, five of the planets--Mercury, Venus, Mars, Jupiter, and Saturn--have been known from time immemorial. There are existing records of them made thousands of years ago. There is no reason why they should not have been thus known, since they have always been as they are now, visible to the naked eye, and all of them save Mercury are as easily seen as the sun or the moon. They do not, of course, exact the instant attention that those great luminaries do, because, being smaller, they are less isolated from the great body of the stars; but they are in their seasons plainly visible, and can then always be seen if one looks at them.