All such theories must necessarily be regarded as working hypotheses only, to be discarded or modified as our knowledge and understanding of the laws of the universe increase. No theory can ever be regarded as final or perfect. The discovery of radio-activity furnishes us with new material for new theories. The sun and the planets may be and probably are far older than we ever dreamed could be possible. It is no longer necessary or reasonable to assume that a greatly extended solar nebula once existed and supplied the planets with heat through gravitational contraction or to place a time limit upon the period required for the formation of the planets and their satellites that is not in accord with the requirements of other sciences. We know today that there exist within the sun powerful repulsive forces, which even under present conditions occasionally eject gaseous matter to heights of five hundred thousand miles or more with a velocity of over two hundred miles per second. Small changes in the velocity of ejection produce great differences in the height of the ejected columns. With an initial velocity of three hundred and eighty miles per second, matter would be thrown from the solar surface to a height of fifty million miles. Were the velocity of ejection three hundred and eighty-three miles per second the height of the column would be five hundred million miles, while a further increase in the initial velocity would send matter away from the sun, never to return. Instead of suns and solar systems evolved from nebulæ we are now more familiar with the idea of nebulæ evolved from stars through some terrific cataclysm as in the case of novas or temporary stars. It is now known that there exist in certain parts of space a number of sharply defined stars surrounded by extensive nebulous envelopes. Are these possibly suns that are going through the process of forming their planetary systems? It is now known that pressure of light and electrical repulsion are forces to be reckoned with in the evolution of stars and nebulæ as well as gravitational contraction. It has long been felt that the peculiar formations existing among the vast irregular gaseous nebulæ could not be explained as gravitational effects alone. _Light-pressure and electrical repulsion_, as well as _gravitation_ are at work within the solar system and the sun is the seat of powerful disturbances which produce periodic outbursts of exceptional activity and which may have produced in the distant past more startling effects than any with which we are familiar at present.