The effect of the close approach of such a sun would be the ejection of a stream of matter from our sun, as we term it, in the direction of the passing body and also in a diametrically opposite direction. This ejection would be continuous as long as the stars remained near one another, the height attained by the ejected stream decreasing as the passing star receded. The result would be the formation of a _spiral nebula_ in which the motion of the ejected particles--planetesimals--would be across the spiral arms, toward and away from the passing star. After the sun had receded so far as to have no further effect upon these ejected particles they would revolve about the sun in more or less elliptical orbits which would gradually be reduced to nearly circular forms by repeated collisions between planetesimals. Larger nuclei would be formed and these would gradually sweep up smaller fragments and become the planets of the present system. Smaller nuclei in the vicinity of larger ones would become their satellites and in the course of many millions of years all of the larger fragments would be swept up by the planetary nuclei and their satellites--leaving only the asteroids, comets and meteors as survivors of the original spiral system. It must be borne in mind that a spiral nebula formed by the close approach of two suns would resemble in form only the great spiral nebulæ that are known to exist by hundreds of thousands in the heavens. These are far too extensive to form anything so small as a single solar system, but might condense into systems composed of many suns--either galaxies or star clusters. Jean's suggested theory of the origin of the planetary system differs in its details from the above, though a passing sun is assumed to be the disturbing force that causes the ejection of a stream of matter which condenses to form the planets and their satellites. The origin of the inner planets is left greatly in doubt by this theory, however, and the system which interests us chiefly--the earth-moon system--is the one about which it is most difficult to arrive at any definite conclusion. Our own sun, it is assumed, was dark and cold, of low density and with a diameter about equal to that of Neptune's orbit at the time of the catastrophe which is placed at some 300,000,000 years ago. In Jean's words, "... The time for arriving at conclusions in cosmogony has not yet come--and it must be left to future investigators armed with more mathematical and observational knowledge than we at present possess to pronounce a final decision." However, since La Place advanced his celebrated nebular hypothesis, great advances in astronomy have been made, and man is in a better position to theorize on this fascinating problem today than he was one hundred and twenty-five years ago.