"QX. Well, when it was discovered that there were millions of times as many planets in the Galaxy as could be accounted for by a dynamic encounter occurring once in two times ten to the tenth years or so, some way had to be figured out to increase, millionfold, the number of such encounters. Manifestly, the random motion of the stars within the Galaxy could not account for it. Neither could the vibration or oscillation of the globular clusters through the Galaxy. The meeting of two Galaxies--the passage of them completely through each other, edgewise--would account for it very nicely. It would also account for the fact that the solar systems on one side of the Galaxy tend to be somewhat older than the ones on the apposite side. Question; find the Galaxy. It was van der Schleiss, I believe, who found it. Lundmark's Nebula. It is edge on to us, with a receding velocity of twelve hundred and forty-six miles per second--the exact velocity which, corrected for gravitational decrement, will put Lundmark's Nebula right here at the time when, according to our best geophysicists and geochemists, old Earth was being born. If that theory was correct, Lundmark's Nebula should also be full of planets. Four expeditions went out to check the theory, and none of them came back. We know why, now--Boskone got them. We got back, because of you, and only you." "Holy Klono!" the old man breathed, paying no attention to the tribute. "It checks--_how_ it checks!" "To nineteen decimals." * * * * * "But still it doesn't explain why you set your traps on that line." "Sure it does. How many Galaxies are there in the Universe, do you suppose, that are full of planets?" "Why, all of them I suppose--or no, not so many perhaps--I don't know--I don't remember of having read anything on that question." "No, and you probably won't. Only loose-screwed space detectives, like me, and crackpot science-fiction writers, like Wacky Willison, have noodles vacuous enough to harbor such thin ideas. But, according to our admittedly highly tenuous reasoning, there are only two such Galaxies--Lundmark's Nebula and ours." "Huh? Why?" demanded Haynes. "Because Galaxies don't collide much, if any, oftener than binaries within a Galaxy do," Kinnison asserted. "True, they are closer together in space, relative to their actual linear dimensions, than are stars; but on the other hand, their relative motions are slower--that is, a star will traverse the average interstellar distance much quicker than a Galaxy will the intergalactic one--so that the whole thing evens up. As nearly as Wacky and I could figure it, two Galaxies will collide deeply enough to produce a significant number of planetary solar systems on an average of once in just about one point eight times ten to the tenth years. Pick up your slide rule and check me on it, if you like."