The Earth with which we are here concerned, because of its
 particular orientation in the Whole Sort of General Mish Mash,
 was hit by a neutrino that other Earths were not.

 A neutrino is not a big thing to be hit by.

 In fact it's hard to think of anything much smaller by which
 one could reasonably hope to be hit. And it's not as if being
 hit by neutrinos was in itself a particularly unusual event for
 something the size of the Earth. Far from it. It would be an
 unusual nanosecond in which the Earth was not hit by several
 billion passing neutrinos.

 It all depends on what you mean by 'hit', of course, seeing as
 matter consists almost entirely of nothing at all. The chances of
 a neutrino actually hitting something as it travels through all this
 howling emptiness are roughly comparable to that of dropping a
 ball bearing at random from a cruising 747 and hitting, say, an
 egg sandwich.

 Anyway, this neutrino hit something. Nothing terribly impor-
 tant in the scale of things, you might say. But the problem with
 saying something like that is that you would be talking cross-
 eyed badger spit. Once something actually happens somewhere
 in something as wildly complicated as the Universe, Kevin knows
 where it will all end up - where 'Kevin' is any random entity that
 doesn't know nothin' about nothin'.

 This neutrino struck an atom.

 The atom was part of a molecule. The molecule was part
 of a nucleic acid. The nucleic acid was part of a gene. The
 gene was part of a genetic recipe for growing . . . and so on.
 The upshot was that a plant ended up growing an extra leaf. In
 Essex. Or what would, after a lot of palaver and local difficulties
 of a geological nature, become Essex.

 The plant was a clover. It threw its weight, or rather its seed,
 around extremely effectively and rapidly became the world's
 dominant type of clover. The precise causal connection between
 this tiny biological happenstance, and a few other minor vari-
 ations that exist in that slice of the Whole Sort of General
 Mish Mash - such as Tricia McMillan failing to leave with
 Zaphod Beeblebrox, abnormally low sales of pecan-flavoured
 ice-cream and the fact that the Earth on which all this occurred
 did not get demolished by the Vogons to make way for a new
 hyperspace bypass - is currently sitting at number 4,763,984,132
 on the research project priority list at what was once the History
 Department of the University of MaxiMegalon, and no one cur-
 rently at the prayer meeting by the poolside appears to feel any
 sense of urgency about the problem.